---------- Forwarded message ---------- Date: Wed, 15 Oct 1997 19:16:15 -0400 (EDT) From: richard hull To: hvlist@Anchorage.ab.umd.edu Subject: Farnsworth/Hirsch Fusor V This is my final planned post in this series. Power supply issues: The power supply must be a floating supply, ideally, but if grounded, it must be a psitive grounded supply. It should also be full wave rectified and, hopefully, filtered. It must be smoothly variable from 0 to 5KV, although 0-2-3KV will work with most demo fusors. Neutron models need a 0-30kv supply of superior design and quality and will not be covered here, although bumping the components up in value and spec. noted here will work for even those levels. ......................... The best supply: A fully isolated and insulated secondary power transformer of about 3KV AC RMS is needed. This means a "plate transformer" Only old hams and hamfests sport these babies. It will weigh a ton (20-50lbs) It must supply at least 60 ma or more of output current. These are quite inexpensive and often are cheaper than a neon sign transformer and much more powerful! Use a four diode bridge rectifier made up from 5kv 1 amp or better diodes. These are specialty items and only hamfests would have them or major distributors of electronic components. Some beefy microwave oven diodes might be good here. They will not be cheap. A custom made series string bridge might be made up from common 1KV 3A diodes, but it is a crap shoot and tedious to construct, if done correctly. A capacitor filter is needed, if possible. I used a 3ufd @ 10 KV paper capacitor I found at a hamfest for $5.00. Next a bleeder resistor of about 5megohms @ 10 watts is used across the capacitor for protection after shutdown. I also added an inline curent limiter resistor of about 62K ohms at 100 watts in series with the output. >From this resistor's HV negative output to (positive) ground, I placed a 0-5KVDC meter made from an old 100 ua meter movement with appropriate divider resistor. This negative (HOT) lead goes to the fusor's inner grid terminal. I also placed a 0-100 millameter in series with the grounded (positive lead) and the outer grid system. This reads system current. The primary of the power transformer was fed from a simple variac autotransformer of about 10 amps rating. This produces a floating supply which will answer many future project needs as it can be quicklly turned into a positive or negative grounded system. ............................ The second best supply with built in current limiting and fixed positive ground: Secure a 9kv or 12kv 60ma neon sign transformer. It is important that it be a 60ma transformer or higher current unit. Use a variac on this transformer's primary to increase and decrease the voltage as in the above system. This set up only requires two diodes, but they should be 10KV 1 amp units. Place the diodes with the cathodes (banded ends) going to each knob of the sign transformer. Connect the anodes together and this point is the HV negative lead and the case of the transforemr, which must be grounded, is the positive lead. From the connected anodes of the diodes to ground place a nice filter cap similar to the one mentioned above. Install meters and bleeder resistor as mentioned above to complete the supply. The current limiting resistor, mentioned above, is not really necessary as the neon transformer is magnetically shunted and will self-limit the output current to the value on the transformer. ***** note, these supplies are absolutely lethal!!!!!************** ELECTROCUTION!! This is the #1 hazard associated with this entire project with implosion being the second. Be very careful, as your first shock with this system will certainly be your last. Your should now be ready to rock and roll. ............................................................... To operate your fusor: Place the positive, grounded lead to the outer grid in the chamber and the negative, hot lead to the central grid terminal. It is real important to ground all outside metal associated with the chamber along with the outer grid! If you have a thermocouple gauge, you must connect its sensor tube's metal body to ground or else possible ion currents at high voltage might crawl into your meter circuitry via the tube's plug and cable. Start your vacuum pump now and let it run for at least 5 minutes. If you have a gauge, you may start gently applying power at about 500 microns. It is most instructive to "ride the system" down with power applied as you can see the various interesting and beautiful regimes taken on by the ion plasma as the system drops in pressure. Note how the current and voltage correspond. You will be amazed that the chamber voltage never exceeds 500 volts at max current until you are down in the 200-100 micron area. Slowly, as the pump down proceeds, the chamber voltage will rise and the current drops for a given voltage. The plasmoid or poissor will not be distinct until below 250 microns and will only appear blue white below 100 microns. At 200 microns, the bugle jet should be distinct out of the side of the poissor. At 100 microns, the electron beam will start to assert itself out of one of the inner grid ports. It is normal as you first pump down for sparkles and small arcs to occur as you "burn off" junk from your hands, etc. For those who haven't worked with vacuum systems and who use a plastic chamber, The normal first pump down time will always be measured in DAYS!! 36 hours is about right for a mint condition25liter/minute pump with a small chamber. Longer is the norm to achieve 30 microns and below. The whole process can be speeded up a bit by powering up for a few minutes and letting the thing "cook". upon killing the power the pump will seem to pump a bit lower and so on, until you hit the magic 30 micron and below level. Note****** Operating at high voltages and currents will definitely make the inner grid glow red hot to white hot in short order!! At lower persssures there is ZERO convective coooling so be very careful. If you silver soldered your inner grids, they may just "blossom" and pop open on you. Study the safe current for your system and don't go over it for any long time period. Sure, you can max the thing out for a few seconds (it is beautiful), but watch that inner grid structure. It might be helpful to realize that the poissor is much, much, much hotter than the inner grid system, as the fine wires just intercept the tiniest fraction of the ions. This is why we really want to make it a "virtual grid" with almost no cross section to bombarding ions. The gas density inside the poissor can be 2-4 orders of magnitude greater that the inter-grid region. Notice I said gas density and not pressure. This is fun stuff! At 30 microns and below, the mean free path in the system is such that a focused needle thin electron beam will be hitting the chamber walls. This is a bad situation, as the beam will melt through the plastic walls of a cheaper chamber. Tom Ligon's plastic chambered system imploded when he inadvertently left the room and the "E" beam softened the polypropylene base of his chamber. It sucked into a hole and ultimately turned the lower chamber inside out! This was at 16 microns. Watch your E beam and check for wall warming. The E beam is readily turned by a magnet, but watch out here, too! A magnet placed at the chamber's surface will actually hasten the process by attracting certain ion beam fragments focusing them to a razors sharpness and heat the walls rapidly. Solution... don't use plastic chambers. The other bummer about plastic is it really outgasses like crazy, and the pump must run every second that you are experimenting. The advantage for plastic in the area of cost, availability, and tapabilty for ports, far outweighs the bad points for a demo type first system. You are now left on your own to have fun and enjoy the fusor. I will have a two hour TCBOR video Educational Series tape ($25.00), entitled "Fusor" in a couple of months. It will chronicle the TCBOR's introduction to this facinating device at the 1997 Teslathon, include a special interview with Tom Ligon, chronicle my early work with the fusor with lots of operational shots and give about 1 hour of tips and construction details for the demo version of the fusor. I'll post more about it when it is done. Naturally, I'll answer questions on this list. Richard Hull, TCBOR