Page 1 - An Interesting Poster to look at from the Tri Alpha Energy Team in California

LANGMUIR PROBE CHARACTERIZATION OF C-2W SCRAPE-OFF-LAYER PLASMAS
Ami M. DuBois, T. Roche, K. Knapp, R. Michel, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
ABSTRACT
The C-2W experiment is a field-reversed configuration (FRC) which uses neutral beam injection and an edge biasing scheme to suppress turbulence and stabilize the plasma. The FRC core is surrounded by open field lines in the edge region of the confinement vessel (CV), called the scrape-off-layer (SOL), which terminate on electrode plates located in the inner and outer divertors. Therefore, settings in the divertors can significantly affect the plasma conditions and stability in the SOL. A motorized triple Langmuir probe with 24 inches of travel has been installed on the CV, 66 cm from the mid-plane to make critical measurements of the SOL plasma parameters. Characterization of the electron density (n ), electron temperature (T ), floating potential, plasma potential, and
TRIPLE LANGMUIR PROBE  Makes time-resolved in situ measurements of v , T , n , v , E by sampling three
OUTER DIVERTOR BIASING
 Electrode biasing aims to control SOL rotation via 𝑬 × 𝑩 velocity shear & suppress instabilities
points along I-V characteristic trace  3 Tungsten tips
feepr
𝐯 − 𝐯 +𝒇
𝐥𝐨𝐠 𝟐
𝐯−𝐯 ++ +
𝑹
r = 44 cm
𝑰 ee𝑰𝒎
ee
 Electrically floating tip (1) measures vf
𝒔𝒂𝒕
𝑻𝒆 =

d = 0.5 mm, l = 5 mm, A = 8.21 mm2
r = 65 cm: -1.3 kV bias
r = 71 cm: 1.3 kV bias
r = 72 cm: Floating
=
edge fluctuations in the C-2W SOL are presented. Initial measurements show SOL T ~20 eV as the FRC is e
 Tips 2 and 3 are floating & biased wrt each other
formed, after which T gradually decreases, and SOL n ~1018 m-3, consistent with Thomson Scattering and FIR diagnostic data measured at the CV mid-plane. Finally, details of a design for a new insertable probe platform system and initial measurements in the C-2W inner divertor will be discussed.
𝒔𝒂𝒕 𝒊
𝒏𝒆 = 𝟎. 𝟔𝟏𝒆𝑨 𝒌𝑩(𝑻𝒆 + 𝑻𝒊)
[1] M. W. Binderbauer et al., AIP Conf. Proc. 1721, 2016 [2] M. W. Binderbauer et al., Phys. Plasmas 22, 2015 [3] P. A. Bagryansky et al., Phys. Rev. Lett. 114, 2015 [4] D. D. Ryutov et al., AIP Conf. Proc. 1721, 2016
[5] M. Tuszewski et al., Phys. Rev. Lett. 108, 2012
[6] W. Lennartsson, NASA Double Layers in Astrophys. 275, 1987 [7] S. Pottinger et al., J. Phys D: App. Phys. 44, 2011
[8] R. Bostrom, IEEE Trans. Plasma Science 20, 1992
𝐯 =𝐯 −𝑻𝒆𝐥𝒏 𝟒𝒎𝒊
t=0.5ms
THE C-2W DEVICE & MOTIVATION
 The C-2W[1] device is a field-reversed configuration (FRC) which uses neutral beam
injection & an edge biasing scheme to suppress turbulence & stabilize the plasma
Triple Probe
Mirror coils
 FRC core - located within separatrix, surrounded by SOL & plasma edge
 SOL - region of open field lines (OFL), terminate on electrode plates in divertors
 Plasma edge - region of OFL that do not make it through mirror region, terminate on CV wall
𝒑𝒊+𝟏 𝒅𝒙
𝒑𝒊
𝒑 𝑬𝒓 =
TYPICAL C-2W PLASMA SHOT
 FRC plasmas last 3 - 4 ms, length ~2 m
 Edge B-field slowly decreases over FRC lifetime
 Te peaks in first 500 μs (during plasma merging)
 Fairly peaked ne early in time with hollow profile developing as FRC develops
 Edge plasma parameters peak during the first 500 μs
 After plasma merging, edge measurements slowly decrease & level out around 2.5 ms - probably due to shrinking of FRC radius
 55 cm maximum insertion depth of probe
 Inserting too deep could inject impurities & cool plasma down, resulting in decreased plasma lifetimes (t20 time)
 No correlation between probe insertion depth & plasma lifetime
𝒇 𝟐 𝟑𝝅𝒎𝒆 𝐯−𝐯
 Overall trend at plasma edge: vf & vp decrease with increasing electrode bias
 Unexpected since field lines in this region do not terminate on electrodes
 Indicative that bias is affecting edge plasmas through the formation of a possible double layer between SOL & wall
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RADIAL PROFILES
Edge Te peaks at ~40 eV (r = 65 cm) & decreases to 0 eV at wall, tracks well with core Te
Abel inversions from FIR line averaged ne currently don’t use TP data as constraint, but match edge ne measurements quite well
Large outward Er generated early in shot, significantly flattens out with almost no Er later in shot
Er approaches 0 V/cm moving inward toward SOL Edge Er x Bz velocity in electron diamagnetic direction
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Double layers are common in plasmas with magnetic mirrors[6], electric propulsion experiments[7], & space plasma boundary layers[8]
Experiments making potential profile measurements closer to SOL will help confirm
TS, FIR
WALL CONDITIONING
Wall conditioning after vent is important for improving plasma conditions & lifetimes
Impurities are reduced shortly after discharge cleaning begins
Core Te & plasma lifetimes increase
t = 0.6 ms
 Mirror coils: generate flared B- field in inner divertor to reduce particle loss from SOL[2] & maintain high SOL T [3,4]
t = 0.5 ms
e
 Electrode plates: control SOL
Discharge cleaning & mirror plasma discharges began after vent to improve wall conditions
Start of titanium deposition on the first wall prior to operations each day
 At plasma edge, vf begins increasing very shortly after Ti deposition begins
 Indicates steepening of IV characteristic trace, or higher electron temperatures
 Edge Te increases with better wall conditions
rotation via 𝑬 × 𝑩 velocity shear[5], suppress instabilities that induce plasma transport & plasma confinement lifetimes
 Inner divertor settings greatly affect edge, SOL plasmas & stability/confinement of core FRC plasma making spatially localized in situ measurements of edge & SOL plasmas crucial
Raw v
Smoothed vp
Radial E-field
p
 New measurements to r = 55 cm will bridge gap between core & edge diagnostics & gain more information on vp & Er closer to SOL