J.C.M. de Haas

Plasma confinement in JET H?mode plasmas with H, D, DT and T isotopes

The scaling of the energy confinement in H mode plasmas with different hydrogenic isotopes (hydrogen, deuterium, DT and tritium) is investigated in JET. For ELM-free H modes the thermal energy confinement time τth is found to decrease weakly with the isotope mass (τth ~M-0.25±0.22), whilst in ELMy H modes the energy confinement time shows practically no mass dependence (τth ~M0.03±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 (τthcore ∝ M-0.16), in approximate agreement with theoretical models of the gyro-Bohm type (τgB ~M-0.2).

Isotope scaling of the H mode power threshold on JET

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.

Studies in JET divertors of varied geometry. I: Non-seeded plasma operation

Results of experiments investigating the performance of the JET Mark IIA divertor are reported and compared with the performance of its Mark I predecessor. The principal effect of reducing the divertor width (increasing closure) was to increase pumping for both deuterium and impurities while reducing upstream neutral pressure. Neither the orientation of the divertor target relative to the divertor plasma nor the width of the divertor had a major influence on core plasma performance in ELMy H modes. Changing the core triangularity and thus the edge magnetic shear modifies the ELM frequency in ELMy H mode plasmas, thereby changing the peak divertor power loading. The integrated performance of the core and divertor plasmas is reviewed with a view to extrapolation to the requirements of ITER. The confinement of JET ELMy H modes with hot, medium density edges is good (H97 ≈ 1) and follows a gyro-Bohm scaling. The impurity content of these discharges is low and within the ITER requirements. When an attempt is made to raise the density with deuterium gas fuelling, the ELM frequency increases and the confinement, especially in the edge, decreases. Good confinement can be achieved in JET either by producing a large edge pedestal, typically in discharges with NB heating or by centrally peaked heating with ICRH schemes. Large amplitude type I ELMs, which are present in all discharges with a large edge pedestal, would result in unacceptable divertor plate erosion when scaled to ITER. Since the power deposition profile due to α heating in ITER is calculated to be intermediate between the JET NB and RF heating profiles, it is likely that operation in ITER with small ELMs in order to reduce first wall loading will result in degraded confinement compared with present day scaling laws.

Recent results from DIII-D and their implications for next generation tokamaks

Recent results from the DIII-D tokamak have provided significant contributions to the understanding of many of the elements of tokamak physics and the application of this understanding to the design of next generation devices including ITER and CIT. The limitations of magnetohydrodynamic stability on the values of plasma beta (the ratio of kinetic pressure to the containing pressure of the magnetic field) that can be attained has been experimentally demonstrated and found to be described by existing theory. Values of beta (10.7%) well in excess of those required for proposed devices (ITER and CIT) have been demonstrated. Regimes of confinement (H-mode) have been established that scale favorably to proposed next generation devices, and experiments demonstrating the dependence of the energy confinement on plasma size have been completed. Understanding of confinement is rapidly developing especially in the areas of bulk transport and the role of turbulence in the plasma edge. Key experimental results in areas of plasma transport and edge plasma phenomena are in agreement with theories based on short wavelength turbulence. Control of the divertor heat loads and impurity influx has been demonstrated, and new progress has been made in the understanding of plasma edge phenomena. Experiments with ion Bernstein wave heating have not found regimes in which these waves can produce effective central ion heating. Electron cyclotron current drive experiments have demonstrated 70 kA of driven current in 400 kA discharges.

Tokamak transport studies using perturbation analysis

Studies of the transport properties of tokamak plasmas using perturbation analysis are discussed. The focus is on experiments with not too large perturbations, such as sawtooth induced heat and density pulse propagation, power modulation and oscillatory gas-puff experiments. The approximations made in the standard analysis of such experiments are made explicit and are discussed. References are given to papers that deal with specific aspects of the theory. Points of agreement as well as discrepancies between different experiments and gaps in the experimental data base are highlighted. The analysis of cross-coupling between electron thermal and particle transport using simultaneous measurements of heat and density pulses in JET is discussed, as an illustration of the potentiality to measure off-diagonal elements of the transport matrix in perturbative experiments.

Simultaneous measurements of electron thermal and particle transport in JET

Radial electron thermal and particle diffusivities have been measured using transient methods yielding chi e and De simultaneously in the same spatial region in the plasma interior. Three methods have been applied, analysis of inward propagation of temperature and density perturbations caused by injection of a small pellet into the plasma, the outward propagation of electron temperature and density pulses caused by sawteeth, and thirdly time-dependent transport analysis applied to non-stationary plasmas. In Ohmically heated deuterium plasmas, limited by the outer carbon belt-limiters, the authors deduce chi e=2.9+or-0.4 m2 s-1 and De=0.4+or-0.2 m2 s-1, giving chi e/De=7.2+or-3. The large value of chi e/De would seem to exclude E*B convection as a dominant mechanism in the observed radial thermal transport in JET.

Operation at high performance in optimized shear plasmas in JET

Heating during the early part of the current rise phase gives a low or negative magnetic shear (= (dq/dr)) in the centre of JET plasmas. Under these conditions the confinement improves with high additional heating power heating during the current ramp-up phase of the discharge. The reduction in the transport manifests itself as a peaking of the profiles with a large gradient region near = 0.55. The best discharges have no transport barrier at the edge of the plasma (L-mode). This allows central power deposition by the neutral beams in JET. A control of the plasma pressure, using feedback of the additional heating power in real-time, minimizes the impact of magnetohydrodynamic instabilities. As a result, these discharges achieve the highest D-D neutron rates in JET; , with , and .

Heat pulse analysis in JET limiter and X-point plasmas

Measurements of the electron heat diffusivity χ of the JET plasma in a wide variety of discharge conditions are reported. χ is deduced from heat pulse propagation analysis, using the sawtooth collapse as the perturbation of the temperature profile. The measurements of the electron temperature are performed with a 12-channel ECE polychromator. In limiter discharges, scans of plasma current, density, input power and Zeff have been studied. It is shown that χ is independent of density and input power, while a clear dependence on Zeff and on the electron temperature Te is demonstrated. The scaling provides a good fit to the JET data. xhp denotes the normalized radius where χ is measured. This radius is coupled to qa. As a result, the xhp-dependence only parameterizes the combined effect of the plasma current and the profile shapes of temperature, density, safety factor, etc. Various transport models are discussed in the light of this scaling. Also, χ measurements in the Ohmic, L-mode and H-mode phases of X-point plasmas are reported. The χ values are on average 20% higher in the H-mode than in the Ohmic phase. In one discharge where χ was measured in all three phases, no significant changes could be demonstrated.

Confinement of high-current steady-state ELMy H-modes with the JET Mark II divertor

The energy confinement time of high-current ELMy H-modes in JET is found to be lower than predicted by the scaling. An unprovoked transition to L-mode is often observed in ELMy H-modes without additional deuterium gas fuelling. This spontaneous H-L transition can be avoided by keeping the input power well above the L-H threshold power. The transition is analysed both in terms of global and local parameters.

Global and local conditions for the L-H and H-L transitions on JET

An analysis of the complete JET D-D threshold database, from 1990 to 1997 inclusive is presented. A JET scaling is derived for the threshold power through the separatrix, estimated by considering also radiation losses from the bulk plasma. Such scalings are applied to obtain a first qualitative estimate of the amount of power above threshold that is needed to obtain steady-state high confinement (H89 > 1.8). The L-H and H-L transitions are also analysed in terms of local edge parameters. The first results on isotope scaling of the H-mode power threshold in D-T and T-T plasmas are also presented, confirming the scaling.