Portable DIY X-Ray Source & Radiography

Intro.

X-rays, "rays of the unknown" as they were first called, marveled all they encountered since their discovery in 1895 by Wilhelm Roentgen. Given my background and familiarity with radiation, physics, and electronics, making an X-ray unit was only a matter of time. Even though many have done this before me, and if boiled down to its essence it is nothing more than applying the correct power to the correct vacuum tube, it is still quite an accomplishment. Much of the wonders of the world lose their appeal when distilled too far, after all, so let's not get too complacent and forget how great it is what we are achieving here.

SAFETY.

Before I go any further, I want to explicitly point out that I possess and employ the use of adequate safety procedures and equipment during all activities related to the construction, operation, and use of this device. Everything from radiation detection equipment to radiation shielding, to electrical and fire safety equipment is employed to ensure a safe adventure in radiography. If you do not *intimately* understand the hazards and means of minimizing said hazards involved in such a device, please never, ever attempt this. You can seriously injure yourself or any bystanders, even ones outside of the immediate operation area. Potentially, you could even kill yourself. THIS IS DANGEROUS.

The Nitty Gritty Basics.

So, how about a paragraph on the internal process of generating X-rays? Simply put we're accelerating Electrons and then smashing them at a very dense metal target. The electric field of the nucleus of the high density target atom slows down and scatters the incident fast electrons. During the braking effect, the electrons emit a continuum spectrum of x-ray photons as a means of releasing energy. This is Bremsstrahlung Radiation, or "braking x-rays". The maximum energy of the emitted photons is the maximum speed of the incident electrons; a 70kV accelerated electron can emit a 70keV photon. Secondly, the electrons bound to the high density target atom can get knocked out of place. When this happens the electrons in the upper levels of that excited atom fall down to fill the void of the ejected electron. The electron that falls to fill the void emits a x-ray photon as a means of releasing excess energy. This is akin to fluorescence, and is indeed referred to as XRF - X-ray fluorescence. The energy of the emitted photon is a characteristic of the atom that released it. Target atoms typically have a few discrete energies emitted by characteristic XRF.

Operational Mode of my Tube.

So, in our Coolidge tube as invented by William Coolidge in 1913, we apply a high positive voltage to the Anode (which is constructed of a high density metal, tungsten in my tube, and surrounded by copper as a heatsink) in respect to the Cathode. The cathode here is actually a filament made of tungsten which we pass a high current at low voltage through, like a light bulb. When the tungsten reaches about 2000C it emits copious amounts of electrons all around it in a cloud. This is often referred to as "boiling off electrons". These electrons are accelerated towards the high voltage anode via electrostatic attraction. As my tube is a modern coolidge tube designed for dental radiography it has a third electrode inside; the Wehnelt Cylinder Electrode. This was invented by Arthur Wehnelt in 1903, and is essentially an electrostatic lens. Its function is similar to that of a screen grid in a standard thermionic valve vacuum tube; to repel or attract electrons from the cathode. Unlike in a standard valve tube, where the grid is between the cathode and the anode, in our x-ray tube it is behind the cathode. Here it acts as an electrostatic reflector and focuser to shape the cloud and resultant beam of electrons. It would also accelerate the electrons towards the anode, but by a negligible amount compared to the effect of the anode's high positive voltage. Since our grid is behind the cathode, it doesn't use a positive voltage in respect to cathode to allow/enhance flow of electrons to the anode, it needs a negative voltage! Think of it this way, instead of pulling electrons closer to the anode, it is pushing them away from it self, towards the anode. I do not currently have a bias supply set up for the Wehnelt electrode in my tube, which would use between -100V and -500V. Fortunately, most dental tubes operate just fine with the Wehnelt electrode held at Cathode potential, the result of course being far simpler implementation of the tube at a reduced clarity in the resultant radiograph. I may add this supply in later, as I already have an idea on how to achieve this inexpensively and easily. It would of course require I modify the existing setup, and if you've seen the photos of my build, you'll know that means messing with a bunch of messy mineral oil.

Datasheet? I don't need no stinkin' datasheet! ...Oh wait, I wish I had one...

Since the datasheet for my tube was unavailable, and it is no longer in production, I had to determine filamentary specifications empirically. Thankfully I had some guidance from the helpful folks at 4HV.org. Among all the challenges involved in making your own x-ray machine, not having the specifications of your x-ray emission device is no small hill amongst the mountains! Not wishing to vaporize or sever my filament, I couldn't just use figured supplied with modern incarnations of my tube and hope for the best. I had to start conservatively and ease the cathode into operation, despite the risks of shortened cathode life that a normal filament cathode endures if run at too low of a voltage. Fortunately, in x-ray heads the amount of X-rays produced, or x-ray flux is controlled via the filament voltage. Emitting more electrons means more electrons strike the anode, which results into more x-rays emitted.

Initial Testing.

