Fast imaging diagnostics on the C-2U advanced beam-driven field-reversed configuration device

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11D428 (2016)
End loss analyzer system for measurements of plasma flux at the C-2U
divertor electrode
M. E. Griswold,a) S. Korepanov, M. C. Thompson, and TAE Team Tri Alpha Energy, P.O. Box 7010, Rancho Santa Margarita, California 92688, USA
(Presented 6 June 2016; received 5 June 2016; accepted 23 July 2016; published online 17 August 2016)
An end loss analyzer system consisting of electrostatic, gridded retarding-potential analyzers and pyroelectric crystal bolometers was developed to characterize the plasma loss along open field lines to the divertors of C-2U. The system measures the current and energy distribution of escaping ions as well as the total power flux to enable calculation of the energy lost per escaping electron/ion pair. Spe- cial care was taken in the construction of the analyzer elements so that they can be directly mounted to the divertor electrode. An attenuation plate at the entrance to the gridded retarding-potential analyzer reduces plasma density by a factor of 60 to prevent space charge limitations inside the device, without sacrificing its angular acceptance of ions. In addition, all of the electronics for the measurement are isolated from ground so that they can float to the bias potential of the electrode, 2 kV below ground. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4961081]
I. INTRODUCTION
The C-2U experiment at Tri Alpha Energy sustains advanced beam-driven field-reversed configuration (FRC) plasmas.1 The FRC core is surrounded by an open field line “scrape o↵ layer” (SOL) that connects the core plasma to divertor electrodes through a magnetic mirror plug followed by a region of expanding magnetic field known as the “expander” (Fig. 1). An end loss analyzer (ELA) was developed to measure the energy lost per ion that escapes to the electrodes, which is an important metric for understanding electron heat transport on the open field lines.2,3 Results on the gas dynamic trap device indicate that open-field-line heat transport can be suppressed with the proper configuration of field lines in the expander region.4,5 The ELA consists of a pyroelectric crystal bolometer that measures total particle power density as well as a gridded ion energy analyzer (GEA) that measures ion current density and can also measure the ion energy distribution.
The ELA mounts directly to the innermost (smallest radius) of the 4 concentric divertor electrodes on C-2U. The mounting location is just outside the location of a plasma gun (Fig. 2) that is used in conjunction with the divertor electrodes to control the radial electric field in the plasma.1 The plasma gun is biased to 2 kV below machine ground, while each of the divertor electrodes are electrically floating. The outer case of each analyzer as well as its measurement electronics are held at Ud, the potential of the electrode to which they are mounted. Although Ud is floating, it is measured to float near the plasma gun bias potential during experiments due to electrical connection through the plasma.
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)mgriswold@trialphaenergy.com
II. ENERGY ANALYZER
The GEA is based on a design that was used to measure end loss from the Tandem Mirror Experiment (TMX) magnetic mirror experiment.7 It consists of a current collector behind four mesh electrodes that are mounted on stainless steel frames and separated by insulating ceramic washers (Fig. 3). The first electrode (#1) is made from a laser cut stainless steel attenuation grid with circular holes of radius 32 μm evenly spaced every 450 μm so that it has a transparency of 0.016. The grid is only 70 μm thick to prevent excessive collimation from its small holes. The following three electrodes (#2-4) are made of thin electroformed nickel mesh pressed against both sides of a frame. The nickel mesh is 70 μm thick and has 300 μm diameter holes and a transparency of 0.826.
Electrodes #1 and #2 are entrance grids that are both held at Ud. Their role is to establish a uniform potential at the entrance to the analyzer and to attenuate the ion current density so that space charge does not impede ion flow between electrodes. In order to establish an equipotential surface, the radius of mesh holes on the entrance electrode should be smaller than the local Debye length, d, which is expected to be in the range 30-180 μm. Electrode #1 has holes with radius 32 μm which satisfy this condition over most of the expected range, but only marginally at the lower end. Electrode #2 mitigates ion defocusing that might occur when the Debye length condition is marginally satisfied at the first electrode8 by reducing the large electric field that would be present behind electrode #1 if it were followed directly by the ion repelling voltage on electrode #3.
The first two electrodes also attenuate the ion current so that it will not be space charge limited anywhere in the device. The strongest constraint occurs in the gap between the ion and electron repelling electrodes, #3 and #4, respectively. These two electrodes have the largest separation of any in the device (3 mm) to prevent surface breakdown from the large voltage di↵erence applied between them (as much as 2.5 kV).
0034-6748/2016/87(11)/11D428/3/$30.00 87, 11D428-1 Published by AIP Publishing.