An Interesting Poster to look at from the Tri Alpha Energy Team in California

Θ-PI Starts
 Improve spatial uniformity
Development of a Pulsed ~100MW Rotating Magnetic Field Ionization System for C-2W
Erik Trask1, I. Allfrey1, E. Granstedt1, V. Godyak2, A. Korepanov1, S. Krause1, J. Leuenberger1, Y. Mok1, R. Smith1, T. Valentine1, W. Waggoner1, and the TAE Team1
1 TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610 2 University of Michigan & RF Plasma Consulting
Abstract
The Rotating Magnetic Field (RMF) ionization system on the C-2W experiment at Tri Alpha Energy has been substantially upgraded from the previous system on the C- 2U facility[1]. This system is used for ionizing gas prior to forming and accelerating Field-Reversed Configurations in the formation sections. Through the use of enhanced power units with increased stored energy, and an improved antenna design for better power coupling, a fully ionized plasma can now be produced in less than 100 us, in a background axial magnetic field in excess of 0.1 T, while at gas pressures in the ~1 mTorr range. The system design, characterization, and experimental ionization parameters will be discussed.
[1]M.W. Binderbauer et al., AIP Conf. Proc. 1721, 030003 (2016) Goal: Improve Ionization System
 Comparisons between C-2U experiments and simulations imply only 10-20% of injected gas was ionized
 RMF and ringing theta-pinch (Θ-PI) systems were used for ionization, almost solely due to the Θ-PI
Solution: Upgrade RMF and Retire Θ-PI
 RMF ionization system is decoupled from formation by geometry
 Axial currents in RMF antenna do not couple to azimuthal formation straps, while azimuthal currents are in opposite directions, reducing net coupling
 Voltage coupling from formation to RMF antenna is ~10% due to finite length of the solenoidal fields
RMF Antennas
Theta-pinch coils and headers
 Increase Capacitive Coupling To Plasma
 Insight: RMF discharge begins with capacitive discharge, switching to inductive mode after the plasma density becomes large enough (thanks to Valery Godyak)
Ionization Test Stand: Develop Antenna Topology
 Test Stand: Components from C-2U formation section
 Quartz tube and fast straps reused
 Axial B field at up to 0.15 T from fast straps connected in series  Old pulsed power equipment drives antennas
 Gas delivery by either backfill or gas puffs
C-2 vs. C-2W RMF System Comparisons
Θ-PI + 8ms
 LamyRidge simulations and fast camera imaging show an annular
RMF: No Light
Θ-PI + 12ms
Θ-PI + 14ms
plasma is produced by the Θ-PI
 Ideal ionization distribution is flat in simulations, with enhanced flux
trapping due to heavy field lines near the wall during reversal
 Decouple Ionization system from FRC formation circuit
 Transient voltage oscillations from the PI circuit cause switches in the main reversal circuit to trigger early. This reduces formation reliability and restricts operating ranges.
 Wide, flat antennas increase the electrostatic field inside the formation section Faster Ionization
 Antenna width increased from 0.05” to 2”
 For the same voltage, electric fields may be 30-60% higher near the walls where ionization is likely to begin
 100% ionization is complete in approximately 70ms
 Average density measured by interferometry
 Oscillatory structure due to rotating plasma relative to line of sight
 Light emission precedes rise in density, dropping off as neutrals
density is reduced
 Average radial density is flat
 Line ratio analysis from
helium doping and fast camera imaging
 See BP11.00047 for details
 Density is present near wall
and axis
 Improved from Θ-PI
Li
Sbi Smr Spi
Cbi Cmr Cpi
Li, Ll: Inductance of Isolation, Load;
Ll
Summary: High Power Ionization System Improves C-2W Initial Conditions
 Circuit is decoupled from reversal, with improved pulsed power reliability
 Ionization percentage is ~100%, with flat radial distribution
 High power circuits are implemented on C-2W (Norman)
Cbi, Cmr, Cpi: Sbi, Smr, Spi:
Capacitance of Bias, Main Reverse, Pre-Ionization; Switch of Bias, Main Reverse, Pre-Ionization Circuit
 Implement different antenna geometries
Two antennas: Quadrupole Two antennas: Dipole Some ionization Better ionization
Antenna Parameters
Material
Topology
Operating voltage / Energy
C-2 / C-2U
Center conductor of high voltage (~20kV) coaxial cable
4 (azimuth) x 4 (axial) saddle coils
~18kV, 0.8 kJ per formation section
C-2W
Copper sheet, insulated with silicone rubber and Kapton tape
4 (azimuth) x 2 (axial) saddle coils
~40kV, ~20kJ per formation section
Test Stand Results: Complete Ionization At > 0.1 T
Four antennas: RMF Best ionization
Upgraded Pulsed Power System
 Pseudo-spark switches: 40kV x 50kA
 High power density
 Oscillatory circuit has not caused issues
 100kV caps
 Voltage reversal does not exceed 40%
 Transmission lines and headers improved  System capability exceeds 45kV (DC)
C-2W Tests: Gas Distribution Inference
 Gas ionization loads antenna, changing damping rate
 Puff gas at earlier times. Time of flight changes the distribution
under an antenna