I first connected my coolidge tube to a small low power pulsed HVDC supply in the form of a VCO controlled modern Flyback transformer. Here the currents available are quite small, and the voltage sags greatly under load. Output is between 15kV and 55kV at several mA to a few hundred uA (respectively); sufficient to cause bremsstrahlung x-ray flux levels needed to do initial emission present/not-present testing but no where near enough to do radiography. With this crude setup I was able to determine that 1.95V is the minimum filament emission voltage. Below this point, while the filament is glowing quite bright, no x-rays are produced. It should be noted that because of the pulsed nature of the DC supply and very low levels of x-ray flux produced I was able to use a standard SBM-20 Geiger Muller tube detector. The detector was faraday shielded with 1mm aluminium to rule out false positives from electromagnetic noise, and to reduce the response to very soft x-rays. Now I knew the absolute minimal filament voltage, and it isn't something absurdly low (like under 1.5V would be).

Planning and Sourcing Parts.

Initial planning was done several years ago when I first decided that I would have a responsible use for a radiography setup. I would either find a suitable x-ray transformer and rectify and filter the output, or I would generate suitable HVDC via a common HVAC transformer such as an AC flyback or NST and then amplify the output voltage using a Cockroft-Walton Voltage Multiplier. I ended up going with the latter as X-ray transformers are expensive and hard to come by, where as CW Multipliers are inexpensive and easy to make, and while still rare, AC flyback transformers are a lot cheaper and easier to source. So, I knew what parts I needed; a CW multiplier, an HVAC transformer, an x-ray tube, a transformer driver, a variable voltage filament supply, an enclosure suitable for high voltage oil potting, and finally a remote trigger or controller for safety.

The CW multiplier is easily made from parts available on eBay. If you derate the current, voltage, and capacity specifications on chinese eBay parts they tend to perform well. I chose a 66% derating factor. 30kV 100mA diodes and 30kV 3.3nF capacitors. The caps are actually right on spec capacitance wise, which is nice.

A good friend of mine, Fiddy, came to me a few years ago asking about having custom AC flyback transformers made to order by a Chinese manufacturer. He was unsure of specifics of the design and wanted me to consult for him on the order. I obliged, requesting only a transformer from the resultant batch as payment. His transformers are quite excellent, and he still sells them from time to time. If you want one, contact him on 4HV.org or laserpointerforums.com , tell him Sig sent you. Anyway, I used his transformer for a few temporary projects over the years but saved it for use in my eventual x-ray machine build.

The x-ray tube took the most time to actually track down inexpensively. A nice couple was selling off some things their father had after he passed, and decided to list two Toshiba D-138B Coolidge tubes on eBay at less than 25% the normal going rate as they did not know much/any information about them. SCORE!

Container/enclosure is a $1 polyethylene bin from walmart.

The transformer driver I actually picked up from eBay a year or two ago. It's an inexpensive chinese made Royer type parallel-LC oscillator designed for ZVS operation of an air-core coil for induction heating. It works equally well for flyback transformers if you size the primary coil correctly and watch the peak voltages and currents. I could have easily designed and built a better driver, but honestly you can't beat chinese-made eBay prices. My driver would have cost me twice as much in parts alone. This driver uses about 160W and outputs about 150W when connected to Fiddy's transformer, quite good efficiency. The peak voltage across the LC tank is about 30V when supplied with 13.8V input, and circulating current is over 150A. A good engineer knows when to use an off the shelf solution.

For filament supply I am using my variable bench PSU, but I do have a variable dc-dc buck converter on order that will fill this role and allow for truly portable operation.

Lastly, I picked up a very inexpensive radio controlled relay on eBay for use as a remote trigger controller. It allows for latching and non-latching operation and uses a 300MHz radio. It works great, has excellent range, and doesn't suffer any interference from both the EM noise from the HV power supply or the incident stray x-rays. I use it in non-latching mode for maximum safety.

For HV insulation I am using Mineral Oil, which is available over the counter as laxative. Yes, I'm quite sure I got strange looks when I bought eight bottles of laxative. The things I do for science!

Minor parts: I am using nylon strapping to secure and aim the coolidge tube, as it is nonconductive and durable. I'm using nylon 6-32 x 2" standoffs as mounting posts. All screws and nuts are 6-32 UNC, both nylon and steel. Nylon zip ties secure the AC flyback transformer. A folded thin polyethylene bag serves as additional primary-core insulation on the flyback as well.

Build Pictures!

The Build is "Finished".

The build is essentially finished. Note all above photos are from before oil was added. I just have to install the DC-DC buck converter for the filament supply once it comes in and that's a 10min job at most. If I decide to implement Wehnelt bias then I'll have to dig in and sever the junction between it and the cathode, and then add a line from the Wehnelt pin to a new bushing, install a new bushing, and mount the Wehnelt supply. Probably an hour's job of tedious, but not hard work. I have ordered new X-ray intensifier screens though and I will see how the results are with them before I put in the effort, time, and money to add a Wehnelt bias.

