1 - Nuclear Sciences and Applications - IAEA

250 JET TEAM
pulses previously. Tuning to hydrogen minority doubled the
neutron rate. Whilst the increased neutron emission could be due
to the formation of a high energy deuteron tail by 2wcd ICRH,
only 12% of the neutrons can be identified beyond 2.8MeV (which
corresponds to the NBI energy) in the liquid scintillator
spectrum.
During the first ^second of ICRH the y-spectrum was
dominated by 12 C(p,p') 12 C (threshold 4.8MeV). Only Ln the las£
second, when the neutron rate had fallen, did C(d,p) C
(threshold 0.36MeV) take over. Thus the increased emission
cannot be due to high energy deuterons. A possible explanation
for the extra neutrons is the effective reduction of the NBI
slowing down rate by the RF fields.
The neutron emission of other high temperature pulses can
be accounted for by the beam-beam, beam-thermal and
thermal-thermal deuteron reactions. Of these, the beam-thermal
interactions are the most important and account, typically, for
60% of the neutrons. The beam-beam neutrons are only significant
during the low density period of the heating pulse and provide
«10-50% of the maximum emission.
The interpretation of the neutron yield measurements
requires that the deuteron concentration in the high temperature
plasmas is rather low, with na/ivsO.25-0.45 being typical.
Analyses of other diagnostics such as visible Bremsstrahlung,
neutron spectra, triton burn-up and charge exchange
recombination radiation give similar values.
5. FAST PARTICLES IN PLASMAS
The energy confinement of ICRH minority ions and the
burn-up of tritons produced by d-d fusion reactions test much of
the important physics of a-particle heating in thermonuclear
plasmas. Not only are the Larmor radii of these ions similar to
those of d-t oc's but also, in the case of the ICRH fast ions,
nfast/ne (*10~ 2 ) , pfast (»i%) and vfaBt/vAlfv4n (>1) are in
the appropriate range. However, d-t a's have an isotropic 1/v
distribution whereas the ICRH minority ions have a
quasi-Maxwellian anisotropic (vji_/V||«30) distribution. The
anisotropy of the ICRH distribution might be expected to provide
rigorous tests for the appearance of loss-cone instabilities,
fishbones and stochastic diffusion.
The energy density contained in fast minority particles
should be TsPrf/2, where Prf is the absorbed power density and
TS the minority slowing-down time on the electrons. The
perpendicular energy contained in the minority particles can be