The First Shots

Posted by Rezwan on Sep 11, 2009 at 08:02 PM
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[NOTE;  First shots taken on Oct. 15].  Once the DPF machine is fully assembled, it will be ready to take it’s “first shots”. 

The first shots are all about seeing if the machine works properly as designed.  When they turn it on, they want to make sure nothing goes “bang”.  Electricity shouldn’t short through anywhere.  But there’s more.  Let’s break it down:

What is a “shot”?

A “shot” takes around 10 to 15 minutes and basically involves:

  1. Pumping the chamber to a vacuum state.  There must be no gases in the chamber other than the gases to be tested.
  2. Pumping in a measured amount of gas (the first series of shots will be taken with Deuterium to calibrate the machine.  The tests with boron and hydrogen come later.)
  3. Charge the capacitors – e.g., press a button, it will take ~1 minute to charge the capacitors up to 25kV (first shots at 25kV.  The machine will be worked up to 45 kV later).  When it reaches 25kV…
  4. “Fire” – this is where Igor pulls the switch.  Actually, Eric or Murali will press the red button. 
  5. Look at readouts from the oscilloscopes and a few other diagnostic instruments attached to the machine.
  6. Pump down to a vacuum (and repeat for next shot).

What is the team looking for in the first shot?

  1. No shorting through the mylar.  This could happen if dust or other impurities are on the high voltage plate.  Such elements can redirect the electricity to penetrate the mylar insulation sheet.
  2. No breakage of the pyrex insulators.  (Called “hat insulators” because of the hat shape).  These insulators are made of pyrex and fitted to the anode.  The fit must not be too tight or too loose as the anode expands and contracts with each charge.  otherwise the pyrex could break.  [IMAGE to be posted next Monday after it is unpacked.]
  3. The amount of current close to what is predicted. 
  4. Possibly…evidence of a “pinch” – although this shouldn’t happen until after ~ 20 shots.
  5. Ideal outcome:  The peak current will be somewhere around 1.2 to 1.3 MA (million amperes)

Rise Time

The capacitors store a fixed amount of energy.  When the researchers “fire” (close the switch), the energy from the capacitor is released around the circuit.  The faster that energy is released, the better.  This is measured by time it takes for the current to reach its peak, called the “Rise time”.  Rise time will be measured immediately by the oscilloscopes.  The researchers are hoping for the most rapid rise time (quicker is better).

After ~20 shots, we may start to see the formation of a plasmoid.  This will be detected by a rapid change in current. 

[Images - to come:
Non pinch pulse curve
Pinch pulse curve – has a precipitous drop where most of the energy is transferred to the plasmoid from the current.]

Diagnostic Instruments Used in first shots

Rogowski coils and oscilloscopes.

  1. A main Rogowski coil goes around the whole set of electrodes and measures the total current in the system.
  2. Each of the 12 capacitors has its own little Rogowski coil which will be used to test each capacitor switch – these are important initially, but won’t be used much after they’ve established that all the capacitors are working properly. 
  3. 2 additional Rogowski coils are placed along the drift tube.  These will only reveal a signal in the first shot if the plasmoid pinches.  One coil is at the top of the tube, and one close to the bottom.  The difference in current measured between the two coils shows the current and energy of the ion beam that the plasmoid produces.
  4. Oscilloscopes – these devices “read” the current and show it on the screen. 

Plasmoid Pinch

Eric doesn’t expect a plasmoid pinch in the first shot.  After 20 or so shots, we may see a pinch.  This is called “conditioning”.  A fresh insulator (the pyrex “hat insulators”) seems to inhibit the current sheath.  “Conditioning” means that after a few shots, the inhibition goes away and pinching begins.  Once it starts pinching, it should pinch on every pulse, but may miss an occasional pulse. 

The pinch will appear as a signature plunging curve on the oscilloscope readouts.  (Image coming soon)

12 Capacitors

When the red button is pushed, a signal will go to the trigger generator, which is mounted on the machine.  The trigger generator generates 12 pulses of electricity at the same time, which go over cables to the 12 switches connected to the capacitors.  They pulse into spark plugs enclosed in a chamber filled with sulfur hexafluoride SF6.  The sparks fire and trigger the energy to flow out of the capacitor and across to the high voltage plate.  The switch is wrapped in a Rogowski coil, so it measures the charge moving across it. The energy from all twelve capacitors goes via metal plates to the central area and is measured by the Rogowski coil there.

Stay tuned for further installments that will break down what happens to the current after this point and what the subsequent shot series will be testing for - this is where the heart of the experiment is.

Looking forward to the day when “the shot heard around the world” has a positive, fusion-based meaning.

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There are (8) comments.

Brian H's avatar

Excellent info, thanks! 

I assume that the ion-current capture part of the rig cannot be tested until actual Helium ions are being generated by p-B fusion.  Is that solenoid part of the original structure?  Or is it added later? 

Edit note: “The sparks fire and triggers”—should be “trigger” (plural).

DerekShannon's avatar

Great write-up, Rez!

Good to see everything’s still on track.  I guess that’s the advantage of the simple technology - no huge superconducting magnets, or finely controlled lasers to tune.  Just plug it in and throw the switch!

I assume you will need to pump down the chamber and leave it for a day or two beforehand to out-gas everything before introducing the D for the first time.

25kV sounds a lot for a first shot, I would have thought 10kV or so would be high enough to find any potential issues without causing damage.  Or only using half the capacitor bank at a time.

Brian H's avatar

I wouldn’t think outgassing is a problem.  This is not a process (AFAIK) requiring pharmaceutical levels of gas purity.  Any minute traces of other elements besides the decaborane wouldn’t have much impact.

I was just looking up what was meant by the conditioning and found this paper from Castillo et al 2007 ‘Neutron yield and pressure evolution during a dense plasma focus device shot series’
Adsorption and desorption of the filling gas, by the electrodes, the insulator and chamber materials of plasma focus devices, have been suggested as probable causes for the fluctuations in their neutron yield. This work describes analysis of data, aimed at looking for evidence to support this hypothesis. Before starting each series of discharges, a vacuum around 10^−6 Torr is achieved. The filling gas, pure deuterium, is maintained under static conditions. A sudden fall of the initial pressure, around 5%, is systematically observed after the first shot in each series, before creeping back at an almost constant rate, in successive shots. On the other hand, for the first shot with fresh filling gas, the neutron yield is always low and systematically increases for the second one. The pressure evolution for the following shots shows no correlation with the neutron yield fluctuations. Thus, except for the first two shots, we find no evidence to support the hypothesis that the neutron yield fluctuations are related to an adsorption–desorption process. It is also observed that a tendency exists for the last shots of each series to yield a larger number of neutrons but with a larger dispersion. This study has been done with both solid and hollow anodes, showing qualitatively similar results in both cases.”

I can understand the release of dissolved hydrogen in the stainless steel and its slow replacement with deuterium affecting the neutron yield over time, but given the relatively high operating pressure I can’t see it being a big effect, after the inital purge.

I’m not sure however of how the pyrex comes into it. They mention that Pyrex is now used over other ceramics as it was found to improve the neutron yield and uniformity, but I couldn’t find out why.  Maybe there ends up being a thin, slightly conducting, film deposited on the surface of the insulator and so helps the initial arc breakdown.  Or maybe its just its smoother surface finish.

Brian H's avatar

Or maybe it transmits the neutrons better.

Brian H's avatar

Yowza!  Update is that pinch achieved on the first couple of shots!

Brian H's avatar

Or by the 6th shot, anyway.  grin

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