After all, you can have the best lighting rig in the world but if your camera is terrible then your pictures will be terrible!

Radiography Awaits!

Well, my cameras are most certainly not terrible. In fact they're quite excellent, however since x-rays are not focusable with glass and are not directly observable with traditional cameras there has to be an intermediary device for converting X-rays into visible wavelengths. Enter the X-ray Intensifier Screen (XIS). (Un)fortunately the XIS I have been saving for nearly a decade is extremely old, from the 1950s or 1960s, and even more unfortunately it is a BLUE EMISSION type phosphor screen. The emission spectra seems to be mostly between 405nm and 450nm, with a peak around 425nm. It is also a "Rapid" screen, meaning it has very large granules of phosphor which will respond faster, resulting in lower exposure times at the cost of image clarity. While this is all well and good for using an actual film with good UV/NUV/Blue sensitivity, it is rather poor for use with a modern digital camera. Even when using my Full Spectrum camera, which can perceive UV through IR, the response will be much lower than if I had a GREEN EMISSION XIS because of the internal Bayer Filter on nearly all digital cameras which has twice as many Green Pixel Sensors than Blue or Red. Still, I'm a competent Photographer, I should be able to work around this, especially since I'm not x-raying living tissue, so dose (exposure time) isn't a concern. I have since ordered some new Green XIS cartridges in both the Fine and Regular speeds, which should greatly enhance results.

My First Radiograph.

Here we see the internals of household mains electrical wall power switch. I hadn't even begun to determine what the right exposure times, anode current/filament voltage/X-ray flux, or positioning setup would be. Here I simply gave things a "best guess" and saw what resulted. Contrast isn't great, there's a ton of noise, the projected image is skewed, etc. As a photographer it makes me cringe, but upon seeing the raw image I was ecstatic! I had achieved a real radiograph. The culmination of years of planning and scrounging parts bore fruit! What was next was a tremendous amount of work...

System Limitations Impose Interesting Operational Paradigms.

I first began by leaving photographic parameters fixed. I needed to work out the radio-electric parameters. According to the basic principles of operation the Anode current is directly proportional to the X-Ray emission flux. Increasing the X-ray flux increases the brightness of the resulting XIS image, increases the contrast of the image (only between full absorb and full transmit regions, not the contrast gradient between different densities), and results in shorter exposure times. Shorter exposure times are one of the holy grails of photography, even more so for radiography.

So, increase the filament voltage; boil more electrons off, increase x-ray output, decrease exposure time. That's all well and good in a professional x-ray unit where the capabilities of the HVDC supply meet or exceed the capabilities of the x-ray tube. Unfortunately, I didn't, and still to an extent don't know the capabilities of the x-ray tube. As such, my power supply cannot supply the full anode current the x-ray tube is capable of drawing. I found this out when during my careful study of and experimentation with the relationship between filament voltage and the resultant radiograph the penetrating ability of the x-rays seemed to diminish above a certain filament voltage. This is clear sign of a "sagging" HV supply. As the current draw increases the CW Multiplier is unable to fully charge its capacitors with the current available from the AC Flyback. The more output current is drawn, the less and less the capacitors are able to charge. The result is the output voltage from the multiplier drops. Less voltage on the Anode means that the resulting Bremsstrahlung x-rays are softer and softer as the peak photon energy drops. This can be useful for imaging low density or very thin subjects, but generally is undesirable as the lower the energy of the x-ray the more easily it is absorbed by living tissue, making the radiation even more dangerous. This is one of those times when you can say your device's bug is a feature, but it's really still just a bug. The only thing that determines if it is a bug or a feature is original intent. Any gamer can tell you that!

Interestingly, while probing the upper end of usable filament voltage / x-ray softness I discovered a hard cutoff effect of the tube. At 3.0V filament the anode current draw is such that either the HV supply completely loads down below minimum B+ rating of the tube, or the x-rays are so soft that they cannot penetrate the envelope with sufficient flux to illuminate the XIS, or the XIS is unable to fluoresce at such a low energy illumination. Since I don't have a means of measuring the anode current or voltage directly I am unable to differentiate between these possible explanations. If I happen upon an analog galvanometer type milliammeter I will investigate further. For now the interest is merely scholarly as I am not going to rework or replace the HV supply, so I will have to live with these characteristics of the device.

This leaves me with a usable filament range of 2-3V, with the apparent "sweet spot" between hardness and flux at about 2.65 Volts DC. X-Ray hardness at this point is sufficient for penetrating thin aluminium quite well, and the flux is at close to 85% of the observed maximum. Below this point I get harder x-rays, but must increase exposure time greatly. Above this point the x-rays get much softer, but the exposure times shorten moderately.

An Important Note about your Anode.

