Overview of JET results for optimising ITER operation

The JET 2019–2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major neutral beam injection upgrade providing record power in 2019–2020, and tested the technical and procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle (α) physics in the coming D–T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed shattered pellet injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design and operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D–T benefited from the highest D–D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER

Neutral pathways and heat flux widths in vertical- and horizontal-target EDGE2D-EIRENE simulations of JET

This paper further analyses the EDGE2D-EIRENE simulations presented by Chankin et al (2017 Nucl. Mater. Energy 12 273), of L-mode JET plasmas in vertical-vertical (VV) and Vertical-horizontal (VH) divertor configurations. As expected, the simulated outer divertor ionisation source peaks near the separatrix in VV and radially further out in VH. We identify the reflections of recycled neutrals from lower divertor tiles as the primary mechanism by which ionisation is concentrated on the outer divertor separatrix in the VV configuration. These lower tile reflection pathways (of neutrals from the outer divertor, and to an even greater extent from the inner divertor) dominate the outer divertor separatrix ionisation. In contrast, the lower-tile-reflection pathways are much weaker in the VH simulation and its outer divertor ionisation is dominated by neutrals which do not reflect from any surfaces. Interestingly, these differences in neutral pathways give rise to strong differences in the heat flux density width λq at the outer divertor entrance: λq = 3.2 mm in VH compared to λq = 11.8 mm in VV. In VH, a narrow channel exists in the near scrape-off-layer (SOL) where the convected heat flux, driven by strong Er × B flow and thermoelectric current, dominates over the conducted heat flux. The width of this channel sets λq and is determined by the radial distance between the separatrix and the ionisation peak in the outer divertor

Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas

The low temperature boundary layer plasma (Scrape-Off-Layer or SOL) between the hot core and the surrounding vessel determines the level of power-loading, erosion and implantation of material surfaces, and thus the viability of tokamak-based fusion as an energy source. This study explores mechanisms affecting the formation of flattened density profiles, so-called ‘density shoulders’, in the low-field side (LFS) SOL, which modify ion and neutral fluxes to surfaces – and subsequent erosion. There is evidence against local enhancement of ionization inducing shoulder formation. We find that increases in SOL parallel resistivity, Λdiv (=[L||νei Ωi ]/cs Ωe), postulated to lead to shoulder growth through changes in SOL turbulence characteristics, correlates with increases in upstream SOL shoulder amplitude, As only under a subset of conditions (D2-fuelled L-mode density scans with outer strike point on the horizontal target). Λdiv fails to correlate with As for cases of N2 seeding or during sweeping of the strike point across the horizontal target. The limited correlation of Λdiv with As was also found for H-mode discharges. Thus, while Λdiv above a threshold of ~1 may be necessary for shoulder formation and/or growth, another shoulder mechanism is required. More significantly we find that in contrast to parallel resistivity, outer divertor recycling as quantified by the total outer divertor Balmer Dα emission, I-Dα, does scale with shoulder amplitude where Λdiv does and even where Λdiv fails. Divertor recycling could lead to SOL density shoulder formation through: a) reducing the parallel to the field flow (loss) of ions out of the SOL to the divertor; and b) changes in radial electric fields which lead to ExB poloidal flows as well as potentially affecting the SOL turbulence birth characteristics. Thus changes in divertor recycling may be the sole process in bringing about SOL density shoulders or in tandem with parallel resistivity

Observations and modelling of ion cyclotron emission observed in JET plasmas using a sub-harmonic arc detection system during ion cyclotron resonance heating

Peer reviewe

Study of instabilities in tokamak plasmas using radiation diagnostics

Title: Study of instabilities in tokamak plasmas using radiation diagnostics Author: Martin Imríšek Department: Department of Surface and Plasma Science Supervisor: Prof. RNDr. Milan Tichý, DrSc. Consultants: Mgr. Vladimír Weinzettl, PhD. RNDr. Jan Mlynář, PhD. Abstract: The thesis focuses on the characterisation of sawtooth instability and its effect on selected edge plasma processes at COMPASS. It is shown that saw- tooth crash precedes the vast majority of L-H transitions, lower occurrence of ELMs and transitions to ELM-free H-mode. The sawtooth instability also af- fects H-L transitions, as their increased occurrence is observed in the middle of the sawtooth cycle. In the thesis, optimised tomography of the electromagnetic emission at COMPASS and JET is also applied to study instabilities in tokamak plasmas. The performance of fast tomographic methods with potential for real- time plasma control purposes is also studied. As the magnetic field configuration plays a vital role in plasma stability and tomographic reconstructions, the thesis also includes simulations of magnetic fields. These simulations were applied to optimise the design of coils for COMPASS-U. Keywords: tokamak, tomography, sawtooth instability, electromagnetic emission ii

