Demo

Impurity-ion rotation dynamics in C-2W
Marcel Nations, Deepak Gupta, Lothar Schmitz, Hannes Lienweber, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Edge biasing in C-2W :
q Impurity-ion rotation measured from Doppler shift of emission lines following charge-exchange between partially ionized oxygen impurities and fast neutrals from DNB
Motivation
Charge-Exchange Recombination Spectroscopy
Impurity-ion azimuthal velocity profiles
qImpurity-ion azimuthal velocities directly inferred from the measured Doppler shift of modeled CX emission line
qImpurities rotate in the electron-diamagnetic direction
• Negative biasing scheme creates radial electric field in the SOL pointing radially
inwards. The opposite occurs when operating with positive biasing
• Impurities rotate faster in the e-diamagnetic direction with increasing negative electrode bias voltage (stronger radial electric field across biased field lines in the SOL)
qRigid-rotor velocity profiles (i.e., uniform angular velocity) inside separatrix, Rs
Effect of neutral beams on impurity rotation
qIn addition to biasing, neutral beams also contribute to the net momentum qNeutral beam currents produce a positive radial electric field, which causes the plasma to rotate in the direction of the beam momentum
qExperiments are in agreement with theoretical model:
• Less beam current à stronger net radial electric field pointing inwards à faster impurity ion rotation in the electron-diamagnetic direction
qOngoing work to develop a circuit model to better understand the effects of biasing and beams on plasma rotation
qThere are three parallel current paths in this circuit, due to beams, ion drag, and electron drag. The ions have rotational inertia, which is represented as a capacitance
Schematic diagram of plasma load
Courtesy of E. Trask, S. Putvisnki
References
1. Tuszewski, M. "Field reversed configurations." Nuclear Fusion 28.11 (1988): 2033.
2. Tuszewski, M., et al. "Field reversed configuration confinement enhancement through edge biasing and
qEdge biasing of annular electrodes in the divertor region is routinely utilized as a boundary control technique to stabilize the FRC in the confinement vessel. The
potential difference between open-field lines in the scrape-off layer creates a radial electric field near the FRC separatrix and, consequently, E×B shear flow
End Biasing in C-2W
SOL
local
Plasma rotation and FRC stabilization:
qTypical FRCs are known to rotate in the
ion diamagnetic direction (same directions
as neutral beam injection), which may lead
to 𝒏 = 𝟐 instabilities if rotation parameter, NB
𝜶=𝒗 ⁄ >𝟏.𝟐[ref.1] Bias 𝜽𝒊 𝒗𝑫𝒊
q Negative biasing produces E×B shear in electron diamagnetic direction that oppose 𝒗𝑫𝒊, slowing down the plasma rotation (𝒗𝜽𝒊) and keeping the parameter 𝜶 < 𝟏 [ref. 2] qObserved low amplitude MHD activity of FRCs in C-2W corroborate with theoretical understanding of biasing
Estimates of radial electric field from impurity rotation
𝑯𝟎 𝒇𝒂𝒔𝒕
+ 𝒁-𝒏
𝑯- + 𝒁-𝒏3𝟏(∗) 𝑪𝒉𝑬𝒙 𝒇𝒂𝒔𝒕
FRC
qMotivation for active spectroscopy
• Local profile measurements with high
spatial resolution
• Added degree of control (signal level
and temporal resolution) with DNB modulation
qTarget: O-VI CX line at 343.4 nm
q Diagnostic uses high speed CCD camera and image intensifier synched with DNB
• Dispersion: 0.0075nm/px (8 km/s/px)
• Inst. Broadening: 0.035nm (43 eV) q Fitting procedure:
1. Fit background signal
2. Add CX model to fitted background 3. Iterate over Doppler width and shift
of CX model to get best fit with total measured signal
• Translate 𝝓(𝒓) along 𝑩-field lines to create 𝑬(𝒓) at plasma edge
• Biasing controls plasma rotation via 𝑬×𝑩 force
Impurity ChERS in C-2W
• 16 optical lines-of-sight (8 shown here) • Ø1” lenses, 600 μm fibers (0.22 NA)
• Full modulation of DNB at 1 kHz
• DNB operated with H/D (40 keV, 8 A)
• Radial profile coverage up to r ~ 50 cm O-VI
Directions of Rotation
FRC
NB
Estimates of E×B drift velocity from radial momentum balance
qThere is a strong need to understand how the applied potentials vary from the electrode surface to the center of the machine
qChanges in potential along field lines (sheath, pre-sheath, etc.) can vary depending on plasma/machine conditions. This affects the resulting radial electric field in the SOL needed for rotation and, consequently, stabilization of the FRC qImpurity rotation can be used as an indirect diagnostic of the radial electric field in the confinement vessel
qAzimuthal rotation of partially ionized oxygen impurities is dominated by E×B drift, and radial momentum balance can be used to estimate the net radial electric field in the SOL
qRadial momentum balance equation valid for impurity-
ions, main-ions, and electrons:
D+
•
• • •
𝐸& 1𝜕𝑃 O6+ 𝑣! =𝑣"×$ + 𝑣% =−𝐵' + 𝑒𝐵'𝑍𝑛𝜕𝑟
impurity-ion velocity dominated by 𝒗𝑬×𝑩 term due to relatively small contribution of diamagnetic velocity term (𝒗𝑫𝑰)
𝒗𝑬×𝑩 increases with bias voltage
𝑬𝒓 @ separatrix ~ -4 to -10 kV/m
The rotation dynamics of the oxygen impurities and are in good agreement with measured main-ion rotation and results from an independent Doppler Backscattering diagnostic
vExB range from Microwave Reflectometer
vExB range from Microwave Reflectometer
Negative Biasing
neutral beam injection." Physical review letters 108.25 (2012): 255008.