I feel I should take a moment to talk about another important limitation on the operation of x-ray tubes; anode dissipation. The anode has to be a tremendous amount of power flowing through it. More so, it has this power directly striking it. Interesting bit of knowledge; a thin piece of wire can conduct hundreds of watts of power supplying an electrode that is emitting a spark/arc/streamer, but if that same wire were to be the point at which the arc is emitting it will overheat and vaporize or melt very quickly. The thermal effects of ions and electrons are not to be underestimated. The tube's anode can only "sink" so much heat, and that heat needs to wicked away and removed from the system. The heat load also needs to be under a threshold that would cause damage to the target surface. In short pulse exposures the heat deposition rate can actually exceed the speed at which the anode material can move heat away from the target spot, which causes ablation and thermal stress on the anode. Given that my tube doesn't have a biased Wehnelt electrode and the fact that my tube is clearly sagging my power supply there is little threat of exceeding the thermal limitations of my tube's anode. Additionally, I have submerged the entire tube in electrically insulating yet thermally conductive oil, aiding removal of heat. If I add Wehnelt bias I will have to revisit the thermal performance, but I doubt it will be an issue.

Well, without further ado, shall we see some more of my radiographs?

Here from L to R is a Bluetooth Audio Receiver, a wallwart SMPS, a bare double-sided PCB.

Here is a glass vial of sodium metal (Na) chunks in mineral oil next to the SMPS above. The vial is on top of a small bismuth metal (Bi) ingot, and the SMPS is on an empty polyethylene wire spool.

Here we can see the internals of a Mobius ActionCam mini-camcorder.

Here we have a somewhat skewed image (in two axes) of a Kill-a-Watt meter. Despite the very noisy/grainy radiograph you can clearly make out the ribbon cable connecting the two boards and the display driver's ceramic potting compound on the center of the upper board.

Here is the bottom half of my LCR/BJT/FET meter and tester. You can see the 9V battery, several potentiometers, and a microcontroller quite clearly. The blobby bit on the bottom right of the pcb is a ZIF interface slot which has a fair bit of steel in it.

Here is a rather clear image of a radiotelemetry receiver unit. You can clearly see the AAA batteries, their contacts, the main PCB, and the internal wiring. I believe this image to show the maximum clarity achievable with my current XIS. The radio-electrical specifics of this exposure are that of my observed nominal operation point. You can see a good differentiation between the densities of the plastic, metal, battery internals, and pcb components. Any harder x-rays and the case would be totally transparent, any softer and the battery internals would not be visible.

Here you can see a minimally enhanced "raw" image from my setup. In post processing I adjusted the histogram to maximize brightness, contrast, and correct for exposure errors. I intentionally left the channel data close to original so that the XIS color could be observed.  Pictured in the radiograph is a precision 1ohm resistor mounted in an aluminium heatsink, on top of which is an EOL HeNe laser tube. You can clearly see inside the "cathode can" of the tube to see where the bore ends, something I've always wondered about. The spool of solder and zeiss lens wipe are illuminated by reflected blue light emitted from the XIS. I've been using the zeiss wipe as a focusing aide to assist with getting the focal plane where I want it since I'm using manual focus of a very wide aperture telephoto lens.

Looking Forward.

I have some fun times to look forward to with the two new XIS and buck converter coming in the mail in the next few weeks. Once they're all here I can position the x-ray unit, object to be imaged, XIS, and camera freely without being tethered to my workbench. This will not only enhance safety and ease of setup and use, but should result in clearer images as the focal plane of the camera will match the image plane of the XIS. I will be able to try first-surface projection as well as transmission projection (which was used in all of the above images). Eventually I plan to save up for RadMax lead sheeting which I can use to lead line a wooden box enclosure that I plan to build for the unit. This would allow me to be in the same room as the x-ray source. Perhaps I'll keep looking for a proper x-ray transformer and make a pseudo-professional unit out of my spare coolidge tube. After all, this build is more of a prototype than a finished project.

One last thing... a tribute to some great men of times past.

May my Grandmother and Mother never read this. I couldn't help myself. After finishing nearly fifty radiographs and completing all of the operational experimentation I could think to do I had one thing left to do. It was against my better judgement, and in fact I said all along I would never do it. I did carefully assess all the risks involved, and while it was and remains still a seriously stupid irresponsible shit-for-brains thing to do, it wasn't calculated to present a likely chance of injury. You probably already know what I am talking about and what I did. How could I not? Were you in my shoes would you have done differently? As much as I like to justify it to myself by reminding myself that all the greats... Roentgen, Tesla, Crookes, Coolidge, the Curies, and countless more did it too, it is glaringly clear that they did NOT know the risks, and I do/did. Still, time has passed since my... lapse in judgement... and no harm was done. I was lucky. That being said... I am never fucking doing that again. I felt like Louis Slotin the whole fucking time. Talk about instant regret. If you don't know who that is, click the link and learn about a great man who met a tragic end. Note; I placed aluminium plates on the x-ray output to filter out the most dangerous soft bremsstrahlung x-rays, leaving only the hardest rays. I also used full-body shielding of 2mm sheet steel to protect myself. I also cut exposure time to half that of normal. So, I present to you...

