Dennis P Brown wrote:
I am very interested in Van de Graaff's since I have built one that stands slightly over five feet tall and has a nearly three foot diameter dome. Would like to find out what you know about those - specifically designs of a high voltage spray (design, and details on the needle array) at the base of the belt; also, pick up needles for the top - location, number, design for both types and belt materials. For instance, return feed polarity spray in the top and best arrangement for wiring in the top relative to the pick up array and return wiring (if any.) Issues of improving performance (excluding a dry box) would be welcome. I have a rather large electro-static deuterium accelerator that I'd like to get working. Built the VdG and a large 60 kV voltage multiplier (for its belt spray - the VdG must have a positive polarity.)
See attachments for details of my latest and by far the best VdG. Notice the construction of the top roller support, by lowering the knurled brown support screws, the belt slacks, and the roller assembly can be pulled out sideways, which is nice when the belt needs changing.
The button on the top corresponds with a switch via an insulating stick, so that the subjects can turn the VDG on themselves:
The combs are made from a strip of FR6 PCB material, the needles are pin needles like the ones used to hold cloth in position prior to sowing it together, and they are just stuck through holes in the PCB material, soldered on the needle head side. Spacing is 10mm, and 15mm from the edge of the belt.
The comb at the bottom is identical, you need 3 combs in total.
One at the bottom, connected to the spray supply, one in the terminal, just before the belt reaches the roller, and one at the top of the belt.
The spray supply is a ~10kV dc supply, I use a solid state oscillator with variable frequency, to tune it for the resonance frequency of the TV flyback transformer. this oscillator drives a half bridge of small fets, and around 10 turns on the free leg of the flyback core. Nothing fancy. Current is limited with around 180Mohm.
The top pickup comb is connected to the roller as well as to the terminal through an 180Mohm resistor. The voltage drop on the resistor effectively places the terminal et a lower potential than the roller, and since the last comb, the downcharging comb, is connected directly to the terminal, corona is created on it's tips. 10KkV is enough to create the corona, and Microamperes will do the trick.
Run the machine in total darkness, allow a couple of minutes for your eyes to adjust, and you will see the corona clearly, as a fine glow on the tip of each individual needle.
See this diagram for complete datails of charging circuit:
http://laplace.ucv.cl/Cursos/TrabajoTit ... -04-05.gif
Belt material, I have had good results with certain belts from the food industry, be sure to not get the ones that come with conductive material in them. A production line in a cookie factory can easily become quite electrified!
Habasit make a product called F-2EQWT, that is what I use.
There is a build thread here If you like, of the first design I made:
http://4hv.org/e107_plugins/forum/forum ... 26708.post
Dennis P Brown wrote:
Gate valves have marginal use with a fusor (see the use for a DP but they are generally all or nothing) - having a throttling valve between a DP/turbo and the chamber is far and away the most useful method to control a fusor for real work.
Having read a lot, but not all, in this forum, I gathered the impression that the connection from DF pump to chamber should be full diametre and as short as possible, so that is what I did, but I can still regroup, and take another half shell, if the gate valve is a waste of time.
In that case, I would appreaciate if you could point me in the direction of a suitable throttling valve
Dennis P Brown wrote:
Issues relative to getting a high enough power/voltage transformer should be something you could help a lot of people here! So, feel free to post on that subject especially how you decide to address that issue!
The only real problem is sourcing a suitable transformer core, winding the coils is easy enough, although it takes a lot of patience.
Here is a picture of a 20kV, 300mA transformer I wound for a Medium sized Tesla coil, it has done museum duty for over 15 years now: