End loss analyzer system for measurements of plasma flux at the C-2U divertor electrode

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11D432 (2016)
Jet outflow and open field line measurements on the C-2U advanced
beam-driven field-reversed configuration plasma experiment
D. Sheftman,1,a) D. Gupta,1 T. Roche,1 M. C. Thompson,1 F. Giammanco,2 F. Conti,2
P. Marsili,2 and C. D. Moreno2
1Tri Alpha Energy, Inc., P.O. Box 7010, Rancho Santa Margarita, California 92688-7010, USA 2Department of Physics, University of Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
(Presented 6 June 2016; received 6 June 2016; accepted 6 July 2016; published online 26 August 2016)
Knowledge and control of the axial outflow of plasma particles and energy along open-magnetic-field lines are of crucial importance to the stability and longevity of the advanced beam-driven field- reversed configuration plasma. An overview of the diagnostic methods used to perform measurements on the open field line plasma on C-2U is presented, including passive Doppler impurity spectroscopy, microwave interferometry, and triple Langmuir probe measurements. Results of these measurements provide the jet ion temperature and axial velocity, electron density, and high frequency density fluctuations. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4960059]
I. INTRODUCTION
Advanced beam-driven field-reversed configuration (FRC) experiments have been carried out on C-2U, an enhanced beam power apparatus based on C-2,1 resulting in improved particle and energy sustainment in the FRC plasma. It has been shown that edge control of the FRC plasma is crucial for the increase of particle and energy decay times.2 Open field line measurements have been performed in FRC devices in the past.3,4 However, these studies were usually limited to electron density and outflow velocity measurements at a single axial point. Thus, radial and axial profiles of electron and ion temperature and particle outflow velocity have largely remained unknown and replaced by numerical estimations. Moreover, experimental conditions obtained on C-2U may result in significantly di↵erent behavior of plasma at the scrape-o↵ layer compared to experiments on other FRC devices. Control of the scrape-o↵ layer plasma parameters is increasingly important for successful FRC temperature scale up. A diverse suite of open field line plasma (henceforth referred to as the “jet”) diagnostics was developed and installed on C-2U. These diagnostic methods provide, for the first time, detailed measurements, in tandem, of electron density, ion and electron temperature, and ion outflow velocity inside the FRC jet.
II. DIAGNOSTIC METHODS
A. Microwave (MW) interferometry
A single chord microwave (MW) interferometer, based on a heterodyned 140 GHz source, was developed and used to measure line integrated electron density at the jet. The MW
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:
sources included 139.9 GHz and 35 GHz Gunn oscillators. Both sources were split into two arms. A 100 MHz modulated reference beam was produced by coupling the first arms of the MW sources through a harmonic mixer, e↵ectively mixing the 139.9 GHz beam and the 4th harmonic of the 35 GHz beam. The same technique was used to produce a 100 MHz modu- lated signal, by mixing the plasma probing 139.9 GHz beam and the second arm of the 35 GHz beam (see Fig. 1). The phase ' of the probing beam is extracted from the quadrature phase detector. The integrated electron density is then calculated as
⌅
where c is the speed of light, "0 is the vacuum permittivity, is the probing wavelength, and m is the electron mass.
The MW interferometer provides a highly sensitive measurement of electron integrated density and fluctuations. A disadvantage of this long wavelength diagnostic is the refraction due to electron density gradients. These gradi- ents occur due to macroscopic motion of the jet column and micro-turbulence. However, the heterodyne method enables multiple fringe count with minimal error, including during increased MW beam refraction, where the signal obtained by the quadrature phase detector is decreased (see Fig. 2). Fast integrated density fluctuations were observed at the jet location, z = 2.25 m. These fluctuations temporally correlate with similar fluctuations observed at the center of the FRC (z = 0), through far infrared (FIR) interferometry. Through time-of-flight measurements, these fluctuations were found to propagate at velocities characteristic of ion acoustic waves (see Fig. 3).
B. Doppler spectroscopy
A Chromex 250-IS spectrometer (3600 g/mm holographic grating) was used to measure ion temperature and axial outflow
4+ velocity. Three axial view chords were used to collect O
n dl = 4⇡c2m" ' , (1) e 0 e2
impurity radiation (see Fig. 4). Fiber optic cables, used to 0034-6748/2016/87(11)/11D432/3/$30.00 87, 11D432-1 Published by AIP Publishing.
dsheftman@trialphaenergy.com