Review of confinement and transport studies in the TJ-II flexible heliac

Abstract

TJ-II is a four period, low magnetic shear stellarator (R = 1.5 m, a < 0.22 m, B 0 ≤ 1.2 T) which was designed to have a high degree of magnetic configuration flexibility. In the last experimental campaign, coupling of the full ECH power (P ECRH ≤ 600 kW) to the plasma has been possible using two ECRH transmission lines which have different power densities. Both helium and hydrogen fuelled plasmas have been investigated. This paper reviews the latest physics results in particle control, configuration effects, and transport and fluctuation studies.

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

“…As TJ-II can be operated without current it was reasonable to assume that vacuum and real magnetic configurations concur. 10 Hence, the alignment procedures described here, together with the response of phosphor screen as a function of ion energy, give calibrated data that can be compared to future theoretical simulations.…”

“…The TJ-II device 10 is a four-period, low magnetic shear, stellarator with a major radius of 1.5 m and an average minor radius of Յ0.22 m. At present, central electron densities and temperatures up to 1.7ϫ 10 19 m −3 and 2 keV, respectively, are achieved in plasmas created and maintained by ECRH using a frequency of 53.2 GHz tuned to the second harmonic, P ecrh Յ 600 kW. Additional heating is available from two NBIs that are operated in co-and counterinjection.…”

“…4,5 Fast ion confinement and loss mechanisms are controlled by effects associated with collisional transport, magnetic configuration peculiarities, heating method, and MHD activity originated by themselves or by other independent mechanisms. [6][7][8][9] A new diagnostic, based on the ionoluminescence of a phosphor screen, has been installed in the TJ-II stellarator 10 for quantifying fast ion losses close to the plasma edge. This detector acts as a magnetic spectrometer where incident fast ions disperse, after passing through a pinhole aperture, onto a phosphor screen with a distribution that depends on ion gyroradius and pitch angle.…”

Results from helical axis stellarators

“…Experimentally, the TJ-II result 26 of T e0 ϭ2 keV, with electron densities up to 1.2ϫ10 19 /m 3 and stored energies up to 1500 J, stands out as the best overall indicator of good confinement in helical axis machines to date. In a heliac, in particular, the drift orbits deviate significantly from magnetic surfaces, causing poor confinement in the absence of radial electric field.…”

“…This is in a region of high pressure gradient, and may be indicative of resistive ballooning instabilities. 26 Neutral beam injection in TJ-II ͑2001͒ and Heliotron J͑FY2000-2͒ should provide access to sufficiently high ␤Ͼ1% to probe this important physics regime more thoroughly.…”