Fast ion confinement in the three-dimensional helical reversed-field pinch

Abstract

Fast ions are well confined in the stochastic magnetic field of the multiple-helicity (MH) reversed-field pinch (RFP), with fast ion confinement times routinely a factor of 5 to 10 higher than thermal confinement time. Recent experiments have examined the behavior and confinement of beam-born fast ions in the three-dimensional (3D) helical RFP state. In lower current discharges, where the onset of the helical state is uncertain, high power neutral beam injection (NBI) tends to suppress the transition to the single helicity mode. In high current discharges (I p ∼ 0.5 MA), where the onset of n = 5 single helicity is quite robust, a short blip of NBI is used to probe the confinement of fast ions with minimal perturbation to the 3D equilibrium. The fast ion confinement time is measured to be substantially lower than fast ions in comparable MH RFP states, and there is a strong dependence on the strength of the helical perturbation. The established helical equilibrium is stationary in the laboratory frame but the locking occurs over the entire range of possible phase with respect to the Madison Symmetric Torus vessel. This effectively scans both the location of the NBI with respect to the helical structure and the pitch of the NBI-born fast ions. Fast ion confinement is observed to be insensitive to this angle, and in fact counter-NB injection into quasi-single helicity discharges shows fast ion confinement times similar to co-injection cases, in contrast to the MH RFP, where counter-injected fast ion confinement time is substantially lower.

Fast ion transport in the quasi-single helical reversed-field pinch

“…Similarly, neutral particle analysis measurements resulting from fast ion charge-exchange losses decrease during the helical state. 6 Both tangential and radial lines of the site show distinct decreases with the growth in the n ¼ 5 core-resonant mode amplitude. The neutral particle analyzer signal is a lineaveraged measurement highly sensitive to particle pitch and neutral density.…”

“…The NBI deposited fast ions in MST mimic alpha particles in a burning reactor and act as a testbed for energetic particle physics in the 3D-RFP. 6 In comparison to alpha particles in a RFP-like reactor, MST's NBI born ions possess a similar gyroradius relative to the device size ðq fi =a $ 0:1), superthermal speed ðv fi =v th $ 13Þ, and superalfv enic speed ðv fi =v A > 1Þ but form a localized pitch-angle distribution due to the injection geometry in comparison to alpha particles' isotropic distribution. 7 The MST device (R 0 ¼ 1.5 m and a ¼ 0.5 m) produces deuterium RFP plasmas with total current I p ¼ 200-600 kA, central field strength jBj ¼ 0:2 À 0:5 T, and confined electron temperature T e ð0Þ ¼ 200 À 2000 eV.…”

Effect of resonant magnetic perturbations on three dimensional equilibria in the Madison Symmetric Torus reversed-field pinch

“…The transition to a QSH state impacts also the confinement of fast ions, as described in Ref. 53. Here, it is shown that the helical equilibrium introduces neoclassical effects that reduce the fast ion confinement time with respect to the MH state.…”

Development towards a fast ion loss detector for the reversed field pinch

“…Indeed, Note: Contributed paper, published as part of the Proceedings of the 21st Topical Conference on High-Temperature Plasma Diagnostics, Madison, Wisconsin, USA, June 2016. a) Electronic mail: bonofiglo@wisc.edu neutral particle analysis and measured fusion neutron flux indicate enhanced fast ion transport in the plasma core at the RFP's transition to a 3D state and with beam-driven, bursting magnetic modes. [7][8][9] By including a FILD in our diagnostic suite, we wish to gain further insight into these events. This paper will discuss the design of a high-pitch FILD for a RFP in Section II and present the initial measurements and findings of the device in Section III.…”