D. Van Eester

Simultaneous Analysis of Ion and Electron Heat Transport by Power Modulation in JET

Heating power modulation experiments using ion cyclotron resonance heating (ICRH) in the (3)He minority scheme have been performed in the JET tokamak to investigate heat transport properties. This RF scheme provides a dominant localized ion heating, but also some electron heating, and therefore both ion and electron heat channels were modulated. This allows us to carry out a simultaneous transport analysis of ion and electron heat transport channels, including transient transport phenomena. This also provides an experimental assessment of the ICRH heat sources of the (3)He scheme. The modulation approach, so far widely used for electron transport studies, has been validated for ion heat transport in these experiments and yields results on stiffness and threshold of the ion temperature gradient (ITG)-driven ion heat transport. The results for the electron channel demonstrate the importance of the ITG-driven, off-diagonal, contribution to electron heat transport in plasmas with significant ion heating.

On the parasitic absorption in FWCD experiments in JET ITB plasmas

Fast wave current drive (FWCD) experiments have been performed in JET plasmas with electron internal transport barriers produced with LHCD. Because of a large fraction of parasitic absorption, owing to weak single pass damping, the inductive nature of the plasma current and the interplay between the RF-driven current and the bootstrap current only small changes are seen in the central current profiles. The measured difference in the central current density for co- and counter-current drive is larger than the response expected from current diffusion calculations, but smaller than the driven currents, suggesting a faster current diffusion than that given by neo-classical resistivity. A large fraction of the power is absorbed by cyclotron damping on residual 3He ions while a significant fraction appears not to have been deposited in the plasma. The strong degradation of heating and current drive occurs simultaneously with strong increases in the Be II and C IV line intensities in the divertor. The degradation depends on the phasing of the antennas and increases with reduced single pass damping which is consistent with RF-power being lost by dissipation of rectified RF-sheath potentials at the antennas and walls. Asymmetries in direct electron heating, lost power and production of impurities, fast ions and gamma-rays are seen for co- and counter-current drive. These differences are consistent with the differences in the absorption on residual 3He ions owing to the RF-induced pinch. Effective direct electron heating, comparable to the indirect electron heating with H-minority heating, occurs for dipole phasing of the antennas without producing a significant fast ion pressure and with low impurity content in the divertor plasma.

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.

RF Physics of ICWC Discharge at High Cyclotron Harmonics

Recent experiments on Ion Cyclotron Wall Conditioning (ICWC) performed in tokamaks TEXTOR and ASDEX Upgrade with standard ICRF antennas operated at fixed frequencies but variable toroidal magnetic field demonstrated rather contrasting parameters of ICWC discharge in scenarios with on-axis fundamental ion cyclotron resonance (ICR) for protons,ω=ωH+, and with its high cyclotron harmonics (HCH), ω=10ωcH+⋅ HCH scenario: very high antenna coupling to low density RF plasmas (Ppl≈0.9PRF-G) and low energy Maxwellian distribution of CX hydrogen atoms with temperature TH≈350 eV. Fundamental ICR: lower antenna-plasma coupling efficiency (by factor of about 1.5 times) and generation of high energy non-Maxwellian CX hydrogen atoms (with local energy E⊥H ≥1.0 keV). In the present paper, we analyze the obtained experimental results numerically using (i) newly developed 0-D transport code describing the process of plasma production with electron and ion collisional ionization in helium-hydrogen gas mixture and (ii) earli...

Review of combined ICRH‐NBI results in TEXTOR

The synergism observed between NBI and ICRH is theoretically interpreted for the interaction at the second and third ion cyclotron harmonic. It is also shown that the performances of supershot‐like discharges obtained with balanced injection can be substantially improved by beam‐RF interaction both at 2 ωCD and 3 ωCD.

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.

ICRF heating at JET: From operations with a metallic wall to the long term perspective of a DT campaign

The first series of experiments with the ITER‐like wall (ILW) will start mid‐2011 with D plasmas and will continue through 2012–13 with H, 4He and D plasmas, and up to 2014–15, when a DT campaign is proposed. In this paper, the previous experience at JET is reviewed to set the scene for the future challenges of ICRF operation including change in the ICRF coupling, W impurity production and evaluation of localized power loads due the RF sheaths. development in a Beryllium/Tungsten environment of ICRF heating schemes for the non activated and the DT phases of ITER.

Recent Experimental Results and Modeling of RF Heating of (3He)-D JET Plasmas: RF as a Tool to Study Transport

D plasmas with 3He minorities have sharp, thin ion‐ion hybrid layers that enable to efficiently excite short wavelength branches that are subsequently damped by fairly well localized electron Landau and TTMP absorption. Depending on the minority concentration chosen, ion minority heating or electron mode conversion damping is dominant. Recent experiments have been devoted to the study of (3He)‐D JET plasmas. One aspect of those experiments—using RF heating as a tool—is the study of the response of the plasma to RF power modulation, allowing to examine the fate of the RF power and to diagnose particle and energy transport. The present paper gives a very brief summary of a subset of these experiments. The focus will largely but not exclusively be on understanding ITB physics. The adopted probing methods are more generally applicable, though.

Development of RF Tools and Scenarios for ITER on JET

The improvement of LH coupling with local puffing of D2 gas, which made operation at ITER relevant distances (10 cm) and with ELMs a reality, has been extended to ITER‐ like plasma shapes with higher triangularity. With ICRF, we developed tools such as (1) localized direct electron heating using the 3He mode conversion scenario for electron heat transport studies, (2) the production of 4He ions with energies in the MeV range by 3 ωc acceleration of beam injected ions at 120 keV to investigate Alfven instabilities and test α diagnostics, (3) the stabilisation and destabilisation of sawteeth and (4) ICRF as as a wall conditioning. Several ITER relevant scenarios were tested. The (3He)H minority heating scenario, considered for the non‐activated start‐up phase of ITER, produces at very low concentration energetic 3He which heat the electrons indirectly. For n3He/ne > 2%, the scenario transforms to a mode conversion scenario where the electrons are heated directly. The (D)H minority heating is not accessible ...