---------- Forwarded message ---------- Date: Wed, 15 Oct 1997 18:08:26 -0400 (EDT) From: richard hull To: hvlist@Anchorage.ab.umd.edu Subject: Farnsworth/Hirsch Fusor IV All, I will digress a bit. Some may be wondering if this is a real thing and to what level it will really operate. Some may doubt the whole idea. Most on the list have little contact with the nuclear end of things and heretofore probably thought neutron production was the thing of fission reactors and giant tokomak fusion rigs costing millions or billions. This is not so!! As my friend Tom Ligon says, "Fusion is Easy!" For those who are doubters, check out Farnsworth's original patent on the fusor. U.S. Patent no. 3,258,402 issued 28 June 1966. Some conceptualization and theory.... If you build the fusor concept demonstrator I am describing with a plastic chamber and a roughing pump (~10 microns) and use atmosphere, you will not get neutrons. You will have an ion multipactor. A thing of marvelous beauty and almost limitless amusement. If you have a vacuum system capable of 10^-6 torr, can snag a small lecture bottle of deuterium and use a pyrex bell jar or stainless steel chamber, you can be producing neutrons. The real hassle with neutron production is all the vacuum incidentals which can drain a bank account real fast. If you are not set up as in this paragraph and are really hot on neutron production, build the concept demo unit and get used to the look and feel of the system and save your nickels and dimes. Remember, the actual fusor construction specifics are no different whether you are ion multipacting or neutron producing. Here is the actual operation scenario: At atmospheric pressure air is basically a non-conductor due to the short mean free path of ions at this pressure. Once ionized they immediately recombine. At reduced pressures (25mm to 10mm) the de-ionization time and mean free path increase. This allows the ions to conduct electricity or carry charge from one place to another. At greater exhaustions (10mm to .01mm) the gas ions become much more like a direct wire connection and form a plasma arc in closed systems with lower voltages. This voltage is greatly reduced over that required to sustain an atmospheric pressure electrical arc. As the exhaustion increases much below 1 micron (.001mm) the gas is now so thin that conduction of any kind of real heavy current is impossible without using extreme voltages. The number of charge carriers is now quite limited. Finally, with ever deepening vacuum levels, the residual trace gas just isn't up to conduction, and all apparent ion glow and current flow ceases. The glow we see is actually mostly caused by recombination and not ionization. So if it's glowin', its dyin'. The continuous application of power to the arc is what keeps it alive. Ions are matter that has been made electrically unhappy! With our concept demonstrator working between 100 and 10 microns, we are not getting hardly any "recirculation" of ions as the mean free path is short. We do have very high density on those ions which do multipact, however. Unfortunately, neutron production in deuterium begins at about 10KV, and as you will note, the demo chamber is hard pressed to reach 3KV before drawing one half amp!!!! This is where the higher vacuum requirement for neutron production comes in. At 10^-6 torr, we can push the chamber to many 10's of kilovolts. By back filling with deuterium to 10^-5 or 10^-4 torr we can now hit 10-30kv chamber voltages with relatively low curent draw and low x-ray production, while still doing fusion. The gas density is low and the mean free path is many chamber diameters. Thus, we get magnificent recirculation as few ions multipact on the first pass through the inner grid system. At some extreme exhaustions, the requirement for an electron source is a must. This is due to the fact that simple ionization is not really possible at these pressures and we must "kick the system in the butt" to excite the atoms. A nice stream of electrons will do the trick here. A filament placed just outside the outer grid helps tremendously. It is usually biased to some slight negative voltage below the outer grid system. (~100-200 volts). It sounds complicated to an amateur but is abysmally easy compared to the megabuck methods traditionally used to get at neutron production, especially fusion! Most of the big boys working in this system are now using actual 10^-6 deuterium pressures and thus must hit 10^-9 vacuums and back fill. Dr. George Miley's group is working at this level and are up to 70-100KV on the chamber, but X-rays are a real hazard here and huge amounts of shielding are necessary for both X-rays and neutrons (up to 10^9 neutrons/sec.) Finally, one must not think of the demo fusor as a bunch of recirculating and then multipacting ions following newtonian orbitals in the chamber. The demo fusor is a crasher! It's mean free path will not allow a lot of recirculation and most of the ions which don't multipact in their pass through the inner grid are recombined in the sea of gas out side of the "poissor" or plasmoid. Most of the fresh ions are created out near the outer grid and go from there. Few ions survive the trip to the central grid system until 100 microns of pressure is achieved. At 10 microns, the mean free path is up to where some small amount of recirculation is just possible. The poissoer ejected electron beam can easily melt a hole in the plastic fusor chamber, so be careful. Next time, final assembly and opertion... Richard Hull, TCBOR