Reply To: What can we do with $189 Billion?

[markus7]

asymmertic_implosion wrote:

Solutions that could be investigated include:

Highest priority – New instrumentation applicable to both 1) the LPP fusor for continuously diagnosing asymmetry problems (sensors for the current and arrival time of each of the sixteen arc filaments and perhaps new sensors for the pinch region?) and 2) critical to the ‘dummy’ fusor test effort, sensors to serve as the ‘goodness’ indicators of testing in the absence of neutron production.

Secondary priorities –
Whatever people can think of
Big geometry variations

1) Wow! This is no small task.

The plasma sheet in most plasma focus devices, if monitored, uses either optical techniques or magnetic probes. The magnetic probes provide information current locally but they are subject to plasma shielding. If a moderate conductivity plasma is generated over the surface of the probes which commonly happens in even ~100 kA devices, the probes need to be corrected for magnetic shielding effects. The key is you have to know the conductivity of the plasma and its thickness. Neither is trivial. The optical emission techniques provide some data on plasma sheet asymmetry but the current or current density is not easy to derive. This is a significant problem that faces many high current plasma devices and people are working on the problem, but it is a very difficult problem.
You can forget about putting diagnostics in the pinch region. They typically screw up the pinch or get badly damaged on the first shot. Emission techniques or laser probing techniques seem to be the only option.

2) Smaller plasma focus devices of the ~300 kA level are reasonably common world wide. Nanyang Technical University in Singapore has two such devices. Kansas State University has a device. A couple companies in the US are using these devices as well. NSTech at the 2 MA level and Alameda Applied Sciences at the ~300 kA level. LLNL also has a ~200 kA device. So the test bed could be available if someone can spark and interest in any of these places. The common diagnostic choices for non-neutron producing reactions would be ion spectrometers which could detect alpha particles from p-11B reactions. They are simple devices in principle but they require alignment using an known ion source. Some papers exist on how to do it but you need a particle accelerator in most cases as the ion source. Other techniques would be nuclear activation using alpha particles as the source. I can imaging building a target that is activated by p-11B alpha particles. I would choose a beta emitter and use a scintillator to count the beta particles as they are produced. In small devices the yield is likely to be extremely low so counts could be a problem, but it might be interesting. My guess would be a money problem. People have specific funded program or internal goals and funding would be required to develop the diagnostic and complete the tests. I would guess ~$50K to develop a single unit activation system for alpha particles and calibrate it to yield. Scientists are expensive.

Thanks very much for your well informed reply.

It is a safe bet that the majority of ideas about focus fusion devices that are easy and potentially productive have already been tried.

However, in my limited reading of the literature, it seems that focus fusion efforts have been, understandably, uniformly ‘focused’ on producing actual fusion, which can be discouragingly challenging and expensive.

I am wondering if sub-problems (such as uniformity of the arc filaments at the exit of the outer annulus) could be more cheaply addressed in units where no attempt is being made to produce actual fusion of any kind (no alpha particles or neutrons at all). More people might be interested in working in the area if they thought they had a realistic hope of solving significant sub-problems.

But someone (perhaps LPP) has to identify the sub-problems unique to focus fusion devices and suggest simple test setups adequate to explore potential solutions.

Using “uniformity of the arc filaments at the exit of the outer annulus” as an example, a lot of geometry configuration and initial magnetic field variations might be cheaply evaluated at perhaps lower currents than the 100 ka you mention as causing shielding effects. Also, note that the goal is symmetry measurements (at least time of arrival and current flow) not absolute values. So if the shielding effect was uniform, then we could still check for symmetry, perhaps at even higher currents. (Of course, if the shielding effect was not uniform, we could misread symmetric currents as asymmetric.)

Yes, the diagnostics in the pinch region must be non-intrusive. I was thinking of optical methods of visualizing the pinch symmetry, or perhaps measuring the current in the positively and negatively charged plasma jets that I assume (?) exit the pinch in opposite directions even when no fusion takes place.

If people thought they could do relatively simple, inexpensive experiments that might usefully sort through a lot of the design space for focus fusion devices without having to actually show fusion, they might be motivated to resurrect some of their old devices.

Costs might be pretty reasonable for simplified experiments.