Particle and heat flux diagnostics on the C-2W divertor electrodes

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10J110 (2018)
Particle and heat flux diagnostics on the C-2W divertor electrodes
M. E. Griswold,a) E. M. Granstedt, M. C. Thompson, K. Knapp, B. Koop, and TAE Teamb) TAE Technologies, Inc., Foothill Ranch, California 92610, USA
(Presented 18 April 2018; received 4 May 2018; accepted 11 July 2018; published online 27 July 2018)
A suite of diagnostics was developed to measure particle and heat fluxes arriving at the divertor elec- trodes of the C-2W experiment at TAE Technologies. The divertor electrodes consist of 4 concentric rings, each equipped with a bolometer, electrostatic energy analyzer, and thermocouple mounted at two opposing azimuthal locations. These probes provide measurements of the power flux to the divertor electrodes as well as measurements of the ion current density, ion energy distribution, and total energy deposition. The thermocouples also provide calibration points for inferring the heat deposition profile via thermographic imaging of the electrodes with a fast infrared camera. The combined measurements enable the calculation of the energy lost per escaping electron/ion pair, which is an important met- ric for understanding electron heat transport in the open field lines that surround the field-reversed configuration plasma in C-2W. Published by AIP Publishing. https://doi.org/10.1063/1.5038752
I. INTRODUCTION A. Machine layout
The C-2W experiment1 at TAE Technologies (Fig. 1) consists of a field-reversed configuration (FRC) core that is surrounded by a mirror-confined “scrape off layer” plasma on open field lines (Fig. 2). The open field lines extend past a mir- ror at either end of the central confinement vessel (CV) and expand into the inner and outer divertor vessels. Inside each divertor vessel, the field lines connect to a set of four concen- tric electrode plates (Fig. 3) that can be biased to introduce a radial electric field in the plasma.
C-2W has four divertor vessels instead of two (as on the previous C-2U experiment3) in order to optimize a trade-off related to how the plasma is formed. At the start of a dis- charge, all of the magnetic field lines bypass the inner divertor and connect to electrodes in the outer divertor [Fig. 2(a)]. This configuration provides a clear path of straight field lines for compact toroids (CTs) that are launched from each formation section into the CV where they collide and merge to form the central FRC. However, once the central plasma is formed, the inner divertor is better situated to control the plasma radial electric field because it is adjacent to the CV and is not sepa- rated from it by stray formation gas. Stray gas creates drag on ions via charge exchange, resulting in a radial current that can “short-circuit” the applied field.
C-2W can be operated in two modes. In the first mode, the magnetic configuration shown in Fig. 2(a) is used throughout the entire discharge. In the second mode, which should enable better performance, the configuration shown in Fig. 2(a) is only used at the beginning of the discharge, after which fast mag- netic coils expand most of the magnetic field lines to connect to the electrodes in the inner divertor, as shown in Fig. 2(b).
Note: Paper published as part of the Proceedings of the 22nd Topical Confer- ence on High-Temperature Plasma Diagnostics, San Diego, California, April 2018.
a) Author to whom correspondence should be addressed: mgriswold@tae.com. b)TAE Team members are listed in Nucl. Fusion 57, 116021 (2017).
B. Motivation for the measurement
One of the major goals of the C-2W experiment is to increase the electron temperature at the edge of the FRC by reducing heat transport along the open field lines that surround it. The familiar Spitzer-Hamm parallel conductivity breaks down in the low collisionality regimes that are typical at the edge of mirror and FRC plasmas,4,5 so heat transport on the open field lines in C-2W should instead be governed by the pro- cess that dominates in mirror machine experiments: convective transport through the mirror.6
An ideal magnetic mirror plasma can confine electron thermal energy well6,7 despite the common intuition that open field lines will conduct heat quickly.8 The confinement works through both magnetic and electric fields: electrons initially scatter into the loss cone faster than ions because of their higher collision frequency, but this differential loss rate estab- lishes an ambipolar potential to enforce quasineutrality. This potential modifies the velocity space loss cone to preferen- tially retain electrons and push out ions until electrons are lost through the mirror at the same rate as ions (Fig. 4). In this situation, the thermal energy carried away by the electrons is only (5−6)Te for each electron-ion pair that escapes to the wall.6
In a real device, the ambipolar potential will be reduced by cold electrons generated at the edge of the machine by the ionization of neutral gas and secondary electron emission from the wall.6,7,9 For each cold electron that enters the mir- ror from the edge, a hot electron can escape without affecting the charge balance of the central plasma. A copious source of cold electrons will effectively eliminate the ambipolar poten- tial barrier, allowing many hot electrons to escape for each ion that escapes from the mirror. In this situation, electrons can carry away as much as kTe × √Mi/me ∼ 40 kTe per ion that is lost.
C-2W’s divertors are designed to control cold electrons at the edge of the machine and reduce electron heat transport to levels that are closer to an isolated mirror. The magnetic field in C-2W’s divertors expands by a factor of Bmax /Bmin ∼ 30,
0034-6748/2018/89(10)/10J110/5/$30.00 89, 10J110-1 Published by AIP Publishing.