D.F.H. Start

Ion cyclotron emission measurements during JET deuterium-tritium experiments

In the course of the Preliminary Tritium Experiment in JET, where combined deuterium and tritium neutral beam injection generated a DT fusion power of 1.7 MW, ion cyclotron emission (ICE) was measured in the frequency range v ≤ 180 MHz. The ICE spectra contain superthermal, narrow, equally spaced emission lines, which correspond to successive cyclotron harmonics of deuterons or alpha particles at the outer midplane, close to tile last closed flux surface at major radius R approximately 4.0 m. Above about 100 MHz the lines merge into a relatively intense continuum. The ICE signal fluctuates rapidly in time, and is extinguished whenever a large amplitude edge localized mode (ELM) occurs. In pure deuterium and mixed DT discharges ICE spectra are similar in form, but on changing from pure D to mixed D+T neutral beam injection at constant power, the intensity of the ICE rises in proportion to the increased neutron flux: this indicates that fusion alpha particles-and not beam ions-provide the free energy to generate ICE. The JET ICE database, which now extends over a range of six decades in signal intensity, shows that the time averaged ICE power increases almost linearly with total neutron flux. The rise and fall of the neutron flux during a single discharge is closely followed by that of the ICE signal, which is delayed by a time of the order of the fusion product slowing down time. This feature is well modelled by a TRANSP code simulation of the density of deeply trapped fusion products reaching the plasma edge. Calculations reveal a class of fusion products, born in the core, which make orbital excursions of sufficient size to reach the outer midplane edge. There, the velocity distribution has a ring structure, which is found to be linearly unstable to relaxation to obliquely propagating waves on the fast Alfven-ion Bernstein branch at all ion cyclotron harmonics. The paper shows how ICE provides a unique diagnostic for fusion alpha particles

Fokker–Planck studies of high power electron cyclotron heating in tokamaks

A Fokker–Planck formalism has been developed to interpret experimental data from high power electron cyclotron heating of tokamak plasmas in which the electron distribution function can be substantially distorted. The Fokker–Planck equation is solved using a 2-D code which incorporates electron trapping, a steady Ohmic electric field and a bounce averaged electron cyclotron heating term. The quasilinear RF diffusion coefficient is calculated using a single particle model which includes the mildly relativistic resonance condition and takes into account localized RF power injection and tokamak rotational transform effects. Comparisons are presented of the calculated and observed soft X-ray spectra from 60 GHz second harmonic electron cyclotron heating experiments on the CLEO tokamak. Good agreement between theory and experiment is found. In addition, predicted RF current drive efficiencies for the COMPASS tokamak are presented.

A kinetic theory of beam-induced plasma currents

A Fokker-Planck treatment of the current induced by a beam of fast ions circulating in a toroidal plasma is developed. The electron Fokker-Planck equation is first reduced to an integro-differential equation which is then solved analytically in the limiting cases of: (a) a large plasma Z and (b) a large ratio of the electron thermal velocity ve to the fast ion velocity vb. In addition, a numerical solution was obtained for the complete range of values of ve/vb and for several values of Z. It is found that the resulting net plasma current has a very different functional dependence upon electron temperature than that given by the conventional theoretical treatment in which the electrons are assumed to be Maxwellian. In particular, for ve > vb and Z = 1, which is the limit appropriate to many present tokamak experiments, the net current is found to be in the opposite direction to the fast-ion current. The theory is compared with recent measurements of this current which were made by using the Culham Levitron, and agreement is found between theory and experiment.

ECRH current drive studies in the CLEO tokamak

Non-inductive current drive using second harmonic ECRH at both 28 GHz and 60 GHz has been studied in the CLEO tokamak. At 60 GHz, RF driven currents of up to 5 kA have been observed at e = 4 × 1018m−3 for 185 kW of injected power, indicating an efficiency of η ≡ eIRFR0/PRF = 0.001 (1020 m−3, A, m, W−1). The RF driven current scaled linearly with total plasma current in the range of 5-15 kA and was maximized when the cyclotron resonance was located near to the centre of the plasma. Sawtooth activity was normally strongly affected and transient sawtooth stabilization was often observed. With detailed theoretical studies it is possible to reproduce both the high absorption efficiencies and the scaling of RF driven current with resonance position seen in the 60 GHz experiments. However, the magnitude of the observed current is a factor of about three below that theoretically predicted. At 28 GHz, no evidence of RF driven current could be detected. Possible reasons for this are discussed.

