Edge radial electric fields were induced in the edge of the TEXTOR tokamak by means of a polarization electrode in order to study their influence on the plasma edge profiles and its confinement. The studies include the generation of H-mode behaviour with either positive or negative polarity. Particle confinement ( T~) of deuterium and of impurity ions as well as energy confinement (73 are investigated. For positive fields which remain below the threshold for the L-H transition, an interesting regime of reduced particle confinement without noticeable energy confinement loss is found. A strong asymmetry in the edge density profiles with respect to the electric field sign is observed at these low polarization voltages. Above the threshold, H-mode behaviour with increased energy confinement and especially particle confinement can be produced with either polarity of the applied electric field. It is, however, found that, whereas the energy confinement in positive H-modes is at least as good as that in negative ones, the ratio T~/ T~ is about three times lower in the former case.

Four new low field side antennae grouped in pairs have been installed on TEXTOR. It is found that the interaction with the wall (density rise, impurity generation) is significantly reduced when operating each pair out of phase (E) as opposed to in phase (0). The beneficial effect in the n configuration is obtained without drop in plasma loading. This experimental property is shown, from theory, to be explained by the judicious choice of the geometrical configuration. A further improvement in the wall interaction is made possible by an appropriate choice of wall conditioning (wall carbonization with liner at 400'C or. above all, boronization). As a result record low values of PYsd/Ptota, were achieved during ICRH. The large reduction in wall interaction during ICRH allows routine long pulse (> 1 s) ICRH operation at the maximum power level available (g 2.5 MW).

Thc cxcilalion of Past-wave global cigenmodcs is investigatcd but11 experimentally for TEXTOR and theoretically using a slab coupling mode. A briefcomparison with a cylindrical model shows that in the TEXTOR situation no essential gcometricdl cflcct is left out by thc slab code. Thc resonant behaviour associatcd with eigcnmodcs is studied fctr several signals: the resistancc and inductance of the antennae, the r.f. magnetic and clectric field components picked up by probes at thc plasma edge, and thc power coupled to a Pdst-wave antcnnil uscd its a receiver It is shown that all thc major features of thesc signals linked to thc existencc of cige"ies can be cnplaincd using the theoretical models. Two possibly counterintuitive conclusions of this study are (aJ that the absence of iesonance peaks in thc rrsislmcc curves o f the rr-phased antennae docs not imply that the cigcnmodes itre not excited a1 similiir lcvcls its for the 0-phasing. and (h) that the variations in local r.f. field magnitude arc ziot closely related l o the rcsanant bchaviuur ofthc antenna resistancc. I . I N T R O D U C T I O N * R ;ind L iiic thc rcsist:mx and inductance per unit length of the antenna. coiisidercd ils il strip line; B prccisc dclinition is givcn by M E S S ~A ~X e, 01. (IOSZ).

I o n c y c l o t r o n h e a t i n p on TEXTOR has now reached t h e ?:exajoule l e v e l , The h e a t i n g s c e n a r i o is normally mode conversion b u t o c c a s i o n a l l y minority h e a t i n g i n 2 D-(H) plasma. i i i t h a p p r o p r i a t e wall c o n d i t i o n i n g by c a r b o n i z a t i o n more than 1 >lid of RF power has been i n j e c t e d f o r lone pulse d u r a t i o n s (?. 1 s). The ICRF heated plasma i s c h a r a c t e r i z e d by 2 q u a s is t a t i o n a r i t y of a l l plasma parameters, l i t t l e i f no impurity i n c r e a s e 2nd 2 l o o p v o l t a g e r e d u c t i o n r e s u l t i n g i n t h e t o t a l power coupled to t h e plasma reaching s i x times t h e remainine ohxic power i n p u t .

An experiment was performed on the TEXTOR tokamak which proves that the plasma can be heated by ion cyclotron resonance using antennas without a Faraday shield. The heating performance of the unshielded antenna was found to be similar to that of a shielded antenna. Although the radiated power was somewhat higher with the unshielded antenna, no impurity problems were encountered. These results are explained on the basis of radiofrequency sheath theory and by the almost perfect shielding of parallel electric fields by the plasma. Implementation criteria for unshielded antennas were obtained.