Modelling of alpha particle slowing down, confinement and redistribution by sawteeth in TFTR using the FPPT code

Abstract

Results from recent deuterium-tritium (DT) experiments on TFTR t o measure the energy spectra and radial profiles of well trapped confined alpha particle distributions using the pellet charge exchange (PCX) diagnostic in quiescent plasmas are compared with a numerical modelling using the Fokker-Planck post-TRANSP (FPPT) processor code and show a classical slowing down behaviour of the alphas (Fisher, R. K., et al., Phys. Rev. Lett. 75 (1995) 846). However, in the presence of sawtooth oscillations PCX experimental data indicate a significant broadening of the trapped alpha radial distributions (Petrov, M.P., et al., Nucl. Fusion 35 (1995) 1437. Conventional models consistent with measured sawtooth effects on passing particles do not provide satisfactory simulations of the trapped particle mixing measured by the PCX diagnostic. A mechanism is proposed for fast particle mixing during the sawtooth crash to explain the trapped alpha particle radial profile broadening after the crash. The model is based on the fast particle orbit averaged toroidal drift in a perturbed helical electric field with an adjustable absolute value (similar to that in Kolesnichenko, Ya.I., et al., Nucl. Fusion 36 (1996) 159). Such a drift of the fast particles results in a change of their energy and a redistribution in phase space. To show the sensitivity of trapped particles to sawteeth, the redistribution in toroidal momentum Pv (or in minor radius) was calculated in two ways. The first is based on the assumption that Pp redistribution is stochastic with a large diffusion coefficient and was taken to be flat. The second way is to apply Kolesnichenko's inversion formula (Kolesnichenko, Ya.I., et al., Nucl. Fusion 32 (1992) 449). Both methods are compared with PCX data. The distribution function in a presawtooth plasma and its evolution in a post-sawtooth crash plasma are simulated using the FPPT code. It is shown that FPPT calculated alpha particle distributions are consistent with TRANSP Monte Carlo calculations. Comparison of FPPT results with PCX measurements shows good agreement for both sawtooth-free and sawtoothing plasmas.

Fast Ion Effects on Fishbones and n=1 Kinks in JET Simulated by a Non-perturbative NOVA-KN Code

“…Note that the increase in the mode frequency can be explained within the framework of the theory of the sawtooth effects on fast ions [12][13][14]. In that theory only low energetic fraction among the trapped fast ions is redistributed within the q £ 1 surface.…”

Alpha particle physics experiments in the Tokamak Fusion Test Reactor

“…In D-T experiments on TFTR, the behavior of fast confined alphas in the presence of sawtooth activity was measured using the PCX diagnostic which showed a strong depletion of the alpha core density and transport of the alphas radially outwards well beyond q = 1 surface after a sawtooth crash, as discussed in Sec. A model based on generation of a helical electric field during the crash time scale, τ cr ~ 10 -5 -10 -4 s, was incorporated in the FPPT code for simulation of the sawtooth redistribution observed by the PCX diagnostic [87]. This electric field leads to a change of the alpha energy due to this toroidal drift motion.…”

“…the Lorentz ORBIT code for alpha loss [8,57], the ORBIT code for TF ripple loss [13,15,17], the FPPT code for sawtooth redistribution [87,90], and the NOVA-K [126,134] and ORNL [136] models for TAE instability studies. The analysis functions of these codes are summarized briefly in Table 8.…”

Transport of energetic ions during relaxation oscillations in plasmas of spherical tori

“…A theoretical study of the effect of sawtooth oscillations on fast ions in CTs resulted in the prediction of the critical energy, E crit , such that the trapped ions with energy higher than E crit are not sensitive to the sawtooth crash (the crash is the relaxation phase of the oscillations), whereas the ions with E < E crit are strongly redistributed by the crash [9]. This prediction was confirmed experimentally [10].…”