Hydrogen recycling and its control on HL-1 tokamak

Abstract

Particle recycling significantly affects the present operation of the HL-1 tokamak device. Recycling behavior and its effect on particle confinement are surveyed under different operational conditions in this paper. Global hydrogen recycling coefficient, R , ranges from 0.8 to 0.9. The fuelling ratio is quite low (20–30%). Various techniques of recycling control have been adopted to improve particle confinement, and consequently, to extend the operating regime. The optimized conditioning procedure, vessel bakeout at 180–200°C concurrent with ac (50 Hz) discharge cleaning with H 2 +Kr mixed gas, produces R as low as 0.5–0.6. Wall modification by titanium film saturated with H 2 , carbonization or inert gas puffing enhances the wall fuelling and increases the time constant of the density decay τ p * by a factor of 3, where τ p * = τ p /(1− R ) and τ p is the particle confinement time. H α emission near the limiter shows strong density and plasma current scaling (α n e 1.5 / I p 2 ). Two relative steps of discharge control during current ramp-up and flat-top phase were taken. The pump limiter decreases R by 13%. The highest density, 1.08×10 20 m −3 , is achieved and the Murakami parameter is raised up to 4.66×10 19 m −2 T −1 by using the above mentioned techniques.