S. Sharapov

JET (3He)-D scenarios relying on RF heating: survey of selected recent experiments

Recent JET experiments have been devoted to the study of (He-3)-D plasmas involving radio frequency (RF) heating. This paper starts by discussing the RF heating efficiency theoretically expected in such plasmas, covering both relevant aspects of wave and of particle dynamics. Then it gives a concise summary of the main conclusions drawn from recent experiments that were either focusing on studying RF heating physics aspects or that were adopting RF heating as a tool to study plasma behavior. Depending on the minority concentration chosen, different physical phenomena are observed. At very low concentration (X[He-3] > 10% electron mode conversion damping becomes dominant. Evidence for the Fuchs et al standing wave effect (Fuchs et al 1995 Phys. Plasmas 2 1637-47) on the absorption is presented. RF induced deuterium tails were observed in mode conversion experiments with large X[He-3] (approximate to 18%). As tentative modeling shows, the formation of these tails can be explained as a consequence of wave power absorption by neutral beam particles that efficiently interact with the waves well away from the cold D cyclotron resonance position as a result of their substantial Doppler shift. As both ion and electron RF power deposition profiles in (He-3)-D plasmas are fairly narrow-giving rise to localized heat sources-the RF heating method is an ideal tool for performing transport studies. Various of the experiments discussed here were done in plasmas with internal transport barriers (ITBs). ITBs are identified as regions with locally reduced diffusivity, where poloidal spinning up of the plasma is observed. The present know-how on the role of RF heating for impurity transport is also briefly summarized.

Recent progress toward high performance above the Greenwald density limit in impurity seeded discharges in limiter and divertor tokamaks

An overview is given of recent advances toward the realization of high density, high confinement plasmas with radiating mantles in limiter and divertor tokamaks worldwide. Radiatively improved mode discharges on the Torus Experiment for Technology Oriented Research 94 (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (Institute for Electrical and Electronics Engineers, Piscataway, NJ, 1995), p. 470] have recently been obtained at trans-Greenwald densities (up to n/nGW=1.4) with high confinement mode free of edge localized modes (ELM-free H-mode) confinement quality. Experiments in DIII-D [J. Luxon et al., Proceedings of the 11th IAEA Conference on Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Association, Vienna, 1987), Vol. 1, p. 159] divertor plasmas with a low confinement mode edge have confirmed the dramatic changes in confinement observed with impurity seeding on TEXTOR-94. Recent experiment with impurity seeding on the Joint Europea...

Gamma-ray imaging of D and 4He ions accelerated by ion-cyclotron-resonance heating in JET plasmas

Gamma-ray images of fast D- and 4He-ions accelerated with third-harmonic ion-cyclotron-resonance heating of 4He-beam were simultaneously recorded for the first time in JET tokamak experiments dedicated to the investigation of burning plasmas with 3.5 MeV fusion alpha (α) particles. Gamma (γ) rays, born as a result of nuclear reactions, 9Be(4He, nγ)12C and 12C(D, pγ)13C, between the fast ions and the main plasma impurities, are measured using a two-dimensional multicollimator spectrometer array, which distinguishes the γ-rays from accelerated D- and 4He-ions. Tomographic reconstruction of the γ-ray emission profiles gives images of the fast-ion population in the poloidal cross-section. The potential of this technique to visualize several energetic ion species and to determine their behaviour in different plasma scenarios is demonstrated.

Characterization of small ELM experiments in highly shaped single null and quasi-double-null plasmas in JET

This paper describes experiments with highly shaped JET H-mode plasmas, which were directed to developing regimes where Type I ELMs are replaced by other edge relaxations, while maintaining the pedestal pressure of Type I ELMy H-modes. It was found that Type II ELMs coexisted with Type I, up to densities of the order of the Greenwald limit, where Type III ELMs appear, and the good confinement was lost. Only at the highest edge collisionality was it observed that Type II ELMs completely replace Type I. At high βp and q95, grassy ELMs replace Type I completely. The MHD spectra characteristics for grassy ELMs are significantly different from those of Type II ELMs. This paper details the experiments, briefly compares the results to those obtained elsewhere and suggests open lines of investigations for the assessment of the potential of grassy ELM regimes as an ELM mitigation technique.

Expanding the operating space of ICRF on JET with a view to ITER

This paper reports on ITER-relevant ion cyclotron resonance frequency (ICRF) physics investigated on JET in 2003 and early 2004. Minority heating of helium three in hydrogen plasmas-(He-3)H-was systematically explored by varying the 3 He concentration and the toroidal phasing of the antenna arrays. The best heating performance (a maximum electron temperature of 6.2 keV with 5 MW of ICRF power) was obtained with a preferential wave launch in the direction of the plasma current. A clear experimental demonstration was made of the sharp and reproducible transition to the mode conversion heating regime when the 3 He concentration increased above similar to 2%. In the latter regime the best heating performance (a maximum electron temperature of 8 keV with 5 MW of ICRF power) was achieved with dipole array phasing, i.e. a symmetric antenna power spectrum. Minority heating of deuterium in hydrogen plasmas-(D)H-was also investigated but was found inaccessible because this scenario is too sensitive to impurity ions with Z/A = 1/2 such as C6+, small amounts of which directly lead into the mode conversion regime. Minority heating of up to 3% of tritium in deuterium plasmas was systematically investigated during the JET trace tritium experimental campaign (TTE). This required operating JET at its highest possible magnetic field (3.9 to 4 T) and the ICRF system at its lowest frequency (23 MHz). The interest of this scenario for ICRF heating at these low concentrations and its efficiency at boosting the suprathermal neutron yield were confirmed, and the measured neutron and gammay ray spectra permit interesting comparisons with advanced ICRF code simulations. Investigations of finite Larmor radius effects on the RF-induced high-energy tails during second harmonic (omega = 2 omega(c)) heating of a hydrogen minority in D plasmas clearly demonstrated a strong decrease in the RF diffusion coefficient at proton energies similar to 1 MeV in agreement with theoretical expectations. Fast wave heating and current drive experiments in deuterium plasmas showed effective direct electron heating with dipole phasing of the antennas, but only small changes of the central plasma current density were observed with the directive phasings, in particular at low single pass damping. New investigations of the heating efficiency of ICRF antennas confirmed its strong dependence on the parallel wavenumber spectrum. Advances in topics of a more technological nature are also summarized: ELM studies using fast RF measurements, the successful experimental demonstration of a new ELM-tolerant antenna matching scheme and technical enhancements planned on the JET ICRF system for 2006, they being equally strongly driven by the preparation for ITER.

