Diagnostic suite of the C-2U advanced beam-driven field-reversed configuration plasma experiment

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11D815 (2016) Absolute calibration of neutron detectors on the C-2U advanced
beam-driven FRC
R. M. Magee,a) R. Clary, S. Korepanov, F. Jauregui, I. Allfrey, E. Garate, T. Valentine, and A. Smirnov
Tri Alpha Energy, Inc., Rancho Santa Margarita, California 92688, USA
(Presented 7 June 2016; received 5 June 2016; accepted 13 July 2016; published online 5 August 2016)
In the C-2U fusion energy experiment, high power neutral beam injection creates a large fast ion population that sustains a field-reversed configuration (FRC) plasma. The diagnosis of the fast ion pressure in these high-performance plasmas is therefore critical, and the measurement of the flux of neutrons from the deuterium-deuterium (D-D) fusion reaction is well suited to the task. Here we describe the absolute, in situ calibration of scintillation neutron detectors via two independent methods: firing deuterium beams into a high density gas target and calibration with a 2 ⇥ 107 n/s AmBe source. The practical issues of each method are discussed and the resulting calibration factors are shown to be in good agreement. Finally, the calibration factor is applied to C-2U experimental data where the measured neutron rate is found to exceed the classical expectation. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4960416]
I. INTRODUCTION
Fast ions play a dominant role in the C-2U advanced, beam-driven field-reversed configuration (FRC) plasma.1,2 They sustain and stabilize the plasma3 and suppress broadband magnetic turbulence.4 The fast ion pressure is therefore a quantity of great experimental interest. It has been found by measuring the ion, electron, and magnetic pressures and solving the radial pressure balance equation. Measurements of the flux of fusion products o↵er a means to calculate the fast ion pressure directly, without the need to solve the pressure balance equation. This method requires an accurate calibration and motivates the present work.
The calibration of fusion neutron detectors with radio- active sources has been well established in the tokamak community for nearly three decades.5–7 Typically, a compact source is moved throughout the confinement vessel volume. Complete coverage is necessary because in a toroidal device, neutrons emitted from the far side of the machine are scattered as they travel through the center stack to the detector. Numerical calculations of neutron propagation8 have been used to calculate the scattering profile with some success, but proper treatment requires detailed modeling of the surrounding structure and computationally intensive codes (e.g., the Monte Carlo N-Particle (MCNP) code9).
The C-2U device is a linear machine, so (other than the vessel wall) there is no solid material between the neutron emitting plasma and the detector. As a result, the signal is a straightforward geometrical calculation and scattering is unimportant. With the requirement to move a point source throughout the confinement vessel volume thus relaxed, we put
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: rmagee@trialphaenergy.com
forward an alternate calibration technique, one that involves no radiological licensing, hazardous material handling protocols, or machine venting: firing high power deuterium neutral beams (NBs) into a high density gas target. The veracity of this simple, straightforward technique is confirmed by comparing to the data from a neutron source calibration.
In Section II, we describe the neutron detectors them- selves. In Sections III and IV, we describe the methodologies of the two calibration methods (beams-in-gas and neutron source, respectively). In Section V, we apply the obtained calibration factor to C-2U experimental data and point to the future work.
II. NEUTRON DETECTORS
Two scintillation neutron detectors are deployed on C- 2U.10 Each is composed of a large volume (230 and 1200 cm3) plastic scintillator (Eljen 200) coupled to a fast-response photomultiplier tube (PMT). The larger volume scintillator is coupled to a Hamamatsu H6614-70 PMT that is capable of operating in a high magnetic field. The smaller volume scintillator is coupled to a Hamamatsu R1104 PMT which must be placed outside of the strong field region. This restricts the scintillator to have a relatively small footprint (3 in. diameter). A light pipe conducts the light from the scintillator to the PMT. The first detector is located at the north end of the vessel and the second in the south. Both calibrations are performed with the detectors in situ.
III. BEAM IN GAS METHOD
The first calibration method is to fire full power neutral deuterium NBs into a high density deuterium gas target. There are 6 NBs, each capable of producing about 100 A of neutral deuterium current at 15 kV (or 150 A of neutral hydrogen
0034-6748/2016/87(11)/11D815/3/$30.00 87, 11D815-1 Published by AIP Publishing.