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

Two-dimensional density profiles of rotating mode structures using the Radon transform on the C-2W experiment
Roger J Smith, Tadafumi Matusmoto, Sean A Dettrick and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Abstract
In 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 [NBI], expander divertors, end bias electrodes and an active plasma control system. Although global MHD modes are largely stabilized, coherent density fluctuations can be occasionally seen in the time traces of the mid-plane 14 chord far-infrared (FIR) interferometer. These benign fluctuations are due to rigid non axisymmetric density structures rotating about the plasma’s axis. Of particular interest are the common FRC n=2 rotating mode and micro-burst mode activity associated with fast ions from NBI. Both are manifested as symmetric line integrated density [LID] radial profiles at any time in a modal period. A new so-called Seesaw mode has been observed that produces lopsided LID profiles that seesaw in the cycle. The high spatial and temporal resolution of the interferometer allows reconstruction of rigidly rotating 2-d density distributions including the Seesaw mode. Inverse Radon transform generated 2-d modal density structures will be presented and discussed along with correlated observations from magnetic Mirnov arrays and electrostatic probes.
[1] H. Gota et al., Nucl. Fusion 59, 112009 (2019).
NB Injector
C-2W Device: Norman
n=2 axisymmetric mode structure. Shot 109347
• Weakest n=2 cycle. Density distribution is very circular.
• Strongest n=2 cycle: a radius of constant LID is clear.
• Density spatial distribution is not circular.
• n=2 cycle near end of sequence, Seesaw behaviour is now evident and a n=3 pattern is present
Core plasma Interferometer
FIR LID profiles suggests plasma is rotating with a rigid structure micro-burst mode
Laser Table
Beam Path
xFIR
FIR chords: 7 vertical beams and 7 tilted by 15
Massive Support Structure
C-2W Vessel
Inner Divertor
Outer divertor Qtz tube formation region
Radon transform (ne(r)
Identifying the cycles (n=2) phase
LID (i)) Possible density for axisymmetric n=2 mode
Summary:
• Radial wobble disguises modes, so use opposing ch differences, n=2.
• N=2 is evident by the antiphase between central and edge chords.
• Amplitude variation with radius as shown is what will characterize the
structure specific to this mode.
• One full revolution of 360 is a two periods .
• A continuous evolution is of the mode is obtained an the Inverse Radon transform can be used at any initial time once T(t) is determined.
• Mixing of two behaviors or two modes with different periods is a problem for this method.
• Complimentary technique of profile fitting with Bayesian analysis is being pursued by a TAE/Google collaboration.
• The Radon transform gives a more integral description of the mode.
Micro-Burst axisymmetric mode structure
LID distribution (1019m-2)
Inputs to Inv. Radon transform at two different angles
r
Possible density structures for non-axisymmetric Seesaw
14 FIR chords
mode (n = 1 or 3)
Inputs to Inv. Radon transform at twodifferentangles
CV and mid-plane interferometer
LID distribution (1019m-2)
14 FIR chords
r
Seesaw non-axisymmetric mode structure
• The LID profiles are seesawing or peaking from one side to the other as the plasma rotates, this could be due to a discrete blob (n=1) wobble or a n=3 spatial pattern.
• Seesaw mode has a n = 3 mode signature and involves the FRC from axial end to end.
• Contours are 1x1017=m-3
• -burst mode exhibits a chirped n=2 mode that saturates and then attenuates at lower frequency.
•
•
the 2 -bursts are resolved and the also the
quiescent period between them.
B. Deng, et al., ’First experimental measurements of a new fast ion driven micro-burst instability in a field-reversed configuration plasma’, Nucl. Fusion 58 (2018) 126026)
First Cycle
Excluded flux radius
Second Cycle
Mode analysis from CV Mirnov probe arrays
Large deformation
Dynamics:
Inbetween two cycles
ne<3x1017m-3 or radial motion induced ne.
Last cycle, LID surface
On axis peak goes away.
Depth between -bursts Small peaking on axis gets
smaller and disappears
Fitted period Vs time
Allows a continuous movie to be made of the dynamics.
Mode is n=2 to start but can change to n=3.
The density perturbation is initially contained inside separatrix but moves outward.
One interpretation is that viscous interaction with SOL slows the rotation once ne moves into SOL.
ne gets stronger and depth increases.