Ion Bernstein wave heating experiment on FTU

The ion Bernstein wave (IBW) is a hot plasma wave with a frequency close to an ion cyclotron harmonic frequency, which couples directly on the plasma thermal ions and electrons . Recent theory and experiments have shown that spatially localized IBWs can induce a velocity shear layer useful for core barrier formation via turbulence stabilization .

An extensive experimental campaign was started to determine the possible heating and confinement effects of the IBW on the FTU plasma. The wave is coupled to the plasma by means of the slow wave launched by a waveguide grill antenna, first utilized in this kind of experiments .

An rf power level up to about 400 kW, limited only by the rf generator, at the frequency of 433 MHz was coupled to the plasma. To enhance the effects of IBW relevant for plasma heating or confinement, a plasma target with Pohm ~PRF was chosen, limiting the plasma current below the value of 350 kA. Moreover, the line-averaged central plasma density was 3 10exp(19) m-3 and the toroidal magnetic field value on the axis was 7.9 Tesla, corresponding to locate to the 4th ion-cyclotron harmonic resonant layer a few centimeters from the plasma center.

The temporal trend of the main plasma parameters during a typical shot is shown in Fig. 1. After the switch on of the rf power of about 350 kW, the line averaged central plasma density increases from 3 10exp(19) m-3 to 5 10exp(19) m-3 (Fig. 1 b). Density profile peaking is also observed by the ratio of the line averaged to the volume average plasma density (Fig. 1 c and 2). The electron temperature increases from about 2.5 keV to about 4.5 keV (Fig. 1 d and 3). The signal in all the channels detecting the H-alfa emission in the horizontal plane decreases, before the peaking of the density profile reaches the maximum value (Fig. 1 e).

After about 100 ms after the rf switch-on, the temperature decreases in correspondence with the onset of a m=2 MHD activity, observed by soft X ray emission, . The radiated power measured by the bolometry is about 60% of the total power input, both during the ohmic phase and during the IBW pulse (Fig. 1 f). An ion temperature increase of at least about 300 eV has been also measured by Doppler broadening of the He-like Fe resonance line, as detected by the bent crystal spectrometer, during the IBW pulse.

Fig.1 Temporal trend of the main plasma parameters during a typical shot during IBW injection: plasma current (a), density (b), peaking density profile (c), horizontal array emission (d), electron temperature from Thomson Scattering (e), total input (continuos) and radiated power (dashed), coupled rf power (g).

Fig.2 Density profiles before and during the IBW pulse. The shadowed band shows the location of the ion cyclotron harmonic resonant layer.

Fig.3 Electron temperature profiles before and during the IBW pulse.

Both plasma density and pressure profiles become steeper than the ohmic ones right inside the major radius 1.03 m, without any significant change in the ohmic power input. These modifications show a formation of a transport barrier in a plasma region a few centimeters away from the ion cyclotron resonant layer, where the poloidal velocity shear layer is expected to be strong enough for turbulence suppression.

No significant change in ne profiles and in the Te,i values have been ever observed, instead, when operating with a different value of the magnetic field BT ~6 T. No IBW propagation into the plasma core is expected at this field, due to a total wave damping at the plasma edge, where the 6th cyclotron harmonic resonant layer is located.

The dominant impurities in the ohmic phase are metallic ones. During the rf injection, no increase of the concentration of these impurities has been observed. However, an increase of the Oxygen and Carbon concentration from about 0.5 to about 1% have been estimated by the visible and U-V spectroscopy. The variation in the radial profile of the effective ion charge Zeff, is shown in fig 4. The central Zeff value remains almost unchanged, while it strongly increases in the outer regions. The behavior observed in the Zeff profiles during the IBW pulse can be explaned by an accumulation of heavy impurities in a region outside the resonant ion cyclotron harmonic layer, produced by a quasi-linear modification of the ion distribution function .

Fig 4 Variation of the effective ion charge along the minor radius during the IBW phase respect to the ohmic phase.

In addition the MHD behavior of the plasma during IBW indicates that the induced barrier phase is completely MHD quiescent, and it is often terminated by an abrupt onset of a m=2 activity.

Experimental Reports