APS 2019 - Marcel Nations - Final

Motivation
q In the C-2W experiment, edge biasing is used to drive rotation, stabilize, and heat a beam-driven field- reversed configuration (FRC) plasma embedded in a magnetic mirror
q Highly charged oxygen impurity ions that exist in the plasma are sensitive to biasing effects and their azimuthal rotational velocity magnitude and direction are dependent on the applied electrode bias voltage and polarity, respectively
q A multi-chord passive Doppler spectroscopy diagnostic targeting the O4+ triplet lines near 278 nm is used to measure impurity ion temperature, density, and azimuthal velocity profiles
q The radial momentum balance equation for impurity ions is utilized to extract the local electric field near the mid-plane of the confinement vessel
q A strong correlation exists between observed rotation and energy accumulated in plasma. Thus, estimates of the electric field magnitude by means of impurity rotation gives valuable insight into how plasma boundary control through edge biasing affects the plasma in the confinement vessel of C-2W
JK +(,)
JL SOL
FRC core
• Translate E(,) along '-field lines to create +(,) at plasma edge
• Biasing drives plasma rotation via +×' force
• O4+ impurity ion rotation is very sensitive to edge biasing effects
• The direction of impurity ion rotation is highly dependent on biasing polarity
Line-integrated measurements
Intensity (W/cm2)
Velocity (km/s)
Temperature (eV)
q Shot-to-shot time history of line-integrated O4+ parameters q Profiles are well correlated with separatrix radius
q Velocity profiles peak near separatrix , whereas temperature profiles are essentially flat in the plasma core
q Heating of impurity ions in the SOL , just outside the separatrix, is often observed for strongly biased shots
q Radial electric field and radial pressure gradient give rise to toroidal drift (drag term neglected here)
q Radial momentum balance equation can be written for multiple species, including impurities
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Measurement of impurity ion (O4+) profiles
Inversion technique for radial (local) profiles
q Local emissivity (MN), velocity (!N), and temperature (4N) matrices are calculated directly:
Electrode biasing and impurity rotation
q Biasing is vital for steady-state NB-driven C-2W plasmas
• Main roles: MHD mode stabilization and turbulence suppression
• Additionalroles:fueling,heatingofSOL,enablingfastionaccumulation
Electric field profile measurements in C-2W from impurity ion radial momentum balance
Marcel Nations, Deepak Gupta, Dmitry Osin, Sangeeta Gupta, Peter Yushmanov, and the TAE Team
a ` a = gL[L\ T T
∑ M[L\ b S TS S S
∑ L[L\ S TS S
TTSS `T==
S
M = L cosX ST ST ST
a T
YT =
S TS
T ST
ST T T
∑ L[L\ Θ S TS S S
∑ L[L\ S TS S
^ ∑ L[L ∑ L cosK X `Ka
q Plasma energy, GHIH, starts increasing at inflection point of transition from initial FRC to NB- driven FRC
q At inflection:
• InitialN=1modeweakens
• Impurityionrotation
frequency sharply increases, indicating biasing voltage penetrating into CV
q Fast ions start accumulating at the onset of bias in CV
where O:, P:, and Q: are the line-integrated brightness, velocity, and temperature matrices, respectively. L:N is the geometric line-of-sight matrix. The angle matrix &:N represents the angles between sightline i and local tangential velocity at emission zone j
R. E. Bell, RSI 66, 558 (1995) R. E. Bell, RSI 68, 1273 (1997)
O4+ velocity and temperature profiles
Velocity
−
_
∑ L[L\ S TS S
O4+ emissivity and density profiles
q Spectral lines are integrated in frequency space: W/(cm2 nm) à W/cm2
q Emissivity profiles calculated by inverting measured line-integrated intensity profiles
Electrode biasing in C-2W
Intensity
2"#$ =20 c 40,20 d0 0ef
q Density profiles calculated directly: M"#$
q Excitation rate coefficients (Open-ADAS) calculated with ne and Te profiles obtained using Bayesian statistics (Minerva)
Density
Temperature
i-ChERS diagnostic in C-2W
q The diagnostic provides spatially- and temporally- resolved measurements of key impurity ion parameters
• Impurity ion velocity (Doppler shift)
• Impurityiontemperature(Dopplerbroadening) • Impurityiondensity(lineintensity)
q Measurements are taken near the center plane of the confinement vessel
q Time resolution: 100 microseconds
q Line-integrated measurements at multiple lines-of- sight
• 15 viewing chords (0-50 cm impact parameter)
• Spatiallyconvolutedsignals(symmetricinversion
method)
• CurrentlytargetsO4+tripletnear278nm
q Initial rotation similar to that of unbiased FRC
q After inflection impurity rotation increases dramatically
q Onset of rotation and onset of energy accumulation highly correlated
O4+ triplet near 278 nm
Emissivity
./"#$ 01"#$ 2"#$
=
3' 01"#$ 2"#$
52"#$ 5,
+ 2"#$
54"#$ 5,
+,~? − A3B/D
"
,&
(
,
4"#$
4+ infersradialelectricfieldsnearplasmaseparatrixof
3' 5(72 2"#$ )
= 01"#$ 4"#$ 5, +
54"#$ 5,
01"#$ 2"#$
Electric field estimates from radial momentum balance
q For O4+ impurities, the contribution of the E×B drift to rotation dominates over the diamagnetic contribution
!9:;
! ~=> − ?> %
&
peak rotational velocities ~30-100 km/s
rDF
q Impurity rotation correlates strongly with global plasma performance parameters
O4+ Thermal vs. Kinetic Energy in steady-state
q O4+ energy almost equally distributed between rotation and temperature due to biasing in the SOL
Courtesy of D. Osin
!#$ '=−++ ./"#$
q Measured O