"Hand Mit Ringen"

Traditional negative image view:

43,825 Days Ago (120 years and six days) Wilhelm Roentgen took the very first Living Tissue Radiograph. He used his wife's hand, though, a matter which I would never attempt. No, I substituted in my hand instead. Never impose on others what you are unwilling to impose on yourself. No judgement placed on our forefathers and foremothers, for they knew not. You can see Roentgen's radiograph here: The First Radiograph 

By the way, I'm almost certain it was purely psychosomatic but for about an hour after I felt what could only be described as a UV burn on the area. There was no redness, no dryness, no changes at all to see. I've had real UV burns, this was far less verifiable, yet felt similar. The short duration and lack of visible changes leads me to believe it was not after all a soft x-ray burn. Also, more than the area in which I felt the sensation received equal, if not increased exposure. In any case reckless stupidity is out of my system, this was the first and last autoradiograph.

Just a little update: It definitely was psychosomatic. No harm was done (miniscule stochastic effects aside). Also, main beam output seems to be about 100Roentgen/hr (30mR/sec)! This puts my total estimated received dose from the one and only exposure at about 125mR. That's 14 months of background radiation, with most of that being only to my left hand.

Sigurthr's Unified SSTC Data File - All the data you need to make your SSTC

I keep getting a lot of nonspecific questions and comments on my old posts and videos for my SSTCs that ask the build specifics of my coils such as parts info, # of turns, etc. I always do my best to answer them but in general unless the question is something very specific like "Help, my X in my Y is/isn't doing Z" or "I can't get X part, will Y part work instead?" I can't provide very detailed responses. This is especially true with build parameters for tesla coils; many factors are interdependent, and many more have a huge range of flexibility. There's rarely ever single correct answer or value.

For all these questions I point towards my Unified Data File, located here: Download Here!

It contains the distilled essence of all my tesla coil builds. It has a fully detailed parts list that links to the exact preferred parts from reliable retailers so that there is no question about what parts to use. Simply pull up that part and if you can't or don't want to get that specific one, use the data on the retailer page to see what would be a good substitute. It has full schematics for all the essential parts of my tesla coils. It even has technical drawings and notes that I made of key points in the design and build process. Located in the included text files are detailed explanations and solutions of common problems and operational parameters too.

So, in an effort to make the data file easier to find (as it was buried within my USSTCC post last year) I'm making this easily searchable post. Cheers, all!

Tesla Coil Related Q&A - First edition.

I get tons of emails asking for technical help regarding Tesla Coils. I'm always happy to answer questions that come my way. I'd like to share the info with more than just one person though, as I get a lot of repeat questions, and even besides that the answers often get buried deep in message boards or in an inbox. So, here's some responses from people who have given their OK to share our correspondences.

Here's a bit of answers to construction questions that were asked. The format of the emails wasn't great for direct posting, but the format of my replies were pretty good for such.

The first step for your design planning, given your goals and flexibility is to examine your primary goal of output size, and the unwritten attributes attached to it.

-You state that you want an arc/streamer length of at least 1 foot and are aiming for about 1kW power level. This immediately means you want an interrupted SSTC or a higher power CW SSTC.

OPERATIONAL MODE
--------CW Operation; Not Interrupted/No Interruptor Added------------

---Running CW (continuous) gives a short, fat, SILENT hot "flame" type discharge that lights up tubes and transmits power readily. CW yields a painless discharge (other than thermal burns) that is safe for powering tubes and other wireless power transfer devices.

---Approximate output arc size per power is 4-6" per kW (1000W) for a coil running in the 100-450KHz range off of fully filtered DC. This is the only mode of operation that can play fully polyphonic hifi audio.

---Unfiltered/partially filtered DC causes a 60 or 120Hz Staccato which increases arc length per kW to around 8-11". Note that the arc will have a loud (not SGTC type "loud" though =P ) low buzz to it similar to hum on a guitar amplifier or if you've seen a video of someone playing with a MOT stack arcing. You can still power tubes and other wireless devices, but power transfer will be around 50%-90% that of fully filtered DC CW. This is determined by the load impedance of the coil (primary impedance at resonant frequency) and the amount of filter capacitance placed after the AC-to-DC rectifier bridge (not inverter bridge) - use a low pass filter ripple formula (ask if you need). -Note that you should never let a tube come in contact with the output streamers when using unfiltered, partially filtered, or interrupted modes.

---The less filter capacitance used, the larger the streamer/arc scales in comparison to full CW, the less heat generated, the less average power transmitted, the louder the output, the more painful the output is for direct contact.

--- Using no filter capacitance yields the greatest streamer length for CW operation (and it isn't technically CW as it isn't constantly running) but causes stress on the feedback system and has the potential to stall randomly as there is no feedback signal every time the mains voltage crosses zero. This is pretty much impossible to predict as it has too many variables, but it should be mentioned. This is easily fixed if it occurs by adding a single capacitor across the DC rails.


