Type-I edge-localized modes (ELMs) have been mitigated at the JET tokamak using a static external n=1 perturbation field generated by four error field correction coils located far from the plasma. During the application of the n=1 field the ELM frequency increased by a factor of 4 and the amplitude of the D(alpha) signal decreased. The energy loss per ELM normalized to the total stored energy, DeltaW/W, dropped to values below 2%. Transport analyses shows no or only a moderate (up to 20%) degradation of energy confinement time during the ELM mitigation phase.

Alfvén spectra in a reversed-shear tokamak plasma with a population of energetic ions exhibit a quasiperiodic pattern of primarily upward frequency sweeping (Alfvén cascade). Presented here is an explanation for such asymmetric sweeping behavior which involves finding a new energetic particle mode localized around the point of zero magnetic shear.The presence of energetic particles in a plasma can alter its behavior from that predicted by conventional magnetohydrodynamics (MHD) theory in two ways. First these particles can perturbatively destabilize a basic MHD mode. Alternatively, a sufficient number of these particles can nonperturbatively alter the very structure of the MHD modes. This latter behavior is relevant to certain shear Alfvénic perturbations often called energetic particle modes (EPM) [1][2][3]. In addition, in recent years there has been a great deal of interest in plasmas with reversed magnetic shear profiles, where transport and MHD stability properties have been shown to improve [4,5]. It is important for fusion experiments in shear reversed fields to understand the collective properties associated with energetic particles. Experiments in JT-60U [6] and JET [7] have investigated reversed shear regimes and have produced energetic particles with ion cyclotron heating (ICRH) [8]. Alfvén modes emerge in these experiments but their spectrum is often puzzling. This paper presents an example of how a purely MHD description is incompatible with the data while a description which accounts for the nonperturbative energetic particle response explains a large part of the data. The interpretation suggests a sensitive method to experimentally determine q min (the minimum safety factor) in reversed magnetic shear tokamaks.The JET experiments exhibit upward frequency sweeping phenomena, named Alfvén wave cascades (ACs) [9] (see Fig. 1a). Each cascade consists of several modes with different toroidal mode numbers and different frequencies.The toroidal mode numbers vary from n 1 to n 6. The frequency starts from 20 40 kHz and increases up to 100 120 kHz which is the toroidal Alfvén eigenmode (TAE) gap frequency. Similar data were obtained some time ago on JT-60U [6]. In both the JET and JT-60U data, the modes with higher toroidal mode numbers exhibit a more rapid frequency sweeping, and the higher n modes re-occur more often than the lower n modes. It is striking that downward frequency sweeping either does not appear, or appears only rarely. In both JET and JT-60U experiments, the minimum value of q decreases in time and a population of energetic ions is created by ICRH heating.ACs resemble the global Alfvén eigenmode [10,11], whose frequency is close to the local value of the Alfvén wave frequency at the zero shear point in minor radius, r r 0 , i.e., 2pf AC ഠ v A ͑r 0 ͒ ϵ jk k ͑r 0 ͒jV A ͑r 0 ͒, where V A is Alfvén velocity and k k is the wave-vector component along the equilibrium magnetic field B 0 . To avoid strong damping, the frequency f AC needs to be somewhat larger than v A ͑r 0 ͒ if v A ͑r͒ has a maxi...

The sensitivity of the stability of the ideal n = 1 internal kink mode is analysed both analytically and numerically in rotating tokamak plasmas. These stability analyses have been carried out including the centrifugal effects of toroidal plasma rotation upon the equilibrium, and also inconsistently when the equilibrium is treated as static. The plasma stability is partially (consistent equilibrium) or wholly (inconsistent treatment) determined by the radial profiles of the plasma density and rotation velocity. It is found that the internal kink mode stability is strongly influenced by small variations in these plasma profiles. Indeed, modest perturbations to the profiles inside the q = 1 surface of only a few percent can result in a stabilising effect upon the kink mode with respect to the static mode growth rate becoming a destabilising effect at the same rotation amplitude, or vice versa. The implications of this extreme sensitivity are discussed, with particular reference to experimental data from MAST.

Persistent rapid up and down frequency chirping modes with a toroidal mode number of zero (n = 0) are observed in the JET tokamak when energetic ions, in the range of several hundred keV, are created by high field side ion cyclotron resonance frequency heating. Fokker-Planck calculations demonstrate that the heating method enables the formation of an energetically inverted ion distribution which supplies the free energy for the ions to excite a mode related to the geodesic acoustic mode. The large frequency shifts of this mode are attributed to the formation of phase space structures whose frequencies, which are locked to an ion orbit bounce resonance frequency, are forced to continually shift so that energetic particle energy can be released to counterbalance the energy dissipation present in the background plasma.