Modeling of bremsstrahlung emission from the confined runaway electrons and applications to the hard x-ray monitor of ITER

Abstract

The importance of runaway electron (RE) detection, analysis of its parameters and suppression or mitigation is well recognized for large size tokamaks such as ITER. One of the wellestablished detection techniques is hard x-ray spectrometry that detects bremsstrahlung emission typically in the MeV range from the REs. It provides space, time and energy resolved measurements, which can also be utilized for the reconstruction of the RE energy distribution function. In this paper, forward modeling has been carried out for the detection of the confined REs and a numerical tool is developed. It calculates analytically anisotropic bremsstrahlung emissivity at each spatial position in the plasma in terms of several plasma, RE and geometrical parameters. The simulation provides line integrated energy resolved spectra of bremsstrahlung photons. The expected bremsstrahlung emission signal during plasma disruptions scenario as measured with the ITER hard x-ray monitor has been simulated for the first time aiming on optimizing the design parameters of this diagnostic. The possible dynamic range for the detection of confined REs is studied as well. The effect of the shape of the runaway distribution function in the momentum space on the observed diagnostic signal is also studied and briefly discussed.

Physics of runaway electrons in tokamaks

“…The main diagnostic for RE detection in ITER is the hard x-ray monitor (HXRM) [284]. It will be available from first plasma until the end of the non-nuclear phase of ITER operation after which it will lose its function due to the high neutron flux.…”

Study of runaway electron dynamics at the ASDEX Upgrade tokamak during impurity injection using fast hard x-ray spectrometry

“…At the DIII-D tokamak, to study the spatial and energy distribution of REs at the current plateau stage, a multidetector array was developed that tangentially views a poloidal cross-section of the tokamak chamber [26]. At the JET tokamak, spectrometric HXR measurements were carried out both at the stage of current ramp-up and plateau [27][28][29][30] and in massive gas injection (MGI) regimes at the current quench phase [3]. In the case of ITER tokamak, a hard x-ray monitor is being designed [31] to detect confined REs in the first plasma scenarios, and radial gamma-ray spectrometers are being designed [17] to detect REs in a subsequent phase of the ITER plasma scenarios.…”

Conceptual design of multichannel fast electron bremsstrahlung detection system to study fast electron dynamics during lower hybrid current drive in ADITYA-U tokamak

“…A forward modeling code reported in references [28,29] To optimize the collimation geometry, pinhole size has been varied over a wide range from 1 mm to 10 mm, and counts per second are estimated at the detector location. In figure 8, along the 𝑥-axis, collimator size is plotted and corresponding estimated counts per second (cps) are shown along the 𝑦 axis.…”

“…The quantity,(𝑑𝑁 (𝐸))/𝑑𝐸 𝑑𝑡, represents the number of hard x-ray counts in the energy interval, 𝐸 to 𝐸 + 𝑑𝐸 per unit time, 𝑑𝑡, entering into the detector volume for a single line of sight passing through the plasma volume is calculated by equation (3.5) [28].…”

“…To estimate the FEB emission level at the detector end and optimize the collimation geometry a synthetic diagnostic module has been utilized which is a part of the PREDICT code [29]. The module is essentially developed for the design of ITER Hard X-Ray monitor/spectrometer for Runaway Electron detection and for signal estimation at the detector location [28,29].…”