E. Righi

Localized bulk electron heating with ICRF mode conversion in the JET tokamak

Ion cyclotron resonance frequencies (ICRF) mode conversion has been developed for localized on-axis and off-axis bulk electron heating on the JET tokamak. The fast magnetosonic waves launched from the low-field side ICRF antennas are mode-converted to short-wavelength waves on the high-field side of the 3He ion cyclotron resonance layer in D and 4He plasmas and subsequently damped on the bulk electrons. The resulting electron power deposition, measured using ICRF power modulation, is narrow with a typical full-width at half-maximum of ?30?cm (i.e. about 30% of the minor radius) and the total deposited power to electrons comprises at least up to 80% of the applied ICRF power. The ICRF mode conversion power deposition has been kept constant using 3He bleed throughout the ICRF phase with a typical duration of 4?6?s, i.e. 15?40 energy confinement times. Using waves propagating in the counter-current direction minimizes competing ion damping in the presence of co-injected deuterium beam ions.

Towards the realization on JET of an integrated H-mode scenario for ITER

ELMy H-mode experiments at JET in 2000/mid-2002 have focused on discharges with normalized parameters for plasma density, energy confinement and beta similar to those of the ITER Q(DT) = 10 referen ...

Recent progress on JET towards the ITER reference mode of operation at high density

Recent progress towards obtaining high density and high confinement in JET as required for the ITER reference scenario at Q = 10 is summarized. Plasmas with simultaneous confinement H-98(y.2) = 1 and densities up to n/n(Gw) similar to 1 are now routinely obtained. This has been possible (i) by using plasmas at high (delta similar to 0.5) and medium (delta similar to 0.3-0.4) triangularity with sufficient heating power to maintain Type I ELMs, (ii) with impurity seeded plasmas at high (delta similar to 0.5) and low (delta less than or equal to 0.2) triangularity, (iii) with an optimized pellet injection sequence, maintaining the energy confinement and raising the density, and (iv) by carefully tuning the gas puff rate leading to plasmas with peaked density profiles and good confinement at long time scales. These high performance discharges exhibit Type I ELMs, with a new and more favourable behaviour observed at high densities, requiring further studies. Techniques for a possible mitigation of these ELMs are discussed, and first promising results are obtained with impurity seeding in discharges at high triangularity. Scaling studies using the new data of this year show a strong dependence of confinement on upper triangularity, density and proximity to the Greenwald limit. Observed MHD instabilities and methods to avoid these in high density and high confinement plasmas are discussed.

ELMy H-modes in JET helium-4 plasmas

ELMy H-modes in helium-4 plasmas provide valuable information on ELMy H-mode physics as well as a possible early low activation operational phase for next-step tokamaks, such as ITER. With this in mind, a series of helium-4 H-mode experiments were performed on JET with pure helium-4 NBI auxiliary heating (up to 12 MW). A set of ELMy H-mode plasmas were produced, in both the Type I ELM regime and a second regime, which showed characteristics similar to the deuterium Type III regime, but with a reverse ELM frequency dependence on power. Sawteeth were also observed, and had similar behaviour to those seen in deuterium. Compared with deuterium plasmas, Type I ELMy H-mode confinement is seen to be 28 ± 6% poorer in helium-4 plasmas and the L–H power threshold 42 ± 10% larger. This is the opposite of the behaviour predicted by experimental isotope mass scalings from hydrogenic plasmas.

High density, high performance high-confinement-mode plasmas in the Joint European Torus (JET)

Recent experiments at the Joint European Torus [Rebut et al., Fusion Eng. Des. 22, 7 (1993)] aim to improve confinement quality in high-confinement-mode (H-mode) plasmas at high densities. Energy confinement time as predicted by the International Thermonuclear Experimental Reactor ITER-H98(y,2) scaling at densities near or in excess of 85% of the Greenwald density limit scaling has been obtained by (i) strong plasma shaping (triangularity 0.35<δ<0.5), or (ii) impurity seeding, or (iii) high-field side pellet injection. Slow peaking of central density without confinement degradation is observed. Loss of sawteeth and core impurity accumulation is prevented by central ion cyclotron resonance heating. In high triangularity and impurity seeded plasmas, reduction of average power loss associated with type I edge localized modes (ELMs) is found which is attributed to the occurrence of additional losses in between ELMs. Broad band magnetic fluctuations are seen which are reminiscent of regimes with small ELMs in ...

Recent Heating and Current Drive results on JET

An overview is presented of the results obtained on JET by the Heating and Current Drive Task Force (TF-H) in the period May 2000—March 2001. A strongly improved Lower Hybrid (LH) coupling was achieved by optimizing the plasma shape and by controlling the local edge density via the injection of CD4. Up to 4 MW have been coupled in type III ELMy H-mode and/or on Internal Transport Barrier (ITB) plasmas with reflection coefficients as low as 4%. Long lasting quasi steady-state ITBs have been obtained by adding the LH current to the bootstrap and beam driven components. Furthermore the use of LH in the pre-heat phase results in electron temperature in excess of 10 keV, deep negative magnetic shear and strongly reduced power threshold for ITB formation. Preliminary results on ICRF coupling are reported including the effect of CD4 injection and the commissioning of the wide band matching system on ELMy plasmas. IC CD scenarios have been studied in H and 3He minority and used to modify the stability of the sawt...

ICRF heating scenarios in JET with emphasis on 4He plasmas for the non-activated phase of ITER

In the initial phase of ITER operation, 4He plasmas could be used in order to avoid activating the machine. The main ICRH scenarios foreseen for ITER 4He plasmas are (3He)4He and (H)4He. ICRH experiments have been carried out on JET using 4He plasmas to validate these scenarios. At the same time, conditions for access to H-mode in plasmas of various isotope compositions from dominantly 4He to dominantly D have been studied. Experiments have also been carried out for the first time in 4He plasmas with the ICRF power added to 4He neutral beam injection at the third harmonic of 4He in order to produce a 4He tail for alpha particle studies.

Influence of plasma proximity to the vessel on the L-H transition in JET ICRF heated plasmas

The ITER ICRF antenna will operate at a considerable plasma-wall distance. In these circumstances it is expected that the ICH&CD system will routinely deliver power near its maximum voltage in the Main Transmission Lines (MTL). On the other hand during ICRF heating of JET plasmas the distance between the plasma separatrix and the ICRH antenna is usually kept to a minimum in order to maximise the power coupled to the plasma. This is necessary in order to compensate for the drastic reduction of the coupling resistance Rc that occurs when the plasma goes into H-mode, due to the depletion of the density in the scrape-off layer in front of the antenna. As a consequence of this the maximum voltage in the MTL line also increases, leaving less headroom to cope with fast transients like ELMs. However it has also been shown [1] that the proximity of the midplane plasma separatrix to the outer wall (RF antenna) increases the power necessary for the transition to H-mode.