Wednesday, November 14, 2007

Thinking About Control

I keep thinking about test reactor control issues - things that need controlling, signals to use for control. I want to leave off things like cooling and other secondary functions which are quite important but are easily controlled and instrumented and happen on relatively long time scales.

Reactor controls are going to be a bitch. Let me run down the list of what needs controlling, some of the interactions and what signals we can use for feedback. This will be in relation to a test reactor about 1 meter across in total dimensions designed for continuous operation burning D-D.

1. Anode (called a grid in the Bussard Reactor) HV
2. POPS frequency, POPS voltage (added to Anode DC)
3. Gas Pressure
4. Electron Injection

Controlling the pumping speed of the turbo pumps is an option but that is rather slow and need only be done to set the base gas flow from the gas pressure controller. We could also vary the magnet current, but again that is slow and is relatively easy to do a set and forget based on the well voltage and the system geometry. So that sets out what we have for plant controls.

What kind of feedback signals can we get?

1. Neutrons - they tell us the fusion rate.
2. Light (PMT amplified) - electron density.
3. X-Ray output with energy binning - who knows what we might learn
4. High Energy Alpha Output - fusion rate
5. Pressure gauges - gas flow
6. High Frequency Current Transformer in the Anode Circuit Ground - you can learn a lot by watching

Burning D-D simplifies things. So far as we know there are no peaky resonance regions in the curve so we don't need to figure out how to keep the reactor anode voltage at some process determined level. So that is one complication out of the way. Second off we can use a high vacuum pressure gauge with out worrying about readings being gas composition dependent.

So we have pressure control. From a quick look at what is out there in terms of pressure measuring equipment it looks like ten measurements a second is a reasonable rate. That means that we probably will have a first order roll off on the loop of about 1 second. That is not too bad as the way the system works helps us. If it takes a certain time T to reach a given pressure it takes 10T to get to 10 times the pressure. Given that fact and the fact that the flow is initially designed to fill the reactor in 1 second control within a 2 to 1 range ( from .6 times the set point to 1.5 times the set point) should be easy. Tighter control of course would be better. We should try to get faster updates from the pressure measuring eqpt. Or we could opt for lower flow from the gas delivery system. Since most of the flow rate will be determined by the capacity of the turbo molecular pumps we could also go for a flow system calibrated to deliver 80% of the required flow and then just have a variable valve to make up the difference.

So gas density should be another set and forget.

That really leaves us only one thing that needs to be controlled on the fly. POPS frequency. Since we plan on impressing it on the Anode the anode current would not be a good place to read it out. If we used an antenna inside the reactor it would have the same problem. What we need is a signal read out that is independent of moving charges. The most likely signal for that is either the X-ray detector or the neutron detector. Since POPS for a drive voltage of 50 KV is expected to be in the 2 to 30 MHz range we will need a neutron measurement or X-Ray measurement system that can give us output at those frequencies. Also the detector efficiency should be such that it can detect at least .3E9 photons or neutrons a second if we are to reliably detect 30MHz. That will be tough. POPS experiments may need to be done with a sweep generator without feedback on small reactors.

Ideally if enough alphas are generated it should be possible to turn off the electron guns and just use the electrons left behind by the fusion alphas as the electron source. Of course if the electron guns must be left on they could be modulated at the POPS frequency and read out could come from the Anode ground current, considerably simplifying matters. The light output (PMT Amplified) could be used as a signal to throttle back the electron guns.

However, we really do not want continuously operating electron guns in a power reactor except for startup. They get in the way mechanically unless they can be placed on the walls of the reactor.

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