---------Interrupted SSTC Operation (aka ISSTC)--------------------
---Running interrupted means you use an interrupter device to pulse the gate drive on and off for varying amounts of time (PW) at varying rates (PRF). Typical rates are 1Hz to 1KHz, with durations ranging from ~15% ON/85% OFF to 95% ON / 5% OFF. Most people use a 555 timer for this job as it is cheap and easy to implement, however a 555 timer is severely limited in PRF range and cannot do PWs below 51% ON. Additionally, if you wish to keep it 50/50 ON/OFF you can't do this with a 555 timer as it is only evenly split at a small fraction of the PRF range. If you want complete control over interrupter operation you either need to run several timer chips in a complicated arrangement with other logic chips, or you use a microcontroller (uC) like an ATTiny or Arduino. I went for the arduino/ATTiny route as you can really customize what you want to do rather easily.

--- Using a common 555 timer based interrupter design you can get approximately ~30-700Hz PRF @ ~50-80% PW. This would give a low buzz to a piercing whine, with varying brightness and streamer length. Using a microcontroller you can have just about any PRF and PW combination you can think of, and control them independently.

--- Increasing PW gives hotter, brighter, whiter streamers. Low PWs give purpleish fainter streamers like that of a small SGTC. PW is proportional to power draw and heat generated.

--- Streamer length to PRF follows a nonlinear curve dependent greatly on atmospheric conditions and resonant frequency. The rule of thumb is above 300Hz streamer length decreases quickly. Most coils have maximum output length around 70-220Hz. Low PRFs can have just as long or longer lengths, but it varies greatly on numerous factors. A new type of SSTC called a "QCW DRSSTC" maximizes this phenomenon. I won't go into them as it isn't applicable here.

---Since you have no size constraints and have limited engineering experience I'd recommend an Arduino over an ATTiny, if you decide you want more than what a 555 can offer. I do have an arduino program written up for use on ATTiny uC's and can easily modify it for an Arduino if you would like. I just have to change which pins do what in the code, the core code is the same.

---The better the DC filtering; the hotter, brighter, and longer the output streamers will be when using an interrupter.

HARDWARE:
Firstly and foremost: Bridge Inverter.
--- Half bridges are simpler, easier, and cheaper to build, but only have half the output of a full bridge, this directly translates to output length! You can counter this by adding a voltage doubler to the mains input, and doubling the amount of filter capacitance you use. Whether this becomes cheaper than building a full bridge depends on your ability to source capacitors for the voltage doubler. A doubler is certainly simpler and easier than going to a full bridge inverter, and it can be added later on at any point *if you plan ahead and size your bridge capacitors accordingly*.

---I recommend a half bridge designed to account for future use of a doubler. NOTE: Change of parts listed in Unified Bill of Materials: "DC Bus Bridge Rectifier: GBPC1502-E4/51-ND" to "GBPC1506FS-ND" (Digikey). This is the same change one has to do if they wish to run the coil on 240V instead of 120V.

--- Bridge Heatsinking. Super important, too big is never too big. Too small becomes expensive fast. Heatsink material should be extruded aluminium with the plate thickness being 1/4" or thicker. Fins should be 1/2" or longer. Overall minimum dimensions should be 4"x 4" per kW for half bridge, or double that for full bridge. EVR used to sell heatsink material at excellent prices. Email or call them to see if it is still available (it should be, but always contact before placing an order as I once ordered and found out they didn't have any at the time due to shop issues).

Individual Q&A

what determines the power? The driver?

For example, in a SGTC you have the NSTs, Rotary gap/ static gap. Those determine power---but on a solid state TC you have one Transformer, a step down to 2A {in your design}.

I thought I covered it in the unified file but I could have glossed over it, it's been a while. In a (SR)SSTC the power draw is determined mostly by the primary impedance. Impedance varies by frequency, and is thus determined by the inductance of the primary. Here's a practical approach to it:

XL = 2pi*f*L, and we will be using Peak values, not RMS, as in a SSTC primary voltage is a square wave, even though the current is a sine. So, assume for example 500KHz and 15uH, half bridge on 120Vac (177Vpk). Half bridge means only half voltage is applied by the bridge, so 89Vpk. XL = 6.28 * 500k * 0.000015. XL = 47.1 ohms.

