Fast ion losses due to toroidal field ripple in JT-60U

Abstract

A previous experiment in JT-60U supported an orbit following Monte Carlo (OFMC) calculation regarding ripple trapped loss, and the present experiment, furthermore, suggests that the OFMC also predicts banana drift loss fairly well. In the experiment presented, the total fast ion losses due to toroidal field ripple were estimated from the decay in neutron emission following a short neutral beam injection (90 keV, D). The neutron decay for co-passing beam injection showed a diffusivity of about 0 m2/s, which indicates no fast ion loss. In contrast, the neutron decay for trapped particle injection exhibited characteristic enhancement of fast ion losses due to toroidal field ripple: the fast ion losses consisted of ripple trapped convection and ripple banana diffusion in the low collisionality regime. The OFMC calculation reconstructed completely the experimental neutron decay irrespective of the total ripple losses and the fraction of banana drift loss. Considering the previous work on ripple trapped loss and this result, it can be concluded that the OFMC code gives a good quantitative estimation of banana drift loss as well as ripple trapped loss

Reduction in Neutral Beam Driven Current in a Tokamak by Tearing Modes

“…Although several experiments have clearly established the existence of beam-driven currents [1][2][3][4] there are no reported measurements comparing profiles of neutral beam-driven currents to those predicted by theory-due to the lack of reliable current profile measurements until now. (The earlier studies [1][2][3][4] compared external measurements of the loop voltage with expected values, but the predicted voltage is insensitive to spatial transport of the beam ions [8].) In this Letter, we present the first detailed measurement of the NBCD profile over a variety of conditions in a large tokamak.…”

“…The confinement of beam ions is scarcely affected by short wavelength microturbulence [6], and diffusion coefficients of ,0.1 m 2 ͞sec are observed [5,7] in plasmas without long-wavelength magnetic field perturbations. In contrast, rapid beam-ion transport is observed in plasmas with large toroidalfield ripple [5,8] or magnetohydrodynamic (MHD) instabilities [5].…”

Measurements of classical transport of fast ions

“…[9][10][11][12][13][14] Also the slowing-down of a barium ion beam with energy 0-40 eV was measured in a Q-machine in which the quiescent plasma ͑T e Ϸ 0.2 eV͒ is confined by a uniform magnetic field. 15 The changes in velocity ͑due to beam slowing-down͒ agree with the predictions of the Fokker-Planck equation within error bars for different ions at different densities.…”

Experimental Check of Deceleration of Neutral Beam-Injected Energetic Ions in the HL-2A Tokamak

“…This method is often called "beam blip" and has been widely used in magnetically confined plasma experiments, i.e. DIII-D, TFTR, JT-60U and CHS [4][5][6][7]. Because in the HL-2A condition the total neutron yield is dominated by neutrons produced by the beam-plasma reaction, the neutron decay rate after beam blip provides in- formation about deceleration and/or dilution of beam ions.…”