Fusion-born Alpha Particles Research Articles

First Gamma-Ray Measurements of Fusion Alpha Particles in JET Trace Tritium Experiments

Gamma-ray spectra from nuclear reactions between fusion-born alpha (alpha) particles and Be impurities were measured for the first time in deuterium-tritium plasmas in the Joint European Torus. The time dependence of the measured spectra allowed the determination of the density evolution of fast alpha particles. Correlation between the decay time of the gamma-ray emission and the plasma parameters in different plasma scenarios was established. Results are consistent with classical slowing down of the alpha particles in discharges with high plasma currents and monotonic q-profiles. In low plasma current discharges and in the discharges with large on-axis current holes (extreme reversal central magnetic shear), the gamma-ray emission decay times are shorter than the classical slowing down times, indicating an alpha-particle confinement degradation in such discharges in line with theoretical predictions.

Alfvén mode stability and wave–particle interaction in the JET tokamak: prospects for scenario development and control schemes in burning plasma experiments**Based on an invited talk given at the 8th International Atomic Energy Agency Technical Meeting on Energetic Particles in Magnetic Confinement Systems San Diego, USA, 6–8 October 2003.

We have investigated the effect of different ion cyclotron resonance frequency (ICRF) heating schemes, of error field modes, of the plasma shape and edge magnetic shear, and of the ion ∇B drift direction on the stability of Alfvén eigenmodes (AEs). The use of multi-frequency or 2nd harmonic minority ICRF heating at high plasma density gives rise to a lower fast ion pressure gradient in the plasma core and to a reduced mode activity in the Alfvén frequency range. Externally excited low-amplitude error fields lead to a much larger AE instability threshold, which we attribute to a moderate radial redistribution of the fast ions. The edge plasma shape has a clear stabilizing effect on high-n, radially localized AEs. The damping rate of n = 1 toroidal AEs is a factor 3 higher when the ion ∇B drift is directed towards the divertor. These results represent a useful step towards the extrapolation of current scenarios to the inclusion of fusion-born alpha particles in ITER, with possible application for feedback control schemes for the various ITER operating regimes.

Alpha-tail production with ion-cyclotron-resonance heating of 4He-beam ions in JET plasmas.

Third-harmonic ion-cyclotron-resonance heating of 4He-beam ions has produced for the first time on the JET tokamak high-energy populations of 4He ions to simulate 3.5 MeV fusion-born alpha (alpha) particles. Acceleration of 4He ions to the MeV energy range is confirmed by gamma-ray emission from the nuclear reaction 9Be(alpha,ngamma) 12C and excitation of Alfvén eigenmodes. Concomitant electron heating and sawtooth stabilization are observed. The scheme could be used in next-step tokamaks to gain information on trapped alpha particles and to test alpha diagnostics in the early nonactivated phase of operation.

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 Alfvén 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.

Analytic expressions of neoclassical transport coefficients for fusion-born alpha particles in a tokamak reactor

Analytic approximate expressions of neoclassical transport coefficients for energetic alpha particles are presented. The energy scattering effect, as well as the usual slowing-down and pitch angle scattering effects, is retained. The formulas are valid for arbitrary aspect ratios and agree well with the previous results, which numerically evaluated the coefficients using an eigenfunction expansion technique. Application of the newly developed formulas to the International Thermonuclear Experimental Reactor Design Activity (ITER EDA) [J. C. Wesley and ITER Joint Central Team, Phys. Plasmas 4, 2642 (1997)] plasma condition is also given.

Variational calculation of alpha-driven bootstrap current in a deutrium–tritium tokamak reactor

A general analytic expression for bootstrap current driven by fusion-born alpha particles is obtained in a deutrium–tritium tokamak plasma using a kinetic variational theory. A self-adjoint form of linearized Coulomb scattering operator has been used, which includes both energy and pitch angle collisions. The formula is applied to predict alpha-driven bootstrap current in a reactor condition similar to that of the International Thermonuclear Experimental Reactor Conceptual Design (ITER CDA) parameters [ITER Documentation Series, Number 18 (International Atomic Energy Agency, Vienna, 1991)]. The result agrees well with results obtained using another technique [Hsu et al., Phys. Fluids B 4, 4023 (1992)].

Bootstrap current induced by fusion born alpha particles

The bootstrap current produced by fusion born alpha particles is obtained, retaining effects of slowing down drag, pitch angle scattering, and arbitrary aspect ratio. The result is presented both as a summation of a rapidly converging series and a simple Padé approximation good for arbitrary aspect ratio. Quantitative results are derived using the International Thermonuclear Experimental Reactor (ITER) [Plasma Phys. Controlled Nucl. Fusion Res. (International Atomic Energy Agency, Vienna, 1989), Vol. 3, p. 214] parameters.

Quasilinear diffusion modeling of the alpha-loss-cone instability in a tandem-mirror reactor

The anisotropic loss-cone distribution of fusion-born alpha particles in a mirror system is capable of destabilizing low-frequency plasma waves. A one-and-one-half-dimensional quasilinear diffusion model is developed to study the nonlinear evolution of the alpha distribution function resulting from enhanced diffusion in velocity space and radial position driven by the unstable Alfvén wave. It is found that pitch-angle scattering of the resonant alphas into the loss cone causes significant losses while radial losses are negligible for a reactor-size plasma. For a typical tandem-mirror reactor, the alpha plasma heating is greatly reduced and ignition in the central cell cannot be achieved.

Analysis of low Za impurity pellet ablation for fusion diagnostic studies

The possibility of using low atomic number (Za) impurity pellets such as carbon, lithium and beryllium as diagnostic probes for fusion grade plasmas is investigated using an ablation model based on the vapour shielding phenomenon. It is shown that pellet injection velocities of approximately six kilometres per second would be required for penetration of a 1 mm sized carbon (diamond) pellet half-way into a CIT-like plasma. Properties of the high density ablation cloud surrounding a low Za pellet are discussed in relation to a diagnostic scheme that uses double charge-exchange interactions of fusion-born alpha particles with the ablation cloud to determine the local alpha energy distribution function. Pellet lifetime results are given and compared with experimental ones.

Anomalous alpha-particle transport in thermonuclear tokamak plasma

Because of the strong localization of the fusion-born alpha particles in velocity and configuration space and their coupling to Alfvén waves in the background plasma, the relaxation of alphas is anomalous. In a finite system, the enhanced electromagnetic fluctuations can produce rapid spatial losses of alpha population and energy. These losses prevent the alpha velocity distribution from attaining a stable collisional equilibrium, thus maintaining a steady-state turbulence level. A self-consistent numerical quasilinear calculation is performed for one of the most dominant low-frequency modes, showing the evolution of the alpha distribution and yielding the anomalous loss rates.

Plasma heating by energetic particles

The interaction of a tenuous energetic test-particle species with a multispecies high temperature plasma is calculated. The Balescu-Lenard kinetic equation is used in order to include collective effects through the dielectric constant. Quantum corrections are made for close collisions. The theory is first applied to the slowing down of fusion born alpha particles in a mirror-confined plasma and theory and numerical results are compared to previous treatments and corrections are found. In Tokamak-like plasmas most of the alpha-particle energy goes into the electrons and the thermalization time is somewhat larger than the plasma lifetime but heating can nevertheless be substantial. Heating of a Tokamak-like plasma by injection of energetic neutrals is shown to be effective at injection energies ≤ 70 keV, possibly doubling the ion temperature when the injected particle density reaches 1% of the plasma density. For analytic estimates, a simple binary collision model for injection heating is given.