Page 2 - Achievement of Sustained Net Plasma Heating in a Fusion Experiment with the Optometrist Algorithm

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Figure 1. Central con nement chamber of C-2U, a plasma con nement experiment comprised of about 10,000 engineering control tags and over 1,000 physics diagnostics channels. Photo copyright belongs to Tri Alpha Energy Inc.
e Optometrist Algorithm is motivated both by human choice experiments2 and by Monte Carlo (MC) optimisation methods3. Human choice experiments are typically used to elicit preference models in complex sit- uations, such as in health care4. MC optimisation methods e ciently explore high-dimensional spaces to discover di cult-to- nd optima.
Application of the Optometrist Algorithm to our plasma fusion problem yielded signi cant bene ts. By delib- erately exploring parameter space, we found plasma with better properties, such as stored energy and con ne- ment times. e broader exploration of parameter space also improved the intuition of the human operators. Finally, the most signi cant outcome was the discovery of an unexpected plasma con nement regime, charac- terised by a factor of two reduction in the energy loss rate and resultant increase in the plasma temperature. e existence of this novel regime will signi cantly facilitate the ascent towards energy generation from fusion. It also validates the technique as a very useful tool in exploring complex, high-dimensional systems. e Optometrist Algorithm may be applicable to discovery of other scienti c or engineering advances.
Our strategy of humans selecting between machine-generated settings is broadly applicable and di ers from typical approaches to optimisation problems in the machine learning community, where strict gures of merit are automatically computed5, 6. As such, the Optometrist Algorithm provides for e cient optimisation in areas where computation of such metrics is not possible. e primary advantage of the Optometrist Algorithm is combining the best of machine and human: the human provides physics intuition while the machine searches high-dimensional space.
Experimental Facility
e experimental fusion apparatus at Tri Alpha Energy7–10, shown in Fig. 1, creates, con nes, and heats a form of plasma called a Field-Reversed Con guration (FRC)11, 12. is experiment combines e cient use of magnetic elds (high β), large orbit ions (orbital radius comparable to system radius) for macro- and micro-stability, and a simply connected divertor (exhaust chamber) for safe removal of power with the goal of eventually enabling a compact power plant with reduced engineering complexity.
e C-2U machine10 on which this experiment was conducted is based on substantial upgrades to the C-2 facility9. e chief upgrade was to the neutral beam system. Particle energy was reduced from 20 keV to 15 keV, but total neutral current and power were increased in excess of 2.5 times that available in C-2. Total power over the 6 neutral beam injectors is more than 10 MW on C-2U. In addition, the injectors are angled inward at 20 degrees to better couple to plasma in the centre of the con nement region. Another upgrade was to the mirror plug magnets. ey remained at 20 kG strength, but could maintain this eld over the full 10 ms duration of the plasma shots, which was not possible in C-2. e nal signi cant upgrade was to the end biasing systems for edge/ stability control, now replaced with Gas Dynamic Trap (GDT) type plasma guns13, capable of biasing voltage and current at 1 kV and 3 kA, respectively. ese upgrades together allowed the achievement of plasma sustainment, limited only by stored energy.
ough the engineering complexity of the C-2U device is greatly reduced when compared to other fusion research facilities, there are still thousands of system parameters that can be adjusted to maximise performance of the machine. ese parameters include the control of a large number of subsystems, such as formation sections (where plasma is made) and high power particle beams (that heat plasma and reduce losses due to turbulence14, 15). Speci c examples of important controlled parameters include voltages and timing of circuits that ionize and accelerate plasma, currents that set magnetic elds in di erent regions and voltages applied at the vessel ends to set boundary conditions.
Scientific REPORTS | 7: 6425 | DOI:10.1038/s41598-017-06645-7 2