1 - Nuclear Sciences and Applications - IAEA

478 LEONOV et al.
2. INSTALLATION AND METHODS
The main parameters of the Libtor tokamak and the discharge used are:
R = 53 cm, a = 11.5 cm, fixed limiter radius = 10 cm, Bo = 2.6 T, Ip = 30 kA,
He = (0.5 - 2) x 10 13 cm" 3 . Two moving molybdenum plates, a large one
(9x9x2 cm 3 ) as a working limiter and a small one (4.5 x 4.5 x 0.5 cm 3 ) as a
probe limiter (single probe), are used as additional limiters placed at the bottom of
the tokamak 90° apart along the torus. The experiment showed that the ion and
electron branches of the single probe characteristics are fairly distinct and the movement
of the probe limiter does not affect the LSL structure.
3. EXPERIMENTAL RESULTS
3.1. The results of the LSL screening properties versus its width on the Libtor tokamak
are shown in Fig. 1. When the LSL width increases up to 3 cm, no great change
in the parameters is to be seen, except for an increase in the impurity fluxes from
the limiter and an increase of heat and current flowing to the limiter. At further
increase of Ar, the rate of impurity influx from the limiter decreases and so does the
flux of light and heavy impurities from the wall. From this, we may conclude that
the total impurity flux also drops. The hydrogen flux from the wall decreases, too.
At the same radius, the average plasma density and the radiated power decrease. The
current between the limiter and the wall changes polarity and becomes negative.
Thus, as in Ref. [1], we can distinguish an optimum limiter radius, which corresponds
to Ar « 4.5-5.5 cm for these discharge parameters. It should be noted
that, when the limiter radius is near its optimum value, the negative limiter-wall
current is maximum and decreases with changing Ar in any direction.
3.2. The purpose of further investigations was to study the distribution of particle
and energy fluxes to the limiter surface at LSL width near optimum (Ar = 4.5 cm).
In so doing, the probe limiter was removed in the shadow of the working limiter —
the LSL width being fixed — and heat fluxes and currents to both limiters were measured.
In Fig. 2(a) the radial distributions of the energy having passed to the limiter
have two distinct zones. At the edge of the limiter a sharper decrease in energy
(e-delay length =3.5 mm) is observed in a zone of 1 cm width. In the zone away
from the limiter edge, the energy density decrease occurs more slowly, with an
e-delay length of »7 mm.
In Fig. 2(b), the radial dependence of the current density having passed to the
limiter is shown for two Bo directions. The electron current passes at the edge of the
limiter; closer to the wall, the current becomes positive. It should be noted that the
position of the negative and positive current zones coincides well with the position
of negative and positive current zones in Fig. 1, i.e. this layer structure not only
characterizes the shadow of the limiter, but the entire near-wall layer, in general. The