1 - Nuclear Sciences and Applications - IAEA

546 ASHRAF et al.
cm from the magnetic axis. The calculated value of ne at which the cut-off
defined by Eqn (1) appears on axis or at the resonance line (nc), is indicated
in each case and clearly the heating effect falls to zero above this value. At
densities just below the cut-off, the ECRH power is absorbed off-axis and as
a consequence the sawtooth inversion radius contracts when the ECRH is
turned on and expands again afterwards as the current profile responds to
changes in Te(r). The time constant of the response (~ 12 ms) is consistent
with classical resistive diffusion of the small current profile changes implied
(A q(o) w 0.03). The difference in the behaviour above cut-off compared
with that in Dili may be due to the edge heating produced by ECRH at
high ne reversing the degradation in r^ in the ohmic discharge in that case.
Current Drive Efficiency
Observation of electron cyclotron current drive at 2wc in the CLEO Tokamak
has recently been reported [8] where the current drive efficiency was found
to be a factor x 3 below theoretical expectations. Recent attempts have
been made to repeat and extend these experiments in DITE using HFS
launch.
These initial experiments were performed at 2wc with a total RF power
of ~ 200kW (net power delivered to the plasma). Plasma currents in the
range 40 — 60kA were employed. For ne < 6 x 10 18 m~ 3 the discharges were
observed to become significantly non-thermal whilst the heating efficiency
decreased rapidly as ne increased beyond 1 x 10 19 m~ 3 ; hence most work
was carried out at ne =(8 — 9)x 10 18 m~ 3 . For such conditions, theoretical
predictions indicate that for quasi-perpendicular propagation (strongest
damping) trapping effects can reduce the driven current by an order of
magnitude. The trapping effects are less severe for more oblique propagation,
resulting in an optimum angle for current drive of 60° to the toroidal
direction. Assuming Teo ~ lkeV the optimised current drive efficiency is
•q = n2oRIrj/Prf = 1.3 x 10~ 3 . Thus for our experimental conditions an
RF-driven current of only 2.6kA is expected (ie.,5% of Ip). The current is
in the same toroidal direction as the launched wave.
The launched waves were directed both parallel and anti-parallel to Ip at
an angle of 60° and during the course of the experiments Ip was also reversed
several times. Both ECE measurements of Teo and magnetic measurements
of /3 consistently indicated that for the same injected power the electron
temperature was higher when the waves were directed anti-parallel to Ip. In
this case the residual microwave power was lower indicating more complete
absorption of the injected power. Furthermore there was good agreement in