So, a 15uH primary running at 500KHz will have an impedance of at least 47 ohms. In reality it is slightly more due to leakage inductance (stray/unavoidable lengths of wire/conductor/leads/traces) and pure ohmic resistance. In addition, you're usually concerned with RMS power when building a coil as your limitation is your power source, not your output power (snuff you FCC!). The coil only draws full load under ground arc, and coupling limits power draw further as it is the second strongest influence on power. I like to underestimate impedance, so you have a larger safety margin. So let's use 45 Ohms continuing our example. Likewise, we'll ignore the reduced draw from coupling (it will never be 100% for air-core transformers like a TC, so extra safety margin built in there). I = V/R; Ipk= 89/45; Ipk= ~2Amps. The primary will draw 2Apk. Now even though the voltage across the primary is half, it is in series with the mains, and currents in series are identical. There's no transformer action in dropping half the voltage in a half bridge, so 2A out means 2A in. Input current will be Primary current (Ipri) plus the effect of power factor. Power factor in most SSTCs I've built is between 0.35 and 0.8 (this is because converting to DC and filtering inherently creates poor power factor, and then you're presenting an inductive load on top of that). Coils running on unfiltered DC have substantially higher power factors. I just use 0.6 as a guideline. Iinput = Ipri/PF. Input current should be around 3.3A, so input power would be around 400Wrms or around 600VA. Output power is of course around 180W. If you add power factor correction to the mains input you'll reduce input consumption and improve efficiency, but not increase output power.

So, to increase power what can we do?
1) Lower f0. Limited by secondary size.
2) Lower Lpri. Limited by coupling (lower L = lower coupling).
3) Increase Vin. Limited by component ratings.
4) Use a Full Bridge. Limited by budget, size constraints.

"When building the Gate transformer do I need a signal generator?"

Nope! You just need to be able to focus on the task at hand with minimal interruptions. There's a point where you've determined which wire is which but have not yet actually marked it as such, an interruption or loss of concentration here usually ends in disaster. It is VERY easy to double check the construction of a completed GDT when you have a 2 channel scope though. You will need a signal of some kind, even if it's just touching the GDT primary leads to a battery momentarily (requires a DSO to use this signal source).

"How do you choose a resonant frequency? Is 300KHz reasonable."

Resonant frequency determines arc length per kW (longer streamer the lower the frequency), some of the heating in the bridge (lower frequency is less heating), some of the load on the logic controller / driver (lower freq, the less load), and most importantly the physical dimensions of the secondary. Yes, 300Khz is reasonable, but I would recommend you aim for around half that for best bang/buck when size isn't a factor.

"I would like a TC that doesn't sound like its sputtering. How do I pick that?"

A coil's sound is determined by the RF envelope, which is determined by operational mode (CW/Interrupted) and interrupter specs (PW and PRF). CW on well filtered DC is nearly silent, with only a lowly audible hiss ("shhhhhhhhhh") sound. Poor DC filtering or no filtering causes a very loud Buzz at the mains-rectifier frequency and many, many harmonics. It will sound like a harsh "BRRRRRRRR" tone. Interrupted mode coils vary greatly in volume and tonal character based on the specifics of their pulse width and pulse rep frequency. A shorter duration ON pulse has more harmonic content and thus sounds brighter, where as a longer ON pulse close to 50% duty sounds duller and warmer. Even if average power draw of the coil is continuous through the PRF range, the volume will chance because we humans perceive sound volume nonlinearly with respect to frequency.

"I noticed you omitted a 555 timer from your design."

555 timers are far too unreliable and imprecise for use in the actual logic controller / driver. They were originally used with a feedback system as an oscillation starter, which ironically does absolutely nothing at all when used with an interrupter, as the interrupter does this role flawlessly. They can still be used for an interrupter though. When used on a CW coil (when not used for interruption) it causes more problems than it is worth because the idea was to provide a signal that should get overridden when real feedback from the coil running takes place. The problem is that how do you ensure it gets overridden and it doesn't override the feedback signal? There's no good answer, and likewise those coils never ran reliably. CW coils without an interrupter still require an oscillation starter sometimes. It really isn't an issue though as it only requires one switch and a resistor OR a change in startup procedure for the coil. There's a whole section on it in my USSTCC unified data file, and it is on the main schematic.

"Now let me understand some components, I have decided to use your logic board.
What exact piece of the puzzle does this include? What other components do I need to design and build myself. "

"I will need my own inverter correct? I will need to rectify the ac current coming from my variac.

Your listed step down transformers, are those the only ones I need? 2A step downs?"
Okay, before listing off what it includes, let me state plainly what it and its associated files do NOT include, in other words things you need to source and build yourself:

1) Bridge Inverter. *
2) AC power cord for Bridge
3) AC power cord for USSTCC's step-down transformer
4) Secondary Resonator
5) Primary Coil
6) hookup wire
7) GDT; wire and core *
8) secondary topload (doesn't have to be fancy - salad bowl!)
*note I do have most of the suggested parts for these listed, things marked with asterisk have detailed instructions and parts lists.

What does the USSTCC Board include? It includes one bare USSTCC printed circuit board and all documentation required for proper construction of a working SSTC and lifetime Q&A help service. You already have this documentation, and you're already using the Q&A service, hehe.

Now, a better question; "What parts of the 'SSTC puzzle' does the completed USSTCC encompass?"

The USSTCC board takes care of the entire Low Voltage power supply supply, feedback processing circuitry, signal amplifier and splitter, add-on (interrupter & modulator) interfacing, and Gate Drive requirements of any SRSSTC operating in any mode. You add a bridge inverter, GDT, and completed resonator and have a fully working SSTC. All of the recommended parts for the bridge inverter and GDT are listed in the BOM files with their ordering numbers direct from reputable suppliers.

