Hanle effect as a magnetic diagnostics for field-reversed configuration plasmas

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11E703 (2016)
A mass resolved, high resolution neutral particle analyzer for C-2U
R. Clary,1,a) A. Roquemore,2 J. Douglass,1 D. Jaramillo,1 S. Korepanov,1 R. Magee,1
S. Medley,2 and A. Smirnov1
1Tri Alpha Energy, Inc., Rancho Santa Margarita, California 92688, USA 2Princeton Plasma Physics Laboratory, Princeton, New Jersey 08540, USA
(Presented 9 June 2016; received 2 June 2016; accepted 21 June 2016; published online 19 July 2016)
C-2U is a high-confinement, advanced beam driven field-reversed configuration plasma experiment which sustains the configuration for >5 ms, in excess of typical MHD and fast particle instability times, as well as fast particle slowing down times. Fast particle dynamics are critical to C-2U performance and several diagnostics have been deployed to characterize the fast particle population, including neutron and proton detectors. To increase our understanding of fast particle behavior and supplement existing diagnostics, an EkB neutral particle analyzer was installed, which simultaneously measures H0 and D0 flux with large dynamic range and high energy resolution. Here we report the commissioning of the EkB analyzer, confirm the instrument has energy resolution E/E . 0.1 and a dynamic range Emax/Emin ⇠ 30, and present measurements of initial testing on C-2U. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4958911]
I. INTRODUCTION
The field-reversed configuration (FRC) experiment at Tri Alpha Energy (TAE) extends the confinement and sta- bility of a deuterium FRC with the use of hydrogen fueled neutral beam injection (NBI).1–4 Up to six neutral beams (E = 15 keV, 150 A, each) are used to inject fast-ions confined in betatron (axis-encircling) orbits. Fast-ions in FRCs are highly non-local since the betatron orbits extend from in- side the field null to outside the separatrix, sampling large changes in neutral density, plasma density, and magnetic field. This results in rich fast-ion dynamics and motivates a spatially and temporally resolved study of fast-ion trans- port, particularly charge-exchange losses. In addition, recent fusion product measurements suggest the existence of super- thermal deuterons in C-2U discharges. One technique we use to investigate confinement properties is deuterium doped neutral beam injection. Thus we are motivated to infer fast ion dynamics with high energy resolution and large dynamic range while simultaneously distinguishing proton and deuteron isotopes.
For this purpose we acquired a neutral particle analyzer (NPA)5 from Princeton Plasma Physics Laboratory with en- ergy resolution E/E . 0.1 and dynamic range Emax/Emin ⇠ 30. In Secs. II–VI we briefly describe its operating principles and how the device was refurbished and calibrated for our uses. We present test measurements of charge- exchange losses from NBI into static, gas-filled C-2U confine- ment chamber and we discuss future developments for the instrument.
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)Electronic mail: rclary@trialphaenergy.com
II. OPERATING PRINCIPLES
The NPA operates in an EkB configuration, permitting simultaneous energy and mass discrimination.6 High energy neutrals are ionized in a helium gas stripping cell and then dispersed according to mass and energy in the field region of the device. The particles are then detected by a system of micro-channel plates (MCPs). Figure 1 Shows the NPA and its control system on C-2U.
Helium gas is bled into the stripping cell at a constant flow rate, maintaining 2 mTorr in the cell. The stripping cell can be operated at steady state without a↵ecting vacuum conditions in the NPA, however, it is gated for 2 min, inclusive of the exper- iment discharge, to avoid He leakage into the C-2U vessel.
Ionized particles are dispersed by the magnetic and elec- tric fields in the y (?B) and x (kB) directions, respectively as
p
y'2⇢=2 2mEand (1)
qB
x ' 2 B2 q , (2)
⇡2 E m
where ⇢ is the Larmour radius, E is the particle energy, E is the electric field, and B is the magnetic field. Thus particles disperse in y with the root of the energy and in x with mass. Note that mass dispersion is independent of particle energy, so particles with the same mass-to-charge ratio are displaced the same.
The MCP system is comprised of three sections along the y-axis, each containing one pair of stacked MCPs to create a dual-gain-stage system. Each section is independently biased and contains 26 anodes, uniformly arranged in two rows of 13. The signal from each anode (channel) is pulled directly into the data acquisition system attached to the MCP vacuum
feedthrough flange.
0034-6748/2016/87(11)/11E703/3/$30.00 87, 11E703-1 Published by AIP Publishing.