9 research outputs found
Edge transport barrier in JET hot-ion H-modes
The effects of changing beam and plasma species on the edge transport barrier are investigated for ELM-free hot ion H mode discharges from the recent DT experiments on JET. The measured pressure at the top of the pedestal is higher for mixed deuterium and tritium and pure tritium plasmas over and above the level measured in pure deuterium plasmas at the same heating power. The pedestal pressure increases with beam tritium concentration for mixed deuterium-tritium beam injection into deuterium plasmas where the measured edge tritium concentration remains low. Alpha heating plays a significant role in the core of such plasmas, and the possible impact on the edge is discussed together with possible direct isotopic effects. Heuristic models for the transport barrier width are proposed, and used to explore a wider range of edge measurements including full power DD and DT pulses. This analysis supports the plasma current and mass dependence for a barrier width set by the orbit loss of either thermal or fast ions, though it does not unambiguously distinguish between them. The fast ion hypothesis could well account for some of the JET observations, though more theoretical work and direct experimental measurement would be required to confirm this. An ad hoc model for the power loss through the separatrix, P(loss)proportional-to-n(edge)(2)Z(eff,edge)I(p)(-1), is proposed based on neoclassical theory, a ballooning limit to the edge gradient and a barrier width set by the poloidal ion gyroradius. Such a model is compared with experimental data from JET. In particular, the model ascribes the systematic difference in loss power between the Mark I and Mark II diverters to the change in the measured Z(eff). This change in Z(eff) is consistent with the observed change in impurity production, which is described in some detail, together with a possible explanation provided by the temperature dependence of chemical sputtering
High fusion performance from deuterium-tritium plasmas in JET
High fusion power experiments using DT mixtures in ELM-free H mode and
optimized shear regimes in JET are reported. A fusion power of 16.1 MW
has been produced in an ELM-free H mode at 4.2 MA/3.6 T. The transient
value of the fusion amplification factor was 0.95+/-0.17, consistent
with the high value of
nDT(0)Ï„EdiaTi(0) = 8.7
× 1020+/-20% m-3 s keV, and was maintained
for about half an energy confinement time until excessive edge pressure
gradients resulted in discharge termination by MHD instabilities. The
ratio of DD to DT fusion powers (from separate but otherwise similar
discharges) showed the expected factor of 210, validating DD projections
of DT performance for similar pressure profiles and good plasma mixture
control, which was achieved by loading the vessel walls with the
appropriate DT mix. Magnetic fluctuation spectra showed no evidence of
Alfvénic instabilities driven by alpha particles, in agreement
with theoretical model calculations. Alpha particle heating has been
unambiguously observed, its effect being separated successfully from
possible isotope effects on energy confinement by varying the tritium
concentration in otherwise similar discharges. The scan showed that
there was no, or at most a very weak, isotope effect on the energy
confinement time. The highest electron temperature was clearly
correlated with the maximum alpha particle heating power and the optimum
DT mixture; the maximum increase was 1.3+/-0.23 keV with 1.3 MW of alpha
particle heating power, consistent with classical expectations for alpha
particle confinement and heating. In the optimized shear regime, clear
internal transport barriers were established for the first time in DT,
with a power similar to that required in DD. The ion thermal
conductivity in the plasma core approached neoclassical levels. Real
time power control maintained the plasma core close to limits set by
pressure gradient driven MHD instabilities, allowing 8.2 MW of DT fusion
power with
nDT(0)Ï„EdiaTi(0) approx
1021 m-3 s keV, even though full optimization was
not possible within the imposed neutron budget. In addition,
quasi-steady-state discharges with simultaneous internal and edge
transport barriers have been produced with high confinement and a fusion
power of up to 7 MW these double barrier discharges show a great
potential for steady state operation. © 1999, Eurato
JET results with the new pumped divertor and implications for ITER
This paper presents an overview of results of the 1994/95 experimental campaign on JET with the new pumped divertor and draws implications for ITER in the areas of detached and radiative divertor plasmas, the use of beryllium as a divertor target the material, the confinement properties of discharges with the same dimensionless parameters (except for the dimensionless Larmor radius) as ITER and the effect of varying the toroidal magnetic field ripple in the ITER relevant range. Discharges with high fusion performance at high current, in steady-state with ELMs and in the ELM-free hot-ion H-mode, are also reported. Limits to operations are discussed and projections to D-T performance are made
High performance Joint European Torus (JET) plasmas for deuterium-tritium operation with the MkII divertor
Planned experiments in the Joint European Torus [Plasma Physics and Controlled Fusion Research, Proceedings, 13th International Conference, Washington, D.C., 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 27] (JET) with deuterium-tritium (D-T) plasmas require high fusion performance for alpha-particle heating studies and for investigation of isotope dependence in conditions relevant to the International Thermonuclear Experimental Reactor [Plasma Phys. Controlled Fusion 37, A19 (1995)]. In deuterium plasmas, the highest neutron rates have been obtained in the hot-ion high-confinement mode (H mode) which is ultimately limited by magnetohydrodynamic (MHD) phenomena when the pressure gradient approaches ideal ballooning and kink stability limits in the vicinity of the edge transport barrier. Results are reported confirming the MkII divertor's increased closure and pumping in this regime, progress in understanding the MHD-related termination is discussed, and the use of ion cyclotron resonance heating (ICRH) in combination with high-power neutral beams to increase the neutron yield is described. In separate experiments internal transport barriers have been established through careful programming of the current ramp and heating waveforms, and neutron emission comparable with the best hot-ion II-modes achieved. Steady-state II-mode discharges exhibiting edge localized modes (ELMs) in reactor-like configurations and conditions have been demonstrated, including cases in which relevant dimensionless parameter values are preserved, ready also for testing in D-T. (C) 1997 American institute of Physics