The experimental characteristics of divertor detachment in the JET tokamak with the Mark I pumped divertor are presented for Ohmic, L-mode and ELMy H-mode experiments with the main emphasis on discharges with deuterium fuelling only. The range over which divertor detachment is observed for the various regimes as well as the influence of divertor configuration, direction of the toroidal field, divertor target material and active pumping on detachment will be described. The observed detachment characteristics such as the existence of a considerable electron pressure drop along the field lines in the scrape-off layer, and the compatibility of the decrease in plasma flux to the divertor plate with the observed increase of neutral pressure and the D α emission from the divertor region will be examined in the light of existing results from analytical and numerical models for plasma detachment. Finally, a method to evaluate the degree and window of detachment is proposed and all the observations of the JET Mark I divertor experiments summarised in the light of this new quantitative definition of divertor detachment.

Results are presented from a series of dedicated experiments carried out on JET in tritium, DT, deuterium and hydrogen plasmas to determine the dependence of the H mode power threshold on the plasma isotopic mass. The Pthr ∝ Aeff-1 scaling is established over the whole isotopic range. This result makes it possible for a fusion reactor with a 50:50 DT mixture to access the H mode regime with about 20% less power than that needed in a DD mixture. Results on the first systematic measurements of the power necessary for the transition of the plasma to the type I ELM regime, which occurs after the transition to H mode, are also in agreement with the Aeff-1 scaling. For a subset of discharges, measurements of Te and Ti at the top of the profile pedestal have been obtained, indicating a weak influence of the isotopic mass on the critical edge temperature thought to be necessary for the H mode transition.

ABSTRACT.Reactor relevant ICRH scenarios have been assessed during D-T experiments on the JET tokamak using H-mode divertor discharges with ITER-like shapes and safety factors. Deuterium minority heating in tritium plasmas was demonstrated for the first time. For 9% deuterium, an ICRH power of 6 MW gave 1.66 MW of fusion power from reactions between suprathermal deuterons and thermal tritons. The Q-value of the steady state discharge reached 0.22 for the length of the RF flat top (2.7 s), corresponding to three plasma energy replacement times. The Doppler broadened neutron spectrum showed a deuteron energy of 125 keV which was optimum for fusion and close to the critical energy. Thus strong bulk ion heating was obtained at the same time as high fusion efficiency. Deuterium fractions around 20% produced the strongest ion heating together with a strong reduction of the suprathermal deuteron tail. The edge localised modes (ELMs) had low amplitude and high frequency and each ELM transported less plasma energy content

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The scaling of the energy confinement in H-mode plasmas with different hydrogenic isotopes (H, D, D-T and T) is investigated in JET. For ELM-free H-modes the thermal energy confinement time τ th is found to decrease weakly with the isotope mass (τ th ~ M-0.25 ± 0.22) whilst in ELMy H-modes the energy confinement time shows practically no mass dependence (τ th ~ M 0.03 ± 0.1). Detailed local transport analysis of the ELMy H-mode plasmas reveals that the confinement in the edge region increases strongly with the isotope mass whereas the confinement in the core region decreases with mass (τ thcore ∝ M-0.16) in approximate agreement with theoretical models of the gyro-Bohm type (τ gB ~ M-0.2).

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Evidence for volume recombination in the JET divertor has been obtained from the population ratios of the excited states of neutral hydrogen. The ratio of the Balmer series lines Dγ/Dα has been used to identify recombination at both the inner and the outer divertor target. At the outer target the onset of recombination is well correlated with the plasma detachment as observed with Langmuir probes in the divertor target. While Te obtained from the target probe characteristics falls to ∼2 eV, Te from the population distribution of the excited states is 0.8 ± 0.1 eV. The peak plasma density viewing across the outer divertor leg just above the target exceeds 2.5 × 10 20 m −3 . When the plasma becomes partially detached from the outer target, the radial distribution of recombination shows a maximum close to the outer divertor strike point.

Ion cyclotron resonance heating (ICRH) experiments have been carried out in JET D-T plasmas using scenarios applicable to reactors. Deuterium minority heating in tritium plasmas is used for the first time and produces 1.66 MW of D-T fusion power for an ICRH power of 6 MW. The Q value is 0.22, which is a record for steady state discharges. Fundamental He 3 minority ICRH, in both 50:50 D-T and tritium dominated plasmas, generates strong bulk ion heating and ion temperatures up to 13 keV. Second harmonic tritium ICRH is seen to heat mainly the electrons as expected for JET conditions. All three schemes produce H-mode plasmas. [S0031-9007(98)06143-2] PACS numbers: 52.50. Gj, 52.55.Fa, 52.55.Pi Ion cyclotron resonance heating is the only method of heating majority ions, rather than electrons, in the dense core of a tokamak reactor. Radiofrequency (rf) power is used to excite a fast magnetosonic wave, to which the high density plasma is accessible. The wave is absorbed at a cyclotron resonance which is positioned in major radius (usually the plasma center) by the choice of magnetic field and rf frequency. The ions damping the wave are often accelerated to suprathermal energies, especially if they are a minority species. This energy is then transferred to the thermal ions and electrons by Coulomb collisions. If the energy of the absorbing ions is less than a critical value, power flows mainly to the thermal ions rather than to the electrons. The critical energy at which the power to the electrons equals that to the ions is given [1] by E crit 14.8AT e ͓Sn j Z 2 j ͞n e A j ͔ 2͞3 where A is the atomic mass of the energetic ions, n e is the electron density, Z is the atomic number, the sum is over the thermal ion species, and T e is the electron temperature. For fast deuterons in a tritium plasma, E crit 14.2T e . In the JET D-minority experiments, T e is about 7 keV and E crit ഠ 100 keV, which is also the deuteron energy at which the deuterium-tritium (D-T) fusion cross section peaks. High fusion power is thus achieved at the same time as equal ion and electron heating.Several ion cyclotron resonance heating (ICRH) schemes in D-T plasmas have been included in the design of the JET system [2] which thus covers a wide frequency band, 23-57 MHz. The same schemes are being considered for the ITER reactor [3]. Three of these scenarios are minority deuterium and minority He 3 at their fundamental resonances and majority tritium at its second harmonic resonance. Recent calculations [4,5] for ITER predict that each method can produce more than 50% ion heating on the route to ignition. The present experiments have demonstrated and assessed the fundamental deuterium scheme, which has never been used previously. Also, the physics and performance of all three methods have been studied for the first time in H mode, D-T plasmas heated predominantly by ICRH. The plasmas were similar to those expected in ITER in terms of shape, safety factor ͑q͒, normalized confinement time, and the behavior of edge localized modes (ELMs), which affec...