A. Fasoli

Bulk ion heating with ICRH in JET DT plasmas

Reactor relevant ICRH scenarios have been assessed during DT 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 ELMs had low amplitude and high frequency and each ELM transported less plasma energy content than the 1% required by ITER. The energy confinement time, on the ITERH97-P scale, was 0.90, which is sufficient for ignition in ITER. 3He minority heating, in approximately 50:50 D:T plasmas with up to 10% 3He, also demonstrated strong bulk ion heating. Central ion temperatures up to 13 keV were achieved, together with central electron temperatures up to 12 keV. The normalized H mode confinement time was 0.95. Second harmonic tritium heating produced energetic tritons above the critical energy. This scheme heats the electrons in JET, unlike in ITER where the lower power density will allow mainly ion heating. The inverted scenario of tritium minority ICRH in a deuterium plasma was demonstrated as a successful heating method producing both suprathermal neutrons and bulk ion heating. Theoretical calculations of the DT reactivity mostly give excellent agreement with the measured reaction rates.

Collisionless magnetic reconnection in a toroidal cusp

Fast collisionless magnetic reconnection is driven and observed in a toroidal magnetic cusp. For low values of the toroidal (guide) magnetic field, reconnection occurs without the formation of a macroscopic current channel. In the absence of a current channel, the measured plasma potential and poloidal flows are consistent with predictions based upon single particle theory. Only for large values of the toroidal magnetic field compared to cusp poloidal field is a current channel present. Also in this case single particle orbit theory provides a good description of the anomalous resistivity observed experimentally.

Fast particles-wave interaction in the Alfvén frequency range on the Joint European Torus tokamak

Wave-particle interaction phenomena in the Alfven Eigenmode (AE) frequency range are investigated at the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] using active and passive diagnostic methods. Fast particles are generated by neutral beam injection, ion cyclotron resonance heating, and fusion reactions. External antennas are used to excite stable AEs and measure fast particle drive and damping separately. Comparisons with numerical calculations lead to an identification of the different damping mechanisms. The use of the active AE diagnostic system to generate control signals based on the proximity to marginal stability limits for AE and low-frequency magnetohydrodynamic (MHD) modes is explored. Signatures of the different nonlinear regimes of fast particle driven AE instabilities predicted by theory are found in the measured spectra. The diagnostic use of AE measurements to get information both on the plasma bulk and the fast particle distribution is assessed.

On the measurement of toroidal rotation for the impurity and the main ion species on the Joint European Torus

The neoclassical moment approach of Hirshman and Sigmar [Nucl. Fusion 21, 1079 (1981)] has been used for the first time on the Joint European Torus [J. Wesson, Tokamaks, 2nd ed. (Oxford Science Publication, Oxford, 1997), p. 581] to deduce the toroidal rotation frequency of the main ion species from that measured for the impurity ions. A significant differential rotation between these two species is found where the second radial derivative of the ion temperature is large. The position of radially localized magnetohydrodynamic instabilities is then deduced from the Doppler shift in the frequency and verified using the cross-correlation between the magnetic and the electron cyclotron emission measurements. Accurate agreement between these estimates of the mode location is only found when using the toroidal rotation frequency of the main ion species. This result fully verifies the neoclassical approach and can be used directly as a diagnostic tool to localize magnetohydrodynamic instabilities.

The effect of plasma shaping on the damping of low-n Alfven Eigenmodes in JET tokamak plasmas

The effect of plasma shaping on the damping of radially extended Alfv?n eigenmodes (AEs) is investigated experimentally during the limiter phase of JET plasma discharges. The AE damping rate is observed to increase with increasing elongation, triangularity and edge magnetic shear. These parameters could therefore be used to control the AE stability in real time.

Experimental test of damping models for n=1 toroidal Alfven eigenmodes in JET

The damping rate of the n = 1 toroidal Alfven eigenmodes (TAE) mode in Ohmic-heated JET plasmas has been measured and compared with the prediction of the kinetic NOVA-K code, which includes electron and ion Landau damping of the global shear-Alfven wave field, collisional damping, and radiative damping. It was found that the calculated damping rate is too small to account for the measured value under these experimental conditions. A relatively weak dependence of the damping rate on the normalized Larmor radius is observed experimentally.

Isotope mass scaling of AE damping rates in the JET tokamak plasmas

The damping of radially extended Alfven eigenmodes in tokamak plasmas is investigated in JET discharges with different ion mass. The comparison of damping measurements with predictions of a gyro-kinetic model indicates that mode conversion into kinetic Alfven waves is the dominant damping mechanism for Alfven eigenmodes in the plasma core.

Energetic particle physics in JET

Results achieved on JET during the 1997-1999 experimental campaigns in the physics of energetic ions and runaway electrons are reviewed. Heating of deuterium-tritium (DT) plasmas by fusion born alpha particles is found to be similar to that achieved by comparable ICRF heating of deuterium plasmas. The stability of alpha particle driven Alfv?n eigenmodes (AEs) in the highest fusion power ELM-free H mode discharges is shown to be consistent with the existing theoretical analysis for AEs. Direct measurements of the trapped alpha particle and knock-on deuteron distribution functions by a neutral particle analyser (NPA) are described. New ICRF heating scenarios tested in JET DT plasmas are presented in view of the possible use of the ICRF heating of ITER-like DT plasmas on route to ignition. The energetic ion pinch in the presence of toroidally asymmetric ICRF waves is studied experimentally on JET. Recent experimentally observed effects of ICRF accelerated ions on sawteeth in JET are reviewed. Detailed time and space resolved X ray images of the electron runaway beam in flight spontaneously generated by disruptions on JET are described.

Stability of Alfvén eigenmodes in optimized tokamaks

Alfven eigenmodes (AEs) with intermediate toroidal mode numbers are modelled using the global gyrokinetic PENN code to determine the stability of high performance tokamak discharges in the presence of energetic particles. A large plasma pressure and a weak magnetic shear in the core give rise to radially extended kinetic AEs, which are stabilized by the high shear at the edge of a divertor (X point) configuration. Large values for the safety factor and the ion Larmor radius in reversed shear operation may however trigger drift kinetic Alfven eigenmode instabilities that could affect the alpha particle confinement in a reactor.

The topology of guiding center orbits in a linear magnetic cusp

Particle orbits for a linear magnetic cusp configuration, characteristic for magnetic reconnection, are analyzed within the guiding center approximation. A class of particle orbits encircling the magnetic X-line is identified and the orbit phase space partition is obtained and characterized as a function of dimensionless parameters controlling the orbit topology. The large majority of particles are found to be trapped in the direction of the X-line implying large neoclassical corrections to the plasma resistivity. This may be the key in explaining the high rates of collisionless magnetic reconnection observed in nature.