ABSTRACT
The article presents a detailed investigation of the fast disruption mitigation valve developed at FZJ Juelich. The essence of this study is the novel direct observation of the piston motion by means of a fast framing camera. The piston stroke and the injection duration are shown to strongly depend on the operational pressure and the used gas. The same is true for the valve throughput. The analysis revealing the leading contribution of the injection duration in this modification is given. The knowledge of the injection duration is also used to reconstruct the characteristic pressure decay rates and the gas outflow rates. The means to increase the gas outflow are discussed. The main found valve characteristics are: (1) valve reaction time, i.e., the delay between the application of the trigger signal and the achievement of reliably observable opening 0.5 mm, is about 0.3 ms; (2) the maximum achieved throughput is 7.5 bar l for argon and 9.5 bar l for helium; (3) the maximum delivery rates are 500 bar l s(-1) for Ar and 1500 bar l s(-1) for He.
ABSTRACT
Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" (delta) of magnetic perturbations: delta<0.5 cm in Ohmic discharges, increasing to delta = 4.4 cm for 0. 6 MW NBI.
ABSTRACT
Confinement quality as good as ELM-free H-mode at densities substantially above the Greenwald density limit ( &nmacr;(e,0)/n(GW) = 1.4) has been obtained in discharges with a radiative boundary under quasistationary conditions for 20 times the energy confinement time. This is achieved by optimizing the gas-fueling rate of RI-mode discharges which tailors their favorable energy confinement and leads to discharges with beta values just below the operational limit beta(n) = 2 of TEXTOR-94, thereby effectively avoiding confinement back transitions or disruptions. In addition, this high-density regime is favorable for helium removal and results in figures of merit tau(*)(p,He)/tau(E) approximately 10-15, relevant for a future fusion power reactor.