[en] This paper analyses the final phase of hybrid scenario discharges at JET, the reduction of auxiliary heating towards finally the Ohmic phase. The here considered Ohmic phase is mostly still in the current flattop but may also be in the current ramp down. For this purpose a database is created of 54 parameters in 7 phases distributed in time of the discharge. It is found that the occurrence of a locked mode is in most cases preceded by a radiation peaking after the main heating phase either in a low power phase and/or in the Ohmic phase. To gain insight on the importance of different parameters in this process a correlation analysis to the radiation peaking in the Ohmic phase is done. The first finding is that the further away in time the analysed phases are the less the correlation is. This means in the end that a good termination scenario might also be able to terminate unhealthy plasmas safely. The second finding is that remaining impurities in the plasma after reducing the heating power in the termination phase are the most important reason for generating a locked mode which can lead to a disruption. (paper)

No abstract available

[en] In order to establish the physics basis for the sustainment of the high integrated performance required in the next-step experimental reactor, JT-60U has been optimizing the discharge control scenarios of high confinement plasmas and extending the operation regimes by utilizing a variety of heating and current drive systems, including the new Electron Cyclotron Heating/Electron Cyclotron Current Drive (ECH/ECCD) system, and the W-shaped pumped divertor. For the integration, current and pressure profile controls are essential and high triangularity δ is beneficial. The range of plasma current with high values of H-factor, β_N and bootstrap fraction was extended to 1.5 MA with nearly full non-inductive current drive by Negative-ion-based Neutral Beam (N-NB) injection into the high-β_p H-mode with Edge Localized Modes (ELMy H-mode). High current drive efficiency of 1.3x10¹⁹ A W^-1 m^-2 was demonstrated for N-NB. High triangularity (0.4-0.5) operation extended the long pulse (∼3 s) high β_N (2.5-2.7) region to low-q₉₅ (∼3). For the reversed shear (RS) mode, feedback control of neutron production rate and stored energy enabled reproducible achievement of the DT equivalent fusion gain Q^eq_DT>1 and sustainment of Q^eq_DT∼0.4-0.5 for ∼1 s. Using the RS mode and the high δ high-β_p ELMy H-mode, the high confinement region has been extended to higher T_e/T_i and also to higher density. Electron heating by Lower Hybrid Range of Frequency (LHRF) and N-NB extended the high confinement region to T_e/T_i>1. Argon gas puffing improved confinement in the high density regime. With the W-shaped pumped divertor, the threshold heating power for the L-H transition was reduced by 30% as compared with the open divertor. Using the pumped divertor, a multiple parameter feedback control including both core and divertor plasma parameters was demonstrated. (author)

[en] The JET gas baking plant allows the vacuum vessel to be heated for conditioning and plasma operations. The vessel was maintained at 320 deg. C for the JET DT experiments (DTE 1). The design of the plant is outlined with particular reference to the features to provide compatibility with tritium operations. The experience of baking gas activation and tritium permeation into the plant are given, Developments to reduce the tritium permeation out of the vessel are considered. (authors)

No abstract available

[en] A crucial and still unsolved problem in tokamak physics is the understanding of anomalous heat transport across the magnetic field via heat conduction of the electrons. All efforts to establish a universal expression for the diffusivity Χ_e as a function of local plasma parameters which is valid also for auxiliary heating have failed so far. The problem has become even greater since it became evident that plasma confinement deteriorates during additional heating in a parameter range that is still very close to ohmic heating conditions. (orig./GG)

[en] We propose a new kind of transit-time interaction in which the reversal (or any localized transition) in the phase velocity of a wave within a wavelength of an antenna results in a high rate of work done by the wave (1/2 Re(j·E*)) near the antenna for conditions where the wave phase velocity is greater than a few times the thermal speed. This enhanced rate of work near the antenna can significantly exceed the far-field value due to Landau damping. For the conditions of typical low-field (< 0.01 T) and low-density (<10¹⁸m^-3) helicon wave-driven plasma sources, where the phase velocity parallel to the magnetic field can be a few times the thermal speed of electrons, it has been demonstrated that this spatial transient overshoot in the rate of work done by the wave is the dominant kinetic energy coupling process to electrons). In this paper it is demonstrated that, within a half wavelength of the antenna, it is the high-energy electrons that gain energy from both the wave and low velocity electrons as a result of this process. An important practical consequence is that the ionization rate of neutral gas can be significantly enhanced above the Maxwellian rate. The phenomenon is not restricted to helicon sources. This process may also explain the production of high-energy electrons in the near fields of antennas used in fusion plasma heating by radiofrequency waves. (author)

[en] Full text: Disruptions have been able to cause considerable damage in larger devices like JET, hence prevention or mitigation is essential. In order to obtain a more precise insight in the occurrence of disruptions in large Tokamaks, a detailed survey has been carried out at JET. Firstly, the average fraction of plasmas that disrupt, or disruption rate, and the so-called disruptivity, i.e. the likelihood of a disruption depending on operational parameters, have been determined. Secondly a survey into the causes of all disruptions at JET was devised. The aim of these studies is to find out what factors determine the occurrence of disruptions and what sets the lower limit. Moreover, the knowledge of the disruption causes can be used to devise better strategies to prevent and mitigate disruptions at JET and possibly in ITER. The average disruption rates for various periods during 25 years of JET operations was found to decrease steadily and recent campaigns show an averaged rate of less than 4% while from 1991 to 1995 this was often higher than 20 - 30%. Higher disruption rates were found during exploratory campaigns while disruptions occurred much less frequently when disruption avoidance was essential, such during D-T operations. This suggests that occurrence of disruptions may partly be connected to less careful operations and human errors and the downward trend could be interpreted as a learning curve of Tokamak operations. To shed more light on this a detailed study into the causes of all disruptions over the period from 2000 to 2007 was conducted. This allowed the visualization of the complex chain-of-events that could lead to disruptions. As expected all disruptions at JET were eventually pushed close an operational limit resulting in the on-set of physics instabilities. But it was also found that the chain-of-events were often triggered (root cause) by rather technical problems such as the failure of sub-systems such as auxiliary heating. Human error was the second highest root cause of disruptions at JET. This paper will discuss the complete set of root causes and disruption classes that were found at JET. This study provided new insights in the processes that cause disruptions at JET and provided useful suggestions on how to prevent them. This work was partly funded by EURATOM, FOM and CCFE Contract of Association and carried out under EFDA. (author)

No abstract available

[en] The diacrode concept has been already successfully developed and tested by Thomson Tubes Electroniques. Demonstration is done that ITER project requirements cannot be met with a single tetrode power amplifier but with one new diacrode, the design of the RF circuit around which is hereafter proposed. (author)