Transient coaxial helicity injection for solenoid-free plasma startup in HIT-II

Abstract

The favorable properties of the spherical torus ͑ST͒ arise from its very small aspect ratio. Methods for initiating the plasma current without relying on induction from a central solenoid are essential for the viability of the ST concept. In steady state tokamaks, the central solenoid can be dispensed with if suitable methods for initiating the plasma current are on hand. Coaxial helicity injection ͑CHI͒ is a promising candidate for solenoid-free plasma current startup in STs and tokamaks. Experiments on the Helicity Injected Torus ͑HIT-II͒ machine at the University of Washington ͓T. R. Jarboe, Fusion Technol. 15, 7 ͑1989͔͒ have demonstrated the capability of a new method, referred to as transient CHI, to produce a high quality closed-flux equilibrium that has been successfully coupled to induction demonstrating that this new plasma current startup method is compatible with the conventional inductive method. This paper presents physics requirements for implementing this method in STs and tokamaks and supporting experimental results from the HIT-II device.

Overview of the physics and engineering design of NSTX upgrade

“…1 as required), and the flux amplification [147] which is the ratio of closed poloidal flux to injector flux = ψ p /ψ inj (black circles) is approximately 0.8. In the calculation of I bb , the flux footprint width d = 11 cm is 20% higher than the minimum value in HIT-II of 9 cm [148] and is a function of device CHI injector geometry. The ratio I P /MI inj (orange stars) is approximately 0.5 and is consistent with the inequality in equation (2).…”

Solenoid-free plasma startup in NSTX using transient CHI

“…The discharge with three capacitors also had increased levels of absorber arc current, which occurs when part of the injector current bridges the upper divertor gap and flows along the upper insulator surface, thereby producing additional contaminants from the absorber region. In principle, as demonstrated on HIT-II [5], the absorber arc can be minimized or eliminated through control of the magnetic field pattern in the upper divertor region to minimize the amount of poloidal flux that connects the upper divertor plates.…”

“…With three capacitors, however, the spectroscopic signals are significantly elevated and so is the radiated power signal. As demonstrated in experiments on HIT-II [5], CHI discharges cannot couple to induction if the radiated power approaches the input Ohmic power. For these plasmas with about 3 -4 V of applied loop voltage and a plasma current of 100 kA, the input Ohmic power is less than 300 -400 kW during the inductive coupling phase.…”

“…However, in a significant development during the past few years, it was shown that for the purpose of plasma startup, axisymmetric reconnection can produce a high quality startup equilibrium. This new method, referred to as transient CHI, was first demonstrated on the HIT-II experiment at the University of Washington [5]. The method has now been successfully used on NSTX for a demonstration of solenoidfree plasma start up and successful coupling to subsequent inductive drive.…”

Solenoid-free Plasma Start-up in NSTX using Transient CHI

“…With these techniques, NSTX discharges should be able to achieve reductions in the inductive flux consumption. It is useful to note that in HIT-II nearly the entire CHI started current is retained in the subsequent inductive ramp [5].…”