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

Correlations between Impurity Ion Tangential Velocities and Electron Density Rotational Modes in the C-2W Experiment
M. Beall, D. Gupta, and the TAE team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
iChERS
C-2W Experiment
 The purpose of the C-2W experiment is to demonstrate control, stability and heating of a Field-Reversed Configuration (FRC) plasma via magnetic controls, neutral beam injection and edge biasing from divertor electrodes
 To this end, it is important to have measurements of key plasma parameters, of which density, temperature, and velocity profiles are of high importance
FIR interferometer
Outermost chord at 35 cm
Initial Results
 All three calculated frequencies are shown in figure b) in example shot a)
 Growth of the N=2 mode has been predicted[1] to be correlated to the parameter 𝜶 = 𝛀 𝛀𝑫𝒊 rising above a threshold of 1-1.4
 Figure c) shows good agreement with the amplitude of the N=2 mode dying away as α drops below 1
 Figure d) shows a shot with edge biasing reducing α over time before termination, and an unbiased shot which retains a value of approximately 2.5 through its lifetime
 Figure e) shows how biasing can reverse the direction of the impurity ions
a)
Outermost chord at -38.6 cm
 System has 15 viewing chords situated with impact parameters every 7 cm across the plasma
 Each chord provides temporally and spectrally resolved data, as shown above, allowing for full profiles of impurity densities, temperatures, and velocities
 The system can also operate in a passive mode with electron excited lines
 For the measurements used in here, the system has a velocity resolution of 4 km/s and a time
b)
c)
resolution of 10 kHz
d)
e)
Future Work
N=2 mode and Rotational Measurements
 The primary density diagnostic for the experiment is the Far-Infrared (FIR) interferometer.
 The system has 14 chords at 433μm sampling the plasma every 8 cm, with a sensitivity of 1016 m-2 at >1 MHz bandwidth, allowing observation of a wide range of plasma modes
 A third laser cavity is scheduled to be installed allowing for simultaneous polarimetry and interferometry
 Half the chords are tilted 15o towards the magnetic axis to allow for measurement of Bz
 The n=2 rotational mode[1] is a well known and observed behavior in FRC plasmas
 Shown to the right, n=2 activity can be observed as a small magnitude coherent oscillation out of phase between inner and outer FIR measurements
 For certain operational regimes in C-2W plasmas, it is observed by multiple diagnostics to develop in a stable and non-destructive manner
 Assuming a rigid rotor model holds true, the plasma rotational frequency can be approximated as the N=2 frequency
 iChERS provides information on the angular frequencies of impurity species (in this case OV), which, while not equivalent to the main ion velocities[2], should respond to plasma controls which effect rotational rates, such as edge biasing
 With ion temperature measurements also from iChERS, it is possible to approximate the ion diamagnetic frequency from
𝑩𝒆𝒏𝒆
for a flat temperature profile and Zeff ≈ 1
 Intermittent nature of N=2 mode makes direct comparison challenging, automation work is in progress to allow for statistical comparison
 To better compare frequencies, impurity density, temperature and velocity profiles can be inverted[3] and used to calculate main ion rates
 Differences between frequencies of bulk plasma motion and rotational modes may provide information on velocity shear and transport
Abstract
The C-2W experiment is a Field-Reversed Configuration plasma device directed towards studies of beam-driven, magnetically confined fusion plasmas. As part of the upgrade from the preceding C-2U device, new magnetic systems, more powerful neutral beams, internal expansion chambers, and biasing electrodes were added to improve energy confinement times, particles temperatures, plasma stability and control. Many interesting dynamics arise from the rotational modes of the plasma, one of which being the so-called N=2 elliptical mode. This plasma deformation arises non-destructively during many high energy pulses, and is observed on a range of diagnostics. The ability of the new iChERS spectrometer to measure temporally and spatially resolved impurity ion velocities and temperatures provides an interesting opportunity to observe the relationship between the plasma rotational velocity and some of its rotational modes. The spectrometer provides a velocity resolution of 4.0 km/s at short wavelengths, such as for 278.1 nm (O-V), and 1.5 km/s at longer wavelengths like 656.52 nm (D).
References
[1] M. Tuszewski, “Review Paper Field Reversed Configurations”, Nuclear Fusion, Vol.28, No. II (1988)
[2] D. Gupta, (2013 November). Measurements of Minority Radial Momentum Balance Equation for The C-2 FRC Plasma. poster session at APS DPP Denver, Colorado
[3] R. Bell “An inversion technique to obtain full poloidal velocity profiles in a tokamak plasma” Rev. Sci. Instrum. 68, 930 (1997)
𝟏 𝑻𝝏𝒏
𝛁𝑷 ≈ 𝒊 𝒆 𝑩𝒆𝒏𝒆 𝝏𝒓
All traces represent different lines of sight from the same discharge