Neutron and hard X-ray measurements during pellet deposition in TFTR

Abstract

ABSTRACT. Measurements of neutrons and hard X-rays are made with a pair of plastic scintillators during injection of deuterium pellets into deuterium TFTR plasmas. Three cases are investigated. During Ohmic heating in plasmas with few runaway electrons, the neutron emission does not increase when a pellet is injected, indicating that strong acceleration of the pellet ions does not occur. In Ohmic plasmas with low but detectable levels of runaway electrons, an X-ray burst is observed on a detector near the pellet injector as the pellet ablates, while a detector displaced 126° toroidally from the injector does not measure a synchronous burst. Reduced pellet penetration correlates with the presence of X-ray emission, suggesting that the origin of the burst is bremsstrahlung from runaway electrons that strike the solid pellet. In deuterium beam heated discharges, an increase in the d-d neutron emission is observed when the pellet ablates. In this case, the increase is due to fusion reactions between beam ions and the high density neutral and plasma cloud produced by ablation of the pellet; this localized density perturbation equilibrates in about 700 us. Analysis of the data indicates that the density propagates without forming a sharp shock front with a rapid initial propagation velocity (>2 X 10 7 cm -s" 1 ) that subsequently decreases to around 3 X 10 6 cm-s" 1 . Modelling suggests that the electron heat flux into the pellet cloud is much less than the classical Spitzer value.

Local-density increment from an ablated deuterium pellet in the TFTR tokamak

“…This is twice the ion acoustic speed for 100 eV D 2 and is reasonably consistent with the speed predicted in Ref. 5 as well as with previous measurements of ablatant speed on TFTR [6].…”

Homogenization of the pellet ablated material in tokamaks taking into account the ∇B-induced drift

“…The equilibration times of the temperatures are ≈100 µs for the electrons and ≈250 µs for the ions (figure 8(a)), much shorter than that of the density (and pressure), which is τ hom ≈ 800 µs (figure 8(b)). This last value corresponds well to the experimental value, estimated to be ≈700 µs in TFTR [22]. The ratio of the kinetic pressures, p 0 /p ∞ , is ≈25 at t = 0, decreasing down to ≈3 in a few tens of µs (figure 8(c)), in agreement with the dynamic measured in AUG [23].…”

“…Then, it decreases rapidly due to the increasing magnitude of the viscous effects. The duration of the fast expansion phase is ≈100 µs with a maximum velocity of ≈1.5×10 5 m s −1 (for comparison, in TFTR, the propagation speed of the density perturbation consecutive to the injection of a pellet has been measured to be >2×10 5 m s −1 , decreasing down to 3 × 10 4 m s −1 after 200 µs [22]). The time histories of L a and Z 0 are displayed in figure 8(e).…”

The behaviour of fast ions in tokamak experiments

“…Sometimes deconvolution of the spectrum of fusion products allows separation of the various contributions to the reaction rate [40]. Injection of a deuterium pellet into the plasma allows separation of the beam-plasma component [428,429], but this technique is seldom employed. Often the thermonuclear emission is negligible but, in some of the most interesting plasmas, uncertainty in the magnitude of the thermonuclear emission complicates interpretation of the measurements.…”