High sensitivity far infrared laser diagnostics for the C-2U advanced beam-driven field-reversed configuration plasmas

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11E128 (2016)
Improved density profile measurements in the C-2U advanced beam-driven
Field-Reversed Configuration (FRC) plasmas
M. Beall,a) B. H. Deng, and H. Gota
Tri Alpha Energy, Inc., P.O. Box 7010, Rancho Santa Margarita, California 92688, USA
(Presented 7 June 2016; received 2 June 2016; accepted 8 August 2016; published online 31 August 2016)
In the prior C-2 experiment, electron density was measured using a two-color 6-chord CO2/HeNe interferometer. Analysis shows that high-frequency common mode phase noise can be reduced by a factor of 3 by constructing a reference chord. In the system upgrade from C-2 to C-2U a 4-chord far-infrared laser interferometer was developed, which demonstrated superior sensitivity (1 ⇥ 1016 m 2 at >1 MHz bandwidth) and solved the under spatial sampling issue of the C-2 interferometer system. Improved density-profile measurement results are presented in this paper, including evidence of fast-ion modified density profile and stabilization of the n = 1 plasma wobble mode. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4962038]
I. INTRODUCTION
The goal of the C-21 and C-2U2 experiments has been
to demonstrate generation and sustainment of field-reversed
configuration (FRC) plasmas. In order to diagnose this kind
of plasma, electron density measurements are essential. In the
C-2 experiment, electron density was primarily measured by a
two-color CO /HeNe interferometer3 with impact parameters 2
of the chords at 3.3, 15, 20, 25, 30, and 35 cm below the machine axis in the midplane.
When the plasma has cylindrical symmetry, the chord measurements on the bottom half of the vessel could generally be mirrored about the center position, as measured by bolome- try, to allow Abel inversion (which assumes axial symmetry).4 The accuracy of the mirroring and inversion process is thereby dependent on the availability and quality of the bolometry data, which is deteriorated by the increased beam power in C-2U. Also, if the plasma moves more than 10 cm away from the machine axis, such as when the n = 1 wobble mode amplitude is large, the interferometer chords no longer have good spatial sampling of the full plasma, resulting in significant errors in Abel inversion. In the case of a downward shift, redundant information is collected about the plasma core, but little to none about the gradient and edge regions. The opposite is true when the plasma core moves up and out of the sightlines of the CO2 chords. This can occur when the n = 1 wobble mode amplitude is large. If accurate center positioning is available, this can allow the impact parameters of the chords to e↵ectively be scanned over the plasma, providing a more thorough profile.2 Similarly, when the n = 2 elliptical-distortion mode develops, plasma axial symmetry is destroyed, and Abel inversion is not applicable. In this case, by modeling the plasma as a rigid rotating ellipse for each half rotating period, 2-D density profile of the plasma, as well as the plasma center position, can be
Note: Contributed paper, published as part of the Proceedings of the 21st Topical Conference on High-Temperature Plasma Diagnostics, Madison, Wisconsin, USA, June 2016.
a)Author to whom correspondence should be addressed. Electronic mail:
mbeall@trialphaenergy.com
obtained.5,6 However, this technique is no longer necessary in C-2U, as the n = 2 mode is almost completely suppressed by edge control via end-on plasma guns.
In the general case of a well-centered plasma, accurate density profile measurements in C-2 still su↵ered from the relatively coarse resolution of the two-color CO2/HeNe inter- ferometer system (referred to as the CO2 system hereafter) of 2 ⇥ 1018 m 2, which also obscures small plasma fluctuation measurements.
In order to alleviate the previously discussed issues for the C-2U experiment a new set of 432.8 μm far-infrared (FIR) chords were added, with impact parameters at 0, +15, +30, and +45 cm above the machine axis in the midplane. The detailed configuration of the FIR diagnostic system is described separately.7 In Section II, the CO2/HeNe interferometer resolution is analyzed, and a method to improve future two- color interferometer resolution is proposed along with a discussion of the superior FIR interferometer resolution. In Section III the improvement of density profile measurements due to the implementation of the FIR interferometer is shown, and new experimental results including evidence of fast-ion modified density profile and stabilization of the n = 1 plasma wobble mode are also presented; followed by a brief summary in Section IV.
II. IMPROVED INTERFEROMETER PERFORMANCE
The two-color interferometer resolution data are shown in Fig. 1(a). Detailed analysis shows that the dominant component is a high-frequency noise, common to all chords, due to the phase mismatch between the CO2 laser and the radio wave source used for the acoustical optical modulator and as the reference signal for the quadrature phase detector.4 By subtract- ing the common mode noise, the resolution can be improved by a factor of 3, as shown in Fig. 1(b). Therefore, for future two-color interferometer systems, it is important to construct a reference chord for measuring the common mode noise.
Even with common mode noise subtraction, the two-color interferometer resolution is still around 7 ⇥ 1017 m 3. On the
0034-6748/2016/87(11)/11E128/3/$30.00 87, 11E128-1 Published by AIP Publishing.