Jet Efda contributors

Studies of burning plasma physics in the Joint European Torus

In burning plasma experiments, the very energetic alpha (α) particles resulting from a deuterium–tritium fusion reaction will be the dominant heating mechanism and will give rise to new physics issues. Recent experiments performed on the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] and aiming to investigate burning plasma physics are reported in this paper. In the presence of very energetic particles, the magnetohydrodynamic stability of plasmas is affected. Sawteeth will be strongly stabilized and may lead to the onset of neoclassical tearing modes (NTMs), which are damaging for the plasma confinement. 4He ions injected at 120 keV by the neutral beam injection system and accelerated by ion cyclotron resonance frequency (ICRF) waves to the MeV energy range have provided the necessary energetic particles to investigate these effects. New scenarios have been used in order to control the stability of the sawteeth even in the presence of fast particles and to prevent or del...

Physics of ECE Temperature Measurements and Prospects for ITER

The physics of the electron cyclotron emission (ECE) temperature measurements is reviewed. The current understanding of the expected ECE spectra in ITER is summarized, for perpendicular as well as oblique propagation. The relevance of the use of oblique ECE for investigating the shape of the electron distribution function at low energies is discussed.

Coupling Of The JET ICRF Antennas In ELMy H‐mode Plasmas With ITER Relevant Plasma—Straps Distance

In ITER, the requirement for the ICRF antenna is to deliver 20 MW in ELMy H‐mode plasmas with an averaged antenna—plasma separatrix distance of 14 cm [1]. Two major problems will have to be solved: the very fast change in antenna loading during ELMs and the decrease of the loading when the plasma is pushed far away from the antenna. JET has the capability to combine these conditions and for the first time, experiments were performed in ELMy H‐mode at antenna—separatrix distance, referred as ROG, varied from 10 to 14 cm. When ROG was increased, the perturbation caused by ELMs was found to decrease significantly and the loading between ELMs was found to deteriorate to very low values. In order to compensate the latter unwanted effect, different levels of deuterium gas were injected in the edge either from the divertor, the midplane or the top of the tokamak. Using this technique, the loading was increased by up to a factor 6 and up to 8 MW of ICRF power were coupled.

Statistical comparison of ICRF and NBI heating performance in JET-ILW L-mode plasmas

After the change over from the C-wall to the ITER-like Be/W wall (ILW) in JET, the radiation losses during ICRF heating have increased and are now substantially larger than those observed with NBI at the same power levels, in spite of the similar global plasma energies reached with the two heating systems. A comparison of the NBI and ICRF performances in the JET-ILW experiments, based on a statistical analysis of ∼3000 L-mode discharges, will be presented.

RECENT RESULTS ON THE DISCREPANCY BETWEEN ELECTRON TEMPERATURE MEASUREMENTS IN HIGH TEMPERATURE PLASMAS IN JET

The long standing disagreement between the central electron temperature measured by Thomson scattering and electron cyclotron emission (2nd harmonic, X-mode) in high performance plasmas observed in ...

D majority heating in JET plasmas: ICRH modelling and experimental RF deposition

Recent experiments in JET have provided information on the potential of using majority RF heating schemes in large plasmas. Adopting a wide range of available diagnostics, the plasma behaviour was monitored. The main results of the experiments are that—due to the poor antenna coupling at low frequency, the low (Ohmic) plasma temperature and the reduced RF electric field amplitude near the ion‐cyclotron resonance layer of the majority ions—ICRH alone is barely capable of heating the plasma. On the other hand, when preheating the plasma using neutral beam injection, the wave‐plasma coupling is noticeably improved and considerable plasma heating, followed by increased neutron yield were observed in several diagnostics. This effect is not only attributed to the lower collisionality of the pre‐heated plasma but also to the Doppler‐shifted IC absorption of the fast beam ions. By studying the response of the plasma to sudden changes in the RF power level, the experimental power deposition profiles were determine...

Recent experiments on alternative dipole phasing with the JET A2 ICRF antennas

Using the JET A2 ICRF antennas [1], experiments were carried out to assess the performance of three different dipole phasing configurations relevant for the operation of the ICRF antenna in ITER. Three similar discharges with dipole (0π0π), “symmetric dipole” (0ππ0) and “super dipole” (00ππ) phasings were compared. In the “super dipole” case, higher coupling was confirmed but lower heating efficiency and much stronger plasma wall interaction were observed, as corroborated, respectively, by the analysis of the diamagnetic energy response to the ICRF power steps and by the observation of a considerable temperature rise of the antenna limiters and septa in the 00ππ case. These observations were found to be in line with simulations of the ICRF wave absorption and with High Frequency Structure Simulator (HFSS) modelling of the RF fields near the antenna [2].

Operational Experience with the Scattering Matrix Arc Detection System on the JET ITER‐Like Antenna

The Scattering Matrix Arc Detection System (SMAD) has been fully deployed on all 4 sets of Resonant Double Loop (RDL), Vacuum Transmission Line (VTL) and Antenna Pressurised Transmission Lines (APTL) of the JET ICRF ITER‐Like Antenna (ILA) and this has been indispensable for operating at low (real) T‐point impedance values to investigate ELM tolerance. This paper describes the necessity of the SMAD vs VSWR (Voltage Standing Wave Ratio) protection system, SMAD commissioning, problems and a number of typical events detected by the SMAD system during operation on plasma.

Simulating the JET ITER-like Antenna circuit

A set of simulation/interpretation tools based on transmission line theory and on the RF model developed by M. Vrancken [2] has been developed to study the ITER‐like Antenna (ILA) at JET. For given tuning element settings, the unique solution of the equations governing the ILA circuit requires solving a system of coupled linear equations relating the voltages and currents at the antenna straps and other key locations. This computation allows cross‐checking predicted values against measured experimental ones. Further more, a minimization procedure allows improving the correspondence with the quantities measured in the circuit during shots, thus coping with unavoidable errors arising from uncertainties in the measurements or from inaccuracies in the adopted RF model. Typical applications are e.g. fine‐tuning of the second‐stage of the ILA circuit for increased ELM‐resilience, cross‐checking the calibration of the measurements throughout the circuit and predicting the antenna performance and matching conditi...

Studies of ICRF Discharge Conditioning (ICRF‐DC) on ASDEX Upgrade, JET and TEXTOR

The present paper reviews the recent results achieved in the ICRF‐DC experiments performed in helium/hydrogen mixtures in the non‐circular tokamaks ASDEX Upgrade and JET and first tests of the ICRF discharges in helium/oxygen mixtures in the circular tokamak TEXTOR. Special emphasis was given to study the physics of ICRF discharges. A new recipe for safe and reliable RF plasma production [〈ne〉∼(3–5)×1017 m−3, Te∼(3–5) eV] with improved antenna coupling efficiency (by 1.5–3 times) and improved radial/poloidal homogeneity was proposed and successfully tested: coupling the RF power in the FW‐IBW mode conversion scenario in plasmas with two ion species. The first results on ICRF wall conditioning in helium/hydrogen and in helium/oxygen mixtures are analyzed.