Requirements for fast particle measurements on ITER and candidate measurement techniques

Recent JET and JT-60U results, have underlined the need to look into the measurement requirements of fast particles diagnostics in ITER. Not only alpha particles but all fast ions (He4,p,D,T,He3) present in the plasma should be diagnosed. Furthermore, the electron diagnostics, monitoring the deviations from a Maxwellian distribution function, play a central role in the electron cyclotron heating and current drive. In the present ITER measurement requirements only alpha particles and He ions are mentioned: space resolution is δr = a/10 (a is the minor radius), time resolution is δτ = 100 ms and accuracy 10-30%. The new proposed measurement requirements for fast particles are spatial resolution ∼a/20 (10 cm on ITER), time resolution (minimum) ∼100 τA (∼100 μs for ITER), density of fast ions: the minority ions could be 4-10% of the plasma density. The candidate diagnostic techniques considered for ITER are amongst others γ-ray spectroscopy, fast ion collective Thomson scattering (CTS) and charge exchange recombination spectroscopy (CXRS). γ-ray spectroscopy detects ions in the energy range 1 < E < 5 MeV and R and D is needed to demonstrate the feasibility of these measurements in the presence of a neutron background. Techniques using ultraviolet spectroscopy of ions such as Krypton are being tested, for the low energy part of spectrum of fast particles. A design study of CTS shows that the requirements are closely met; the system includes a high power gyrotron operating in a region not yet tested in current applications (i.e. using a gyrotron at a frequency below the first ECE harmonic). For CXRS the spatial resolution and accuracy are not achievable for r/a < 0.3 and R and D is needed to assess the minimum figures in terms of accuracy possible for measurements close to the centre. The measurement requirements deduced from the need for detecting the effects of interaction of fast particles with MHD instabilities put forward important new objectives to the candidate diagnostic systems. In particular diagnostics for fast particles in the energy region 200 < E < 1300 keV and diagnostics for escaping fast particles need urgent development