Gas fueling studies in the PDX tokamak: II Page: 5 of 20
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4
We define the compression ratio, C, as the ratio, of the
pressure in the divertor chamber, P^, to the pressure in the main plasma
chamber, PM. The previous open divertor geometry was unable to sustain a
significant difference between Pp and P^. The compression ratio was of the
order of unity for this configuration for both H2 and Dj fueling, over a wide
range of gas influx rates (1*200 Torr £/s). However, only midplane qas
injection was investigated with the open divertor.
The closed divertor geometry, in contrast, can sustain a significant
compression ratio. Table 1 summarizes the achieved compression ratios in the
four cases shown in Fig. 2. Note that since the midplane pressure curves ar»
similar for all cases, the differences in C are the result of differences in
divertor chamber pressure.
The data show that C can be doubled (from 10 to 20) by injectinq the
gas into the divertor chamber instead of the usual midplane location. The
value of C could be lowered by gettering in the divertor chamber. For
example, only moderate compression ratios (C = 3) were measured in the lower
divertor which was gettered. In the case of H2, the isolation of the divertor
from the main chamber can be compromised by excessive gas feed, presumably
through too large a neutral or plasma ion flow through the divertor, and the
compression ratio collapses. This situation, termed a "blow-throuqh" in the
PDX literature[3), is identified by a large rise in midplane neutral pressure
and a concurrent rise in midplane emission.
No significant difference in neutral pressure distribution is evident
between and fueling, except for a sliqhtly larger compression ratio for
Dj in the case of midplane fueling. Figure 3 shows the only pressure curves
which were obtained with He. Even with gas injection in the divertor chamber,
only a moderate compression ratio of 5 is sustained with He.
The fuelinq efficiency of the closed geometry relative to the
previously investigated open geometryM) was studied by measuring the gas flow
required to maintain a constant plasma density (Fig. 4). Fueling with H.,
injected into the divertor chamber in the closed geometry was found to have
the same efficiency as fueling from the midplane with the open geometry,
whereas, in the comparison of the two injection locations in the closed
geometry, midplane fueling was found to require approximately twice as much
gas as divertor fueling.
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Dylla, H. F.; Bell, M. G.; Fonck, R. J.; Jaehnig, K.; Kaye, S. M.; Owens, D. K. et al. Gas fueling studies in the PDX tokamak: II, article, January 1, 1984; New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc1068439/m1/5/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.