Deng-APS-DPP-2020-poster_4

Density Fluctuations Measured by FIR Interferometer System in C2W FRC
C. Deng1, M. Beall1, M. Kaur1,2, E. Parke1, R. Smith1, K. Zhai1, and TAE Team1
1TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610, 2University of California Irvine, Irvine, CA 92697 Coherent and Broadband Density Fluctuations Measurement Capabilities
Coherent modes
Method:
(1) Radon transformation: tomographic reconstruction assuming plasma rotation [3]
(2) Using spectral analysis to get time averaged amplitudes and phases on 14 chords, (a) use magnetic probe to determine the mode number or (b) use Abel inversion for the assume mode numbers and find the best fit to measured Amplitude and Phase, both involve Abel inversion with reconstructed pathlength array for different mode n number.
∫69RSRTIE E*,*'XY 6Z*,* ... E*,['XY 6Z*,[ 69* ]*,* ... ]*,[ 69* ...=............=............
∫ 69RSRV IE E\,*'XY 6Z\,* ... E\,['XY 6Z\,[ 69[ ]\,* ... ]\,[ 69[
Where: ∫ 69RSRT...V IE are interpolated from measured mode amplitudes (Amp(x)) and phase (q(x)):
real(∫ 69RSRT...V IE )=Amp(x)*cos(q(x)), imaginary(∫ 69RSRT...V IE )=Amp(x)*cos(q(x)), x: chord impact constant; M:
chord number, N: number of rings for Abel inversion, n: mode number. Assuming mode amplitude 69 =
69 ^ . New pathlength array L are calculated, use SVD to solve the equations to get fluctuation mode radial profile.
Density broadband fluctuations:
Method: (1) Spectral analysis: frequency spectrum of ∫ 69IE for each chord, and the amplitudes in certain frequency ranges are the square root of integration of power spectral density(PSD) in the frequency ranges.
(2) correlation analysis: obtain relations (coherence and phase) to other signal (magnetic signal)
Coherent Modes
Disruptive modes, n=1 wobble mode for example, are well suppressed by Neutral Beam Injection and end electrode biasing [1].
(1) The high performance FRC is obtained with NBI and 1.5kV biasing and a benign low frequency density
Abstract
In the TAE Technologies current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams, advanced divertors, end bias electrodes, and an active plasma control system. The density profile measurements and characterization of fluctuations such as low frequency n=1 and n=2 rotational modes are made using the powerful 14 chords FIR interferometer system. A new investigation into higher frequency density fluctuations measured by far-forward scattering methods is presented in this effort. These data are analyzed for a variety of plasma parameters and machine settings such as biasing voltages, looking towards correlations between density fluctuations and plasma confinement variations.
[1] H. Gota et al., Nucl. Fusion 59, 112009 (2019).
Broadband Density Fluctuations with Different Biasing Voltage
Time evolutions of plasma parameters of three shots with different Vbias
Power spectral density of ∫69/IE
Relative higher frequency ne fluctuations vs Vbias (from many shots)
Total plasma stored energy increases with Vbias
Time traces of Vbias and filtered ∫ 6/ IE
Profiles of higher frequency ne fluctuations
Laser Table
Beam Path
C-2W Vessel
FIR Interferometer System
Far-forward scattering measurement re-confirms the conclusions that have been demonstrated both theoretically[4] and experimentally[5] :
Higher biasing voltage improve Beam Driven FRC plasma performances by
(1) Suppressing n=1, 2 modes (2) Suppressing broadband density turbulence
Massive Support Structure
fluctuation mode is observed. (shown on the right)
Mode characters:
• Mode frequency is ~ (10-60)kHz
• Mode frequency increase with total temperature,
• Amplitude measured by RMS (Root Mean Square)
Time traces of Te, ne, and Rs (shot 114534)
∫ 6/ IE spectrogram, RMS of the mode and total temperature
and Future Plans
(1) 14 Chord FIR interferometer system can be used to measure the density spatial distribution and its fluctuations with !" < 0.5'()*
(2) Density fluctuation coherent mode radial profiles may be calculated from the measured amplitude and phase with knowing mode numbers:
(3) The measurements shows higher biasing voltages :
• suppress the coherent n=1,2 modes, produce stable high performance plasma
• suppress broadband density turbulence, improves plasma confinement.
Future plans:
(1) Find relation between the broadband density confinement with plasma transport (2) Identify the coherent modes
(3) Search for zonal flows using envelop analysis
References:
[2] B. Deng, et al; RSI 89 (2018)
[3] R. Smith, et al; presentation in APS DPP conference 2019 [4] M. Tuszewski, et al; Phys. Rev. Lett. 108 (2012
[5] L. Schmitz, et al; Nature Communications (2016)
Summary
14 chords with 7 chords in the mid-plane and 7 chords tilted 15o from the mid-plane
14 chords of interferometer signals are used for density profile measurement,
7 vertical chord polarimetry signals are used to measure toroidal magnetic field component while 7 tilted chords polarimetry signals are used to measure poloidal (z) magnetic field component [2].
The 14 chord interferometry signals are used to measure density fluctuations by Far-Forward Scattering scheme, with upper bound perpendicular wavenumber !" < 0.5'()* , which is determined by the optical
width to the detection: p/w, w is the diameter of beam at the focusing lens before the detector or the diameter of the lens
Density fluctuations can be measured from information on:
(i) Phase +,-./01/023/./0 = ∫ 67 + 69 dl
(ii) Amplitude –scattering :;<=../0,-> ∝ :,-<,@/-.69A ∝ (C(DEFGHIJ)A
reduced with increase of Ttotal after Ttotal > 650eV
(2) Line-integrated fluctuating density mode amplitude (spectral analysis) and phase (correlation analysis) can
be calculated:
PSD of ∫ 6 IE (x=4cm) /
Measured and modeled amplitude and phase
Density profile at 13ms and 3x mode radial profile
•
Line-integrated fluctuating density amplitude have nodes and phase flipped at ^~ ± 0.28(
1. 14 chords.
3. 1MHz bandwidth.
2. 8cm chord spacing 4. L" < M. NOP)Q
profile inversion conform: n=2.
• The mode peaks outside of Rs (=0.33m), at greatest density gradient region (4) The RMSs of n=2 modes from shots with different biasing voltage show:
à n is even, and most likely 2
(3) Best fit to the measured data from mode radial
RMS of n=2 mode for 3 shots with different Vbias
The n=2 mode can be suppressed by increasing biasing Voltage