The only electrical components you need to pick and determine source of yourself are the AC power cords/plugs, GDT wire (22ga solid core, insulated (not enameled) to >300V, 2 or 3 colors), rosin core eutectic solder, and primary coil wire (10ga finely stranded copper, insulated to >300V - silicone if possible, cheap on eBay used for RC hobbies).

The physical components you'll need are coilforms, polyurethane, various mounting hardware (screws, nylon standoffs), etc.

I've never used an arduino before, whole new can of worms.

Re: Arduino; easy as pie, really. You pick up an Arduino board, install the drivers and IDE (operating) program on any windows pc, I email you the code, you copy paste it into the IDE, plug in the Arduino to usb, hit upload, done. Then it's just a matter of attaching two 25k pots, a resistor, and the fiber optic transmitter. The F.O. receiver connects right into the USSTCC expansion port. Fiber runs between the two and just inserts in the F.O. parts. High end interrupter completed and installed.

Can I use thermal paste like arctic alumina instead of sil-pads?

Arctic alumina works, but only in conjunction with silpads, not instead of; it can't withstand the high voltages reliably.

Can I just ground the TC to mains earth at the socket?

While some people swear by it, it isn't safe and doesn't provide very good performance to do this without at least carefully checking the mains earth wiring on the electrical branch you'll be using and adding RF balancing nodes to that mains branch. You place correctly rated class-Y and class-X capacitors between Line and Earth (Y), Neutral and Earth (Y), and between Line and Neutral (X) which allows RF to have an equal potential across the three conductors, essentially allowing any common mode filter to keep the RF out of where it shouldn't be. One of these nodes should be placed directly at where the coil gets plugged in, and one should be placed at where any sensitive equipment is to be plugged in. You should still unplug any equipment you can't afford to replace. This is still second rate compared to a proper RF earth ground line.

My 4046PLL based SSTC isn't behaving

It's a well known and documented flaw in the using of the 4046PLL for SSTCs. It's finicky as hell and sometimes you have to adjust the lock range and middle point while the coil is running just to get it going again. Make sure none of the coil's surroundings have changed too, as this will change the loaded resonant frequency of the secondary. Likewise, the resonance point changes with operating voltage, so mind that as well.

It's likely locked onto an upper harmonic. Unfortunately this is a tricky problem to solve, as I mentioned before the 4046 is far out of it's designed application in SSTC use *(this is why myself and most others abandoned the chip for SSTC use. The big boys moved on to FPGA software based PLLs, and I went oldschool with VCOs). This problem pops up most times on working coils when someone changes the PLL chip as it is often triggered by small variances that are well within spec. Switching manufacturers or even batches has been known to prompt it.

One thing you can do is disconnect the bridge, and set the center point of the PLL's VCO to the known resonance frequency of the secondary. Then install a bandpass LC or LCR filter network between the feedback source and the feedback input of the driver. You should set the bandwidth of the filter to be as close to the bandwidth of your secondary as possible (if known). You can empirically measure an estimation of the bandwidth by scoping the secondary connected to a low power variable oscillator (or remotely monitor power draw with coil running at full power). Otherwise you can use a rule of thumb guess, but I'd need to know the f0 to give you a ballpark figure. But basically you see you're unable to rely on the PLL's intrinsic loop filter to reject harmonics and actually lock on the desired frequency, so you have to help it along with an external filter. If adding the filter does not immediately remedy the issue the next step is to place the entire driver inside a faraday cage and see if that helps.

There's a trick that sometimes works (far less well than the bandpass filter, but doesn't require the use of phase shift compensation like a filter - I might have forgotten to mention that above; if your filter network induces a sizable phase shift you'll need to correct it via additional series passive reactive components.) where you set the PLL sweep range to +-(1/2*Phi*Bandwidth). So if your f0 is 390KHz, and your bandwidth is 20KHz, you'd have a 32KHz PLL sweep range centered on 390KHz (0.5*1.618*20). The idea being there are no natural harmonics within this range for the PLL to lock to. It isn't bulletproof though.

My CT feedback isn't working, and I've tried reversing polarities!

Don't use a parallel resistor in the secondary of the feedback transformer; this makes it a CT current transformer, and SSTCs use in-phase feedback, not 90deg shifted feedback like DRSSTCs. Remove that resistor and place a roughly 10-100k one in series with the secondary side to make it a VT (voltage transformer), then feed that in to the antenna in connection. If you ground the opposite terminal of the VT/CT secondary it forms a DC short which prevents startup oscillation (normally achieved by noise) without manually pulling the Enable line on the UCCs (modulation inputs) to Vcc. So leave the other terminal of the VT floating or use a capacitor to tie it to ground. Or you can use an interrupter or a momentary push button switch to ping the enable circuit. There's a schematic in my USSTCC files showing it.