Measuring dynamic fast ion spatial profiles with fusion protons in the Madison Symmetric Torus

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10I104 (2018) Measuring dynamic fast ion spatial profiles with fusion protons
in the Madison Symmetric Torus
R. M. Magee,1,a) J. K. Anderson,2 S. Korepanov,1 L. Frausto,1 J. Boguski,2 P. J. Bonofiglo,2 J. Kim,2 and R. McConnell2
1TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, California 92610, USA 2University of Wisconsin–Madison, 1150 University Ave., Madison, Wisconsin 53706, USA
(Presented 18 April 2018; received 23 April 2018; accepted 27 May 2018; published online 17 July 2018)
Neutral beam injected fast ions play a dominant role in both the field reversed configuration (FRC) at TAE Technologies and the Madison Symmetric Torus (MST) reversed field pinch (RFP), making fast ion diagnosis a major pillar of both research programs. And as strongly self-organized plasmas, the FRC and RFP similarly exhibit dynamic relaxation events which can redistribute fast ions. Recently, a collaboration between TAE Technologies and the University of Wisconsin was conducted to develop a method for measuring a fast changing fast ion spatial profile with a fusion proton detector and to investigate commonalities between the two plasmas. The steerable detector was designed and built at TAE and installed on MST. The fusion proton emission profile resulting from injection of a 25 kV deuterium neutral beam is measured with better than 5 cm spatial resolution and 100 μs temporal reso- lution over the course of several 10s of shots. The fast ion density profile, forward modeled by tracing the orbits of the 3 MeV protons through a reconstructed magnetic equilibrium, is observed to flatten during global magnetic tearing mode activity, dropping by 30% in the core and increasing by a similar amount at the edge. The equilibrium profile is observed to be consistent with measurements made with a collimated neutron detector. Published by AIP Publishing. https://doi.org/10.1063/1.5037349
I. INTRODUCTION
Minority populations of suprathermal ions, whether born from neutral beam injection (NBI), RF heating, or processes natural to the plasma (e.g., magnetic reconnection), often play an outsized role in plasma dynamics, so accurate diagnosis is critical.
Measurements of the flux of fusion products are ideal for fast ion diagnosis for a number of reasons: the sensitivity of the cross section to energy ensures that the signal is dominated by the highest energy ions; the emission does not require a high density of neutrals, so conditions in the hot core can be probed absent a neutral beam target; the particles are not confined by the magnetic field due to their high energy or, for the case of neutrons, lack of charge; and the detectors are passive and non-perturbative.
On the TAE field reversed configurations (FRCs) C-2U1 and C-2W,2 neutron and proton detectors are used in a comple- mentary fashion. Calibrated, scintillator-based neutron detec- tors3 operated in current mode provide high time resolution, volume integrated measurements of the neutron flux from deuterium-deuterium fusion during deuterium NBI. The rela- tively low neutron flux (107 s−1 cm2) precludes collimation, as has been done on other machines, so multiple, single-chord fusion proton detectors,4 operated in pulse-counting mode, are used to reconstruct the axial emission profile, as has been done in previous studies.5,6
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)Electronic mail: RMagee@TAE.com
The focus of the present work is on the development of a steerable proton detector, capable of resolving the dynamic radial profile of the fusion proton emission from a single port. MST7 provides an ideal test bed for this diagnostic devel- opment: highly reproducible, fusion-grade plasmas at a high repetition rate (>100 shots/day) and a high flux of NBI-induced deuterium fusion products (1010 s−1 cm2; MST NBs operate at 25 keV, whereas C-2U/W NBs operate at 15 keV). Addition- ally, MST plasmas, like C-2U/W plasmas, exhibit fast, discrete relaxation events which redistribute fast ions, allowing us to simulate the FRC measurement in the RFP. Furthermore, both plasmas exhibit Alfve´n and energetic particle mode activity, opening the possibility to studies of physics commonalities between the two devices.
II. HARDWARE A. Detector
The detector is a large area (50 cm2), partially depleted Passivated Implanted Planar Silicon (PIPS) detector from Can- berra Industries, Inc.8 The detector is negatively biased with 60-90 V, and the signal is amplified with a transimpedance amplifier with a gain of 106 V/A. Typical amplified signal pulses have a FWHM of 200 ns and an amplitude of 1 V. The signal is digitized at 60 MS/s. All pulse counting and pulse shape analysis is done digitally. The details of the electrical circuit are described in an earlier publication.4
B. Collimators
Detachable collimating heads are constructed by bundling small-diameter, seamless, smooth-bore stainless steel tubing in
0034-6748/2018/89(10)/10I104/4/$30.00 89, 10I104-1 Published by AIP Publishing.