Gas fueling studies in the PDX tokamak: II Page: 6 of 20
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5
The behavior of the ionization light emission over the same density
range as the pressure measurements is shown in Fig. 5 (for emission) and
Fig. 6 (for Hell emission). At the highest densities ne > 3 x 10^3 cm 3 a
saturation or even a decrease in D (or Hell) emission is noted for the
g
measurements in the divertor,
iv. Discussion
The closed divertor geometry allowed a significant neutral pressure
difference to be sustained between the edge plasma region at the plasma
nidplane and divertor chamber. Not surprisingly, the compression ratios
(P0/PM) could be increased by injecting gas directly into the divertor chamber
or by applying less divertor pumping. The measured compression ratios are in
good agreement with the predictions of a numerical model of the PDX divertor
using the neutral transport code DEGASI6).
We have analyzed the set of data which is most complete, the case of
oivertor fuelinq with D.,, by combining a simplified model of H emission!?],
with the DEGAS code.
For the electron density and temperature (T^) ranqes expected in the
divertor, the light emission (T) varies approximately as
r = K • nfin0Tea, where a = 5.3, T# < 5 eV
1.0, 5 < Te < 60 eV,
where n0 is the density of atomic hydrogen, and K is a constant. Assuming nQ
is proportional to neutral pressure, Tg can be computed from the data shown in
Figs. 2d, 5, and from the measured divertor density shown in Fig, 7. The
predicted dependence of Te on ne is shown in Fig. 8. The curve has been
scaled at one point to an experimental value of T0 = 15 eV at n& = 2.5 x 1013
cm that was measured with a Langmuir probe positioned on the separatrix in
the divertor(9]. The predicted value of T0 is found to decrease from 60-70 eV
at ne = 1 x 101j cm”3 to 6-8 ev at ne = 3 x 1013 cm-3. Above ne = 3 x 1013
cm-3, T& drops below 5 eV, and so, as r varies as Te5-3, the rate of fall in
T# is smaller, dropping to 4-5 eV at ne = 4 x 1013 cm-3. A second
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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/m1/6/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.