Dependence of the confinement of fast ions generated by ICRF heating on the field configuration in Heliotron J

Abstract

A formation and confinement experiment for fast ions is performed using the ion cyclotron range of frequencies (ICRF) minority heating scheme with a proton minority and a deuteron majority in Heliotron J, a low-shear helical-axis heliotron. The effect of the magnetic configuration on the fast-ion confinement is one of the most important issues in helical devices. In this paper, the effect of bumpiness, one of the Fourier components of the field strength, on the trapped fast-ion confinement is clarified by using ICRF minority heating. The role of the bumpiness is a key issue for the design principle of the magnetic field of Heliotron J, where the particle confinement is controlled by the bumpiness. Here, the bumpiness or the bumpy ripple is defined by the Fourier harmonic ratio εb ≡ B 04/B 00 in (Mizuuchi T et al 2006 Fusion Sci. Technol. 50 352), where B 04 is the bumpy component and B 00 is the averaged magnetic field strength. The proper bumpiness causes deeply trapped particles to be confined in the small grad-B region. High-energy ions are produced up to 10 keV by injecting an ICRF pulse into an electron cyclotron heating target plasma where ion temperature at the centre T i(0) = 0.2 keV, electron temperature at the centre T e(0) = 0.8 keV and line-averaged electron density . For the study of the configuration dependence of the fast particle confinement, three configurations are selected; the bumpy ripples are 0.01, 0.06 and 0.15 at the normalized minor radius ρ = 0.67. The measured tail temperatures by using a charge-exchange neutral energy analyser are 1.10 keV, 0.88 keV and 0.50 keV for the ripples of 0.15, 0.06 and 0.01, respectively. The heating efficiency of the bulk ion is also better in the high bumpy case. A Monte-Carlo analysis also indicates good confinement of the high energy ions in the high bumpy case although the difference is not very large compared with the experiment.

Numerical Analysis of ICRF Minority Heating in Helitoron J

“…3 of Ref. [6]). The magnetic field and the ICRF frequency are selected such that the ECH resonance is located on the plasma axis.…”

“…Good confinement of fast ions and high-efficiency of ICRF heating for a high bumpiness have been achieved [6,7]. of the energy spectrum for three different bumpinesses.…”

“…The toroidal ripple (bumpiness) of the magnetic field strength is one of key parameters for enhancing confinement in the Heliotron J configuration [3]. The fast-ion velocity distribution has previously been investigated using fast protons generated by ion cyclotron range of frequencies (ICRF) proton-minority heating in Heliotron J with special emphasis on the effect of the bumpiness and the heating position [4][5][6][7]. In the present study, the role of the bumpiness is investigated by measuring fast ions generated by ICRF heating and by performing Monte-Carlo simulations.…”

“…In the present study, the role of the bumpiness is investigated by measuring fast ions generated by ICRF heating and by performing Monte-Carlo simulations. Initial results for Monte-Carlo simulations of fast ions have been obtained and the effects of the configuration have been discussed [6]. This paper presents energy spectra and compares the effective temperatures of minority ions.…”

A flexible luminescent probe to monitor fast ion losses at the edge of the TJ-II stellarator

“…Therefore it is a primary challenge for experimentalists to estimate ͑or gather information on͒ the population of such ions and their distribution function, either with regard to their properties in the plasma interior or the loss component, since collisions with mechanical structure parts in the vacuum chamber could accelerate damage or contribute to plasma pollution by impurities. The behavior of fast ion confinement in medium sized stellarator devices can be benchmarked by using either the suprathermal ion population created by microwave heating 1,2 or by monitoring the nonthermalized fast ions associated with the NBI system. 3 Questions related to the dynamics of fast particles in driving magnetohydrodynamics ͑MHD͒ phenomena or in the generation of electric fields in plasmas constitute high-priority research efforts in plasma devices such as tokamaks and stellarators.…”