1 - Nuclear Sciences and Applications - IAEA

344 DeGRASSIE et al.
CJ
CO
a
23
21
19
APERTURE
229kA
lp = 209 kA
v ;
c ' Y - • ' •• ; •••..•••••••':•-.•*.
0 30 60 90 120 150 180 0 30 60 90 120 150 180
POLOIDAL ANGLE (DEGREE) POLOIDAL ANGLE (DEGREE)
FIG. 3. Mappings of field structure at the two times indicated in Fig. 2. The m = 7/n = 2 island is
seen to pass by the limiter aperture during the period of increased plasma collection.
This effect is clearly a resonant phenomenon, as expected, in that
the increase occurs only at q values near the resonant q value. (At
q — 3.2 on the limiter front face, q ~ 3.5 at the limiter aperture.) Varying
the limiter radius requires a different Ip to achieve the resonant q as
expected. Reversing the direction of In places an x point on the aperture
rather than the o point for all conditions the same. This reversed
situation gives no observable pressure rise.
Interestingly, the pressure increase is greater in helium discharges
than in hydrogen. Empirically, TEXT has found better particle confinement
time with helium compared to hydrogen, indicating a lower intrinsic
cross-field transport in helium. Any transport mechanism which
moves particles across the field lines competes with the desired RID
particle control.
Measurements with the infrared camera confirm that heat is diverted
off the limiter face in the presence of the magnetic island. Also,
a fiber optic threaded down the limiter shaft viewing the region inside
the aperture collector shows a significant increase in the ha recycling
h'ght due to the enhanced plasma collection.
Although the pumped limiter enhancements are qualitatively as
predicted, one cannot justify the magnitude given the small island
widths in TEXT. The measured intrinsic cross-field diffusion should
transport an ion across an island long before it circulates the island
due to motion parallel to the field lines, thus washing out any particle
channeling action. Presently, our working model is that island induced
perturbations in the electrostatic field cause ions to circulate the islands
much more rapidly due to the SE X B drift, being independent
of parallel motion and transporting impurities as rapidly as the pro-