Experimental studies of instabilities and confinement of energetic particles on JET and MAST

In preparation for next step burning plasma devices such as ITER, experimental studies of instabilities and confinement of energetic ions were performed on Joint European Torus (JET) and on Mega-Amper Spherical Tokamak (MAST) with innovative diagnostic techniques, in conventional and shear-reversed plasmas, exploring a wide range of effects for energetic ions. A compendium of recent results testing capabilities of the present-day facilities for burning plasma relevant study is presented in this paper. Alpha tail production using 3rd harmonic ion-cyclotron resonance heating (ICRH) of He-4 beam ions has been employed on JET for studying He-4 of the megaelectronvolt energy range in a neutron-free environment. The evolution of ICRH-accelerated ions of He-4 with E >= 1.7 MeV and D with E >= 500 keV was assessed from nuclear gamma-ray emission born by the fast ions colliding with Be and C impurities. A simultaneous measurement of spatial profiles of fast He-4 and fast D ions relevant to ITER was performed for the first time in positive and strongly reversed magnetic shear discharges. Time-resolved gamma-ray diagnostics for ICRH-accelerated He-3 and H minority ions allowed changes in the fast ion distribution function to be assessed in the presence of unstable toroidal Alfven eigenmodes (TAEs) and sawteeth. A significant decrease of gamma-ray intensity from protons with E >= 5 MeV was detected during the tornado modes. This was interpreted as tomado-induced loss of fast ions with the drift orbit width, Delta(f), comparable to the minor radius of tokamak a. Experiments performed in the opposite case, Delta f/a = 500 keV, have shown excitation of numerous Alfven eigenmodes without a significant degradation of the fast ion confinement. The stabilizing effect of fast particles on monster sawteeth was experimentally found to fail in low-density plasmas with high power ion cyclotron resonance frequency (ICRF)-heating. The transition from the monster to short-period grassy sawteeth was investigated with different ICRF phasing, which controls the pinch-effect and radial distribution of ICRF-accelerated ions. Instabilities excited by super-Alfvenic beam ions were investigated on the spherical tokamak MAST. Due to higher values of beta and a higher proportion of fast ions on MAST than on JET, a wider variety of modes and nonlinear regimes for the Alfven instabilities were observed, including the explosive TAE-regimes leading to the formation of hole-clump pairs on the fast ion distribution function. The MAST and START data showed that TAE and chirping modes decrease both in their mode amplitudes and in the number of unstable modes with increasing beta.

Application of ICRF waves in tokamaks beyond heating

Note: 30th EPS Conference on Controlled Fusion and Plasma Physics, St Petersbourg, Russia, July 2003 (invited talk) Reference CRPP-CONF-2003-042 Record created on 2008-05-13, modified on 2016-08-08

Analysis and modelling of power modulation experiments in JET plasmas with internal transport barriers

Understanding the physics of internal transport barriers (ITBs) is a crucial issue in developing ITER relevant advanced tokamak scenarios. To gain new information on ITBs, RF power modulation experiments, mainly devoted to the study of electron heat transport through ITBs, have been performed on the JET tokamak. The main physics results have been reported in [1]. The present paper describes in detail the data analysis and numerical modelling work carried out for the interpretation of the experiments. ITBs located in the negative shear region behave as localized insulating layers able to stop the heat wave propagation, thus implying that the ITB is a region of low diffusivity characterized by a loss of stiffness. Various sources of spurious effects affecting the interpretation of the results are analysed and discussed. First principle based models have so far failed to predict the temperature profile in the first place, which prevented their application to modulation results, while empirical transport models have been set up and reproduce the major part of the data.

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...

Heating, current drive and energetic particle studies on JET in preparation of ITER operation

This paper summarizes the recent work on JET in the three areas of heating, current drive and energetic particles. The achievements have extended the possibilities of JET, have a direct connection to ITER operation and provide new and interesting physics. Toroidal rotation profiles of plasmas heated far off axis with little or no refuelling or momentum input are hollow with only small differences on whether the power deposition is located on the low field side or on the high field side. With LH current drive the magnetic shear was varied from slightly positive to negative. The improved coupling (through the use of plasma shaping and CD4) allowed up to 3.4 MW of PLH in internal transport barrier (ITB) plasmas with more than 15 MW of combined NBI and ICRF heating. The q-profile with negative magnetic shear and the ITB could be maintained for the duration of the high heating pulse (8 s). Fast ions have been produced in JET with ICRF to simulate alpha particles: by using third harmonic He-4 heating, beam injected He-4 at 120 kV were accelerated to energies above 2 MeV taking advantage of the unique capability of JET to use NBI with 4 He and to confine MeV class ions. ICRF heating was used to replicate the dynamics of alpha heating and the control of an equivalent Q = 10 `burn was simulated.