An Interesting Poster to look at from the Tri Alpha Energy Team in California
P. 1

                                                                                                                      Ion Flow Measurement on the C-2W Open Field Line Plasma
D. Sheftman, N. Bolte, D. Gupta, M. C. Thompson, and the TAE team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
           Accurate operation and high performance of the open field line plasma surrounding the Field Reversed Configuration (FRC) is crucial to achieving the goals of successful temperature ramp up and confinement improvement on C-2W. Of particular importance is the control of the end loss plasma flow from the FRC core towards the divertor plates and mitigation of the flow of impurities back into the FRC plasma. A high-resolution Doppler spectrometer was used to measure impurity ion flow at different locations across the open field line plasma from a position in the mirror region between the central FRC confinement vessel and the inner divertor. Impurity ion velocity profiles are presented and ion temperature and flow profiles are derived.
Abstract
   Light collection system includes a retractable mirror assembly in the vacuum, at the mirror location, collecting light through a lens to a fiber array. Fiber bundle is 15m long and spectrometer is placed on the floor adjacent to C-2W machine, to enable maximum transmission of UV radiation. 3 chords available for each shot.
 Spectrometer is a Princeton Instruments IsoPlane 320 with astigmatism correction. Double cylindrical lens system used to relay spectral image to PMT arrays.
    Minimal resolution achieved - 0.02nm
Calibration drift
Lens system
PMT arrays Amplifiers
DAQ
Equivalent to 1mrad of grating rotation
      Due to changes in the environment temperature, mechanical motion of the spectrometer induces a change in the wavelength calibration, which shifts velocity measurement.
Solution: Use calibration lamp, or measurement of static plasma at wavelength range where calibration lamp is not available
 Sample results
   Ion velocity calculated from Doppler shift:
𝑉 = −𝑐 𝜆 − 𝜆0 𝑖𝜆
Flow from Confinement toward Formation- positive velocity from Formation inwards- negative velocity
 Jet spectrometer
  Impurity ion velocity profiles, O4+ 650nm, t=0.2ms:
   Shot 107901
Maxwellian distribution
Impurity ion flow velocity:
Shot 107837
Shot 107837
 𝐼 𝑣
= 𝐴 ∙ 𝑒𝑥𝑝
𝑣 − 𝑣 2 0
𝜎 𝑇 𝑖
Thermalization time (ni~1013 cm-3, T~250eV) = 0.2ms
At t=0.2ms impurity ions are thermalized and a Maxwellian distribution is observed
  Velocity trends indicate flow of impurities at t<0.5ms from the formation and/or inner divertor towards the confinement vessel. The effect of this behavior on plasma performance is still unclear.
Slow jet outflow (M<<1) from confinement, due to use of strong mirror fields.
Ion flow velocity expected to increase as ions reach closer to electrode plates.
The latter region has yet to be diagnosed through spectroscopy.
Ion temperature:
Shot 107761
Note: Possible leakage of radiation through light collection assembly may result in over-estimation of ion temperature and under- estimation of ion velocity. Leakage is corrected for the next experimental campaign
Shot 107901
  Shot 107837
    Results and Analysis
   Characterization of ion flow to inner divertor electrodes is important to ascertain effectiveness of field line flaring.
 Effect of mirror fields and other machine parameters on ion confinement can be inferred from the end loss flow
 Characterization of impurity flow may reveal energy loss mechanisms
For layout of C-2W, see poster PP11:00078
Jet spectroscopy on C-2U
Doppler spectroscopy measurements revealed a thermally uniform and slow outflow due to use of mirror plugs and possible dense and collisional plasma in the jet region
(See Rev. Sci. Instrum. 87, 11D432 (2016))
    Background – the need for jet spectroscopy
 Measurement of strong O4+ 278.1nm line to provide higher signal for impurity flow measurements, which enables measurement at later times and during cleaner vessel conditions
 Thorough analysis of ion flow vs. variable mirror fields and inner divertor flaring fields
 Modulated Neutral beam or gas puffing can enable measurement of main ion flow through charge exchange
 Spatial Heterodyne Spectrometer will be added to measure ion flow at the inner divertor flaring field region.
Future Work
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