A.E. Shevelev

Energy resolution of gamma-ray spectroscopy of JET plasmas with a LaBr3 scintillator detector and digital data acquisition.

A new high efficiency, high resolution, fast γ-ray spectrometer was recently installed at the JET tokamak. The spectrometer is based on a LaBr3(Ce) scintillator coupled to a photomultiplier tube. A digital data acquisition system is used to allow spectrometry with event rates in excess of 1 MHz expected in future JET DT plasmas. However, at the lower rates typical of present day experiments, digitization can degrade the energy resolution of the system, depending on the algorithms used for extracting pulse height information from the digitized pulses. In this paper, the digital and analog spectrometry methods were compared for different experimental conditions. An algorithm based on pulse shape fitting was developed, providing energy resolution equivalent to the traditional analog spectrometry method.

Gamma ray spectroscopy at high energy and high time resolution at JET

In fusion plasmas gamma ray emission is caused by reactions of fast particles, such as fusion alpha particles, with impurities. Gamma ray spectroscopy at JET has provided valuable diagnostic information on fast fuel as well as fusion product ions. Improvements of these measurements are needed to fully exploit the flux increase provided by future high power experiments at JET and ITER. Limiting aspects are, for instance, the count rate capability due to a high neutron/gamma background combined with slow detector response and a modest energy resolution due to the low light yield of the scintillators. This paper describes the solutions developed for achieving higher energy resolution, signal to background, and time resolution. The detector design is described based on the new BrLa3 scintillator crystal. The paper will focus on hardware development, including a photomultiplier tube capable of stable operation at counting rate as high as 1 MHz, the magnetic shielding, and the fast digital data acquisition system.

Development of gamma-ray diagnostics for ITER

Gamma-ray spectrometry is a diagnostic tool for fast ions in large tokamak plasmas. The information provided allows us to identify and simultaneously distinguish the presence of fast α-particles and other ions (H, D, T, 3He) to obtain information on their energy distribution and relative densities, in addition to performing a tomographic radial profile reconstruction of the γ-emission sources. The lack of vertical diagnostic ports in ITER makes the implementation of tomographic neutron and γ-ray systems more complicated. The use of a vertical divertor port for the implementation of a vertical viewpoint is currently suggested. The strong magnetic field (~2 T) found there makes it hard to use conventional multi-dynode photomultipliers as light detectors. Instead, the use of micro-channel plate photomultipliers is suggested here. Investigations of the magnetic field impact on the performance of the γ-spectrometer with a micro-channel photomultiplier are carried out. A high-speed pulse height analysis technique, which allows us to trace gain changes in the photomultiplier tube, is developed at the Ioffe Institute. The tests demonstrate the feasibility of γ-spectrometric measurements in the divertor port provided that micro-channel photomultiplier detectors and the developed high-speed technique are used.

Reconstruction of distribution functions of fast ions and runaway electrons in fusion plasmas using gamma-ray spectrometry with applications to ITER

Gamma-ray spectrometry on ITER can provide information both on confined fusion alpha particles for optimization of plasma heating and runaway electrons, which is important for safe reactor operations. For the purpose of deconvolution of gamma-ray spectra recorded in fusion plasma experiments the DeGaSum code has been developed. The code can be applied for processing of both spectra of monoenergetic gamma rays, which are born in nuclear reactions produced by alpha particles and other fast ions, and continuous bremsstrahlung spectra generated by runaway electrons in the MeV range in the plasma and reactor structure materials. Gamma-ray spectrometer response functions and bremsstrahlung spectra generated by electrons in the MeV energy range are calculated and used in the DeGaSum code. The deconvolution of the discrete spectra allows the identification of nuclear reactions, which give rise to gamma rays, and the calculation of their intensities. By applying the code for continuous hard x-ray spectra, the runaway electron energy distribution can be inferred. It can provide the maximal energy of runaway electrons with accuracy, which satisfies the ITER project requirements. The code has been used for processing of spectra recorded in JET experiments. An application of the deconvolution technique for gamma-ray emission measurements on ITER is discussed.

