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Registered Member #894
Joined: Thu Jul 12 2007, 02:38PM
Location: Macon , France
Posts: 12
Antonio wrote ...
It seems possible to make a pelletron using as chain a string of beads, alternating conductive and insulating beads in an a strong insulating string. I can easily find chrome-plated plastic beads with 2 cm of diameter that look interesting for a demonstration machine. For more current, parallel strings can be used.
Take a look on this Tandem in Heildelberg ( Deutschland ) : 3 chains Pelletron
Registered Member #894
Joined: Thu Jul 12 2007, 02:38PM
Location: Macon , France
Posts: 12
Most air-insulated VDGRF's require only 7 kV DC at 1 mA to achieve full charge on a belt width of 1 inch through 20 inches.
Be absolutely sure that the spray screen is equal to or less than 80% of the belt width. This prevents charge from "leaking" at the corners of the screen directly to the metal pulley. Short the HV terminal to ground and observe this in a very dark room --- you should see a completely uniform blue corona across the entire belt --- and not just corona spray at the edge of the screen to the pulley (poor performance).
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
An Aside for you all pelletrons will give much smaller current to terminal than a VDG. Our 6Mv tandem VDG has been converted to a pelletron (2chains) and all the resistors down the acceleration tube had to be changed upward by a factor of five so that the machine can reach a bit more than 6Mv. the reason for he change is that peletrons have much lower ripple (in the terminal volts) the VDG's. before conversion terminal ripple @ belt speed was around 60kV, now with the peletron it is 1kV or less
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
johnf wrote ...
before conversion terminal ripple @ belt speed was around 60kV, now with the peletron it is 1kV or less
Interesting, Do you know which material was used for the belt?
I am surprised to hear that the ripple was 1%. The Round Hill generator built in 1934-36 had a regulation of 0.1%. This superior figure was achieved by a combination of attention to details in the belt and the charging system. I would have thought that these results were bettered by a contemporary belt system.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
Finn Figures quoted are before applying GVM feedback and the unit is an EN tandem (Van Der Graaf Corp) Belt condition was the major reason for change. There was a large excursion in terminal volts at the belt join as it went round. The tandem is used for C14 dating with ratios of C12, C13, C14 being taken from samples with 5mg of carbon in the form of graphite. The system is occasionly used for high energy implantation last week being an example with a silver beam @ 65MV ie silver atoms @ 10+ charge the carbon foils didn't last long with 6nA on target longest implantation = 60 minutes
BTW i looked at the terminal volt deviation today under GVM control and the external derippler and terminal volts held to within + - 10 volts @ 5,250,000 volts for the ten minutes i was in the control room To get an idea on what I /we do I spend half my time between Isoscan and the ion beam analysis lab
Registered Member #894
Joined: Thu Jul 12 2007, 02:38PM
Location: Macon , France
Posts: 12
sparky wrote ...
Along the same lines as a Van De Graff - I saw a very interesting device the other day... rather than using a belt to create the static it uses a powder blown through an acrylic tube network via an air compressor to make a very decent charge. It had a 3 ft dome attached to it ---- looked like a hybrid Van De Graff to me .... maybe someone here has seen something similar before. ???
One of the most curious modification of the Van de Graff generator had been conceived around 1936 in France by Marcel Pauthenier and all. In fact, they proposed to use a flow of charged dust particles circulating in a closed insulating pipe instead of the classical belt of the original apparatus. The dust was composed of glass spheres of a few microns in diameter. A blower produced a 60 in.p.s flow of these particles in the loop-pipe. For charging the dust there was a 'ionizer', which was composed by several wires parallel to the pipe. The wires were negatively charged (12,000 volts) by a kenotron rectifier and the electric fields of the wires ionized the gas molecules. The positive ions were immediately attracted by the wires, while negative ones were repelled in the direction of the walls of the pipe and charged the dust particles. Great care had to be taken for adjusting the various parameters (diameter and speed of the particles, voltage of the wire, diameter of the ionizing tube) so that the dust did not precipitate on the wall of the tube but could continue its journey after having been charged. At the top bend of the pipe the charged dust entered in a kind of centrifugal collector, which was connected with the spherical terminal electrode of the generator. In the collector the charges of the particle were transferred to the electrode.
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Inside the Van de Graaff
