DIAGNOSTICS OF CONFINED ENERGETIC IONS

Diagnostics of energetic ions and energetic ion-driven instabilities are crucial for burning plasmas. Development of such diagnostics is challenging as the diagnostics need to perform accurate simultaneous measurements of several populations of energetic ions, confined and lost, under the harsh conditions of D-T plasma. In addition, reliable techniques for detecting Alfvenic instabilities coupled to the energetic ions via wave-particle resonances are required as these instabilities may affect the transport and losses of energetic ions.

This section reviews the diagnostic techniques for energetic ions employed on tokamaks.

Measurements of Energetic Ion Distribution with a Neutral Particle Analyser

Measurements of confined energetic ions are often carried out by analysing flux of neutrals escaping from plasma. The well-known instrument for this is the neutral particle analyser (NPA), which relies on the neutralisation of the energetic ions in charge-exchange reactions with some electron donors in the plasma. When the electron donors are naturally present in the plasma, for example, carbon or beryllium impurities, the NPA measurements are passive. If a source of electron donors is used, for example, NBI, then the measurements are active. The escaping neutrals are re-ionised in gas cells or with stripping foils at the entrance to NPA, and deflected by magnetic and electric fields to determine their energy and mass. The NPA measurements are intended: (1) to provide experimental testing and validation of theories of ICRH and NBI heating scenarios, (2) to quantify the link between energetic ions and MHD instabilities, and (3) to investigate confinement and slowing down of charged fusion products.

For example, consider NPA for high energy, 0.3<£ (MeV)<3.5, hydrogen and helium isotope fluxes [3.10] on JET. This NPA is located at the top of the torus with its vertical line-of-sight intersecting the ICRH power deposition region and lower energy (Octant 4) NBI at the plasma centre. The energy distribution function/(£) integrated along the line-of-sight is measured for ions with pitch angle <5 x 10^-3.

Procedures for the deduction of the local effective temperature and minority density in the case of an anisotropic distribution function of ICRH-accelerated ions from the measured/(£) were developed for NPA in such geometry in Ref. [3.11]. The NPA measurements of the energy distribution functions of ICRH-accelerated ions have resulted in corroboration of a linear increase in the perpendicular temperature of the minority ions with the ICRH power density [3.11], as predicted by the Stix model [3.5]. Typical NPA measurements are shown in Figure 3.3 for JET discharge, in which both H-minority ICRH and D-NBI were used [3.12]. Figure 3.3 demonstrates a reduction in the tail temperature of the H-minority distribution function from 243 to 160 keV, after deuterium NBI with starting energy of 140 keV starts at 12 s. Starting from that time, a high energy tail builds up to

FIGURE 3.3 Evolution of ICRH-accelerated proton distribution function during combined ICRH and deuterium NBI. Distribution function of the second harmonic ICRH-driven beam deuterons is also shown.

173 keV in the deuterium energy distribution, implying a channelling of the ICRH power from the first harmonic heating of H-minority to the second harmonic ICRH of the D beam.

Measurements of fusion alpha-particles were made with the NPA during JET D-T campaign in 1997 [3.13], in a high fusion yield H-mode discharges with NBI heating only, comprising 80 keV D atoms and 140 keV T atoms, giving a D-T fuel mixture of np/(n_D+nj)~0.5. The measurements were carried out with the NPA which was set up to detect ⁴He atoms in eight channels in the energy range 0.8-3.0 MeV. For the DT plasma analysed, two surprising findings were observed: (1) the flux of neutrals corresponding to the helium mass-to-charge ratio was an order of magnitude higher than expected, and (2) the low energy flux of neutrals corresponding to helium was seen almost immediately after NBI power was switched on, that is, well before the fusion-generated alpha particles could slow down to the low energy range of the NPA measurements. The analysis [3.13] has identified the excessive flux as that of high energy deuterons accelerated by close elastic collisions (knock-on) between fusion alpha particles and thermal D ions [3.14]. Despite the low density of knock-on deuterons compared to that of alpha particles (0.0025 d(E)/f_a(E)< 0.07), a flux of deuterium atoms to the low-energy NPA channels exceeding the helium flux could be produced because of the much higher (about a factor of 100) neutralisation probability for the single-charged deuterium ions than for double-charged alpha-particles. The measured distribution functions of the helium-like ions [3.13], were later shown to be in a satisfactory agreement with the Fokker-Planck code FPP-3D, which incorporates neoclassical transport and classical slowing down of fusion alpha- particles and the knock-on deuterons [3.15]. Figure 3.4 illustrates the comparison.

FIGURE 3.4 Distribution functions of fusion alpha particles and knock-on deuterons: deduced from NPA measurements (points) and modelling assuming classical confinement and slowing down (curves). Circles and squares show the results when the maximum and minimum values for neutralisation probability are used.