Overview of results obtained at the Globus-M spherical tokamak

Experiments and simulations to achieve high values of plasma parameters at the Globus-M spherical tokamak (ST) at moderate absolute auxiliary heating power (up to 0.8 MW) and high specific heating power (up to 2–3 MW m−3) are described. Important distinguishing features are the low edge safety factor range, which is unusual for STs, 2.7 < q < 5 and small plasma–outer wall space (3–5 cm). High ion heating efficiency with neutral beam injection (NBI) was demonstrated. Results of numerical simulation of fast ion trajectories are described and fast ion generation during the NBI and ion cyclotron resonance heating is discussed. Investigations on their confinement and slowing down are also presented. Reasons for achievement of high IC heating efficiency are outlined. Reliable H-mode regime achievement is described. Transport ASTRA modelling demonstrated that during NB heated H-mode ion heat diffusivity remains neoclassical and the particle diffusion coefficient inside transport barrier decreases significantly. Analysis was performed of divertor tile and special probe surfaces after irradiation by plasma during a large number of shots (3000–10 000 shots). Mixed layer composition is measured and deuterium retention in different tokamak first wall areas is estimated. Plasma jet injection experiments with upgraded plasma jet are described. Jet penetration to the plasma centre with immediate increase of density and temperature drop is proved and analogy with pellet injection is outlined.

Investigation of beam– and wave–plasma interactions in spherical tokamak Globus-M

The experimental and theoretical results obtained in the last two years on the interaction of neutral particle beams and high-frequency waves with a plasma using the spherical tokamak Globus-M are discussed. The experiments on the injection of low-energy proton beam of ~300 eV directed particle energy are performed with a plasma gun that produces a hydrogen plasma jet of density up to 3 × 1022 m−3 and a high velocity up to 250 km s−1. A moderate density rise (up to 30%) is achieved in the central plasma region without plasma disruption. Experiments on high-energy (up to 30 keV) neutral beam injection into the D-plasma are analysed. Modelling results on confinement of fast particles inside the plasma column that follows the neutral beam injection are discussed. The influence of the magnetic field on the fast particle losses is argued. A neutral beam injection regime with primary ion heating is obtained and discussed. The new regime with fast current ramp-up and early neutral beam injection shows electron temperature rise and formation of broad Te profiles until the q = 1 flux surface enters the plasma column. An energetic particle mode in the range of frequencies 5–30 kHz and toroidal Alfven eigenmodes in the range 50–300 kHz are recorded in that regime simultaneously with the Te rise. The energetic particle mode and toroidal Alfven eigenmodes behaviour are discussed. The toroidal Alfven eigenmode spectrum appears in Globus-M as a narrow band corresponding to n = 1. The first experimental results on plasma start-up and noninductive current drive generation are presented. The experiments are carried out with antennae providing mostly poloidal slowing down of waves with a frequency of 920 MHz, which is higher than a lower hybrid one existing under the experimental conditions. The high current drive efficiency is shown to be high (of about 0.25 A W−1), and its mechanism is proposed. Some near future plans of the experiments are also discussed.

Application of deconvolution methods to gamma-radiation spectra of thermonuclear plasma

The possibility of application of deconvolution methods to solving the problem of reconstruction of gamma-radiation spectra of thermonuclear plasma is demonstrated. The DEGAS code is created, which implements the modified algorithms of deconvolution. The results of the code application to the test spectra, the discrete spectra of radiation sources, and the gamma spectra measured during the experiments at the JET tokamak using methods for additional plasma heating are presented. The possibility of reconstruction of spectra with low statistics is shown.

Alfvén oscillations in ohmic discharges with runaway electrons in the TUMAN-3M tokamak

Studying the mechanism of Alfvén wave generation in plasma is important, since the interaction of these waves with energetic particles in tokamak-type reactors can increase the losses of energy and particles with the corresponding decrease in the efficiency of plasma heating and, under certain conditions, lead to the damage of structural elements of the system. Despite the previous detailed investigations of the excitation of Alfvén waves by superthermal particles in regimes with additional heating, the physics of Alfvén mode generation in discharges with ohmic heating of plasma is still not sufficiently studied. We have established that a significant factor inf luencing the development of Alfvén oscillations in ohmic discharge is the presence of runaway electrons. A physical mechanism explaining this relationship is proposed.

Noninductive plasma generation and current drive in the Globus-M spherical tokamak

Experimental results on the generation and maintenance of the toroidal current in the Globus-M spherical tokamak by using waves in the lower hybrid frequency range without applying an inductive vortex electric field are presented. For this purpose, the original ridge guide antennas forming a field distribution similar to that produced by multiwaveguide grills were used. The high-frequency field (900 MHz) was used for both plasma generation and current drive. The magnitude of the generated current reached 21 kA, and its direction depended on the direction of the vertical magnetic field. Analysis of the experimental results indicates that the major fraction of the current is carried by the suprathermal electron beam.