Fokker-Planck calculations of rf heating in tokamaks

Abstract A bounce-averaged Fokker-Planck code has been developed for studying electron cyclotron heating in tokamak experiments. The equation solved and the numerical scheme used are discussed, and the advantages of the implicit time advancement emphasised. The numerical problems encountered while developing the code are described. The distribution functions predicted by the code are shown to be in good agreement with those produced in experiments.

A 2-D Fokker-Planck treatment of the ICRF enhancement of beam-driven currents

A 2-D Fokker-Planck treatment of the interaction between an ICRF wave having k|| = 0 and fast ions arising from neutral-beam injection is undertaken. Fast-ion collisions with the background plasma are treated using a collision operator which takes account of slowing down on both electrons and ions, together with pitch-angle scattering on the thermal ions. Energy diffusion due to electron collisions is neglected. The RF interaction is represented by the usual quasi-linear diffusion operator. The Fokker-Planck equation is solved in the weak RF field limit by treating the effect of the wave interaction as a perturbation of the fast-ion distribution. The current carried by the injected fast ions is found to be increased by the wave interaction in most circumstances. The optimum efficiency (additional current drive per wave power absorbed) of the current enhancement is found to be about twice the optimum efficiency of generation of the straightforward beam-driven current.

Impurity radiation in DITE during neutral injection

The large increase in radiation during neutral injection in DITE is identified as being primarily due to the charge-exchange recombination of the impurities on the fast neutrals. The time dependence of the increase in line eion and its dependence on injection power and plasma density is shown to fit the theory. – The line emission and the total radiated power are found to be toroidally asymmetric, being larger in the region of beam deposition. A theoretical explanation for this asymmetry is given. The recombination of impurities by thermal neutrals is found to be important in low-density Ohmic discharges, and the implications of this are discussed.

The coupling between the resistive-g and ion temperature gradient instabilities in a sheared magnetic field

The shear in the magnetic field has been found to couple the resistive-g and ion temperature gradient instabilities to form a single, strongly growing mode. This result is first obtained from the structure of the turning points of these two instabilities and is confirmed by a numerical solution of the full radial eigenvalue problem. The dependence of the growth rate of this instability on shear, curvature, collisionality and ion temperature gradient is discussed – Comparisons are made between characteristics of this instability and the low-frequency fluctuations observed on the Culham Levitron.

Identification of ion cyclotron emission from fusion alpha particles during JET deuterium-tritium experiments

Summary form only given, as follows. In the course of the preliminary tritium experiment (PTE) in JET, where combined D and T neutral beam injection generated a DT fusion power of 1.7 MW, ion cyclotron emission (ICE) in the frequency range up to 180 MHz was measured using an ICRF heating antenna as probe. The ICE spectrum showed a number of superthermal narrow, equally spaced emission lines which correspond to deuteron or alpha -particle ion cyclotron harmonics at the outer mid-plane edge (major radius R approximately 4 m). DD and DT power spectra are similar in form, and show split lines which, above approximately 100 MHz, merge into a continuum. With tritium injection, the ICE power increased by a factor comparable to the increase of neutron flux, indicating that 3.5 MeV fusion alpha -particles provide the free energy for generating ICE. The ICE power increases almost linearly with neutron flux over a range of six decades. The evolution of the ICE follows the rise of the neutron flux, but is delayed by a time comparable with the slowing-down time of the alpha -particles. The ICE intensity is anti-correlated with large amplitude edge-localized modes.

Comparison of theory with electron cyclotron current drive experiments on WT-2

A simple analytical model of the non-Maxwellian electron velocity distribution, whose parameters are constrained by the observed plasma current and soft X-ray emission spectrum, is used in conjunction with a fully three-dimensional ray tracing code to calculate the absorption of radiation in the electron cyclotron range of frequencies by WT-2 discharges. This gives a quantitative, theoretical description of the way in which a drifting population of energetic electrons appears to dominate WT-2 current drive experiments. Calculation of the proportion of source power absorbed by the plasma helps to determine the efficiency factor of the current drive mechanism in this context. The strong dependence of current drive on the extraordinary mode launch direction is given theoretical support.