Observation of Sawtooth Oscillations in the COMPASS Tokamak

The sawtooth instability in tokamak plasmas results in periodic relaxations of the core plasma density and temperature. The physics of sawtooth is still not fully understood. It is predicted that fusion-born alpha particles will lead to long sawteeth. However, longer sawteeth can seed other instabilities which cause further degradation of plasma confinement. This paper provides brief introduction into sawtooth physics and observations of sawtooth instability in COMPASS. Furthermore, evidence of triggering transition to high confinement regime by sawtooth crash is presente

Studium nestabilit tokamakového plazmatu pomocí radiačních diagnostik

Title: Study of instabilities in tokamak plasmas using radiation diagnostics Author: Martin Imríšek Department: Department of Surface and Plasma Science Supervisor: Prof. RNDr. Milan Tichý, DrSc. Consultants: Mgr. Vladimír Weinzettl, PhD. RNDr. Jan Mlynář, PhD. Abstract: The thesis focuses on the characterisation of sawtooth instability and its effect on selected edge plasma processes at COMPASS. It is shown that saw- tooth crash precedes the vast majority of L-H transitions, lower occurrence of ELMs and transitions to ELM-free H-mode. The sawtooth instability also af- fects H-L transitions, as their increased occurrence is observed in the middle of the sawtooth cycle. In the thesis, optimised tomography of the electromagnetic emission at COMPASS and JET is also applied to study instabilities in tokamak plasmas. The performance of fast tomographic methods with potential for real- time plasma control purposes is also studied. As the magnetic field configuration plays a vital role in plasma stability and tomographic reconstructions, the thesis also includes simulations of magnetic fields. These simulations were applied to optimise the design of coils for COMPASS-U. Keywords: tokamak, tomography, sawtooth instability, electromagnetic emission iiiNázev: Study of instabilities in tokamak plasmas using radiation diagnostics Autor: Martin Imríšek Katedra: Katedra fyziky povrchů a plazmatu Vedoucí: Prof. RNDr. Milan Tichý, DrSc. Konzultanti: Mgr. Vladimír Weinzettl, PhD. RNDr. Jan Mlynář, PhD. Abstract: Práce se zaměřuje na charakteriyaci pilovou nestabilitz a jejího vlivu na vybrané plasmové procesy na tokamaku COMPASS. Pád pilové nestability na tokamaku COMPASS předchází dominantní většíně L-H přechodů, sníženému výskutu nestability ELM a přechodům do H-modu v režimu bez nestabilitz ELM. Pilová nestabilita má také vliv na H-L přechod, který nastváv se yvýšenou pravděpodobností zhruba uprostřed jejího cyklu. Práce se také věnuje tomo- grafické rekonstrukci elektromagnetického zaření na tokamaku COMPASS a JET včetně aplikace optimalizovaných algoritmů ke studiu nestabilit v tokamakovém plasmatu a testování rychlých tomografických metod s možným využitím k řízení plasmatu. Práce také obsahuje simulace magnetických polí, které hrají důležitou roli ve stabilitě plasmatu a ve výpočtu tomografických rekonstrukcí. Tyto simu- lace byly také využity pro optimalizaci designu cívek tokamaku COMPASS-U. Klíčová slova: tokamak, tomografie, pilová nestabilita, electromagnetické...Matematicko-fyzikální fakultaFaculty of Mathematics and Physic

Calculation of edge ion temperature and poloidal rotation velocity from carbon III triplet measurements on the COMPASS tokamak

A high-resolution spectroscopic system for the measurements of the CIII triplet at 465 nm was installed at the COMPASS tokamak. The Doppler broadening and shift of the measured spectral lines are used to calculate the edge ion temperature and poloidal plasma rotation. At first, the spectroscopic system based on two-grating spectrometer and the calibration procedure is described. The signal processing including detection and removal of spiky features in the signal caused by hard X-rays based on the difference in the behaviour of Savitzky-Golay and median filters is explained. The detection and position estimation of individual spectral lines based on the continuous wavelet transform is shown. The method of fitting of Gaussians using the orthogonal distance regression and estimation of the error of estimation of the rotation velocity and ion temperature is described. At the end, conclusions about the performance of the spectroscopic system and its shortcomings based on summary of results calculated from 2033 processed spectral lines measured in 61 shots are drawn and the possible enhancements are suggested

First dedicated observations of runaway electrons in the COMPASS tokamak

Runaway electrons present an important part of the present efforts in nuclear fusion research with respect to the potential damage of the in-vessel components. The COMPASS tokamak a suitable tool for the studies of runaway electrons, due to its relatively low vacuum safety constraints, high experimental flexibility and the possibility of reaching the H-mode D-shaped plasmas. In this work, results from the first experimental COMPASS campaign dedicated to runaway electrons are presented and discussed in preliminary way. In particular, the first observation of synchrotron radiation and rather interesting raw magnetic data are shown