Investigation of RF-sheath effects and parasitic absorption during ICRH

1. Investigation of RF-sheath effects and parasitic absorption during ICRH T. Hellsten Association Euratom-VR, EE, KTH, Sweden, T. Hellsten TG H&CDRF Dec 2008 Brussels

2. Early JET results Degradation of the heating performance was seen in JET with the A1 antenna with large misalignment angles in experiments with reversed plasma currents (old JET). • Monopole phasing. Fraction lost power 40% Clear evidences of RF-sheath effects have been seen under conditions when the heating is degraded. Impurity radiation varies with coupling resistance. T. Hellsten TG H&CDRF Dec 2008 Brussels

3. Motivation of study RF-sheath Parasitic absorption is expected to be small in ITER and should not affect the heating performance, but could lead to a faster wear and tear of the antennas. T. Hellsten TG H&CDRF Dec 2008 Brussels

4. Early JET results Correlation between the Be radiation, antenna voltage and coupling resistance. When coupling resistance is high radiation has a minimum. High coupling resistance the antenna couples to waves with low |nf|. The plasma is cleaned up by sawteeth and ELMs M. Bures et al, PPCF 33 (1991)937 T. Hellsten TG H&CDRF Dec 2008 Brussels

5. Variation of coupling resistance for weak single pass damping Analyse the effects when coupling to plasmas with weak single pass damping. Coupling model: standard plane slab model of the vacuum wave field with a model for the location of the eigenmodes with respect to plasma parameters, varied reflection coefficient and phase of the reflected wave. Calculated the coupling for a large number of eigenmodes. Sheath losses modelled by fitting losses to Bures experiments with the A1 antenna with large misalignment angle. T. Hellsten TG H&CDRF Dec 2008 Brussels

6. Coupling restance Antenna voltage, Normalised wrt P0.5 Sheath losses Fraction of power lost Variation of coupling resistance for weak single pass damping Mono-pole 90° Dipole The model reproduces the peaks seen in experiments during ramp up of magnetic field (Hellsten &Appert EPS 1986). Individual modes only seen at lowest single pass damping 0.5% 90° phasing single pass damping of 5% The peaks in coupling resistance come from coupling low |nf|. T. Hellsten TG H&CDRF Dec 2008 Brussels

7. Variation of coupling resistance for weak single pass damping Good coupling is seen when the antenna for a given phasing couples to modes with low |nf| for which the coupling resistance is high, consistent with the observation by Bures et al. The worst behaviour is seen when the antenna couples to modes with high |nf| for which the coupling resistance is low. This can be understood by the fact that the phasing has the most important effect on the rectified sheaths. A low coupling resistance require a higher voltage for the same power. What is bad is monopole phasing and low coupling resistance. Weak single pass damping makes the coupling low between the compound resonance peaks consisting of low |nf|-waves. Improvement can be obtained by broadening the low |nf| peaks. T. Hellsten TG H&CDRF Dec 2008 Brussels

8. Sheath losses versus single pass damping for 5° misalignment angle Sheath losses versus misalignment angle Top 0.5% single pass damping, 5% and 50% T. Hellsten TG H&CDRF Dec 2008 Brussels

9. Variation of coupling resistance for weak single pass damping That improvement can be obtained by broadening the low |nf| peaks is consistent with the experiments by Heikkinen et al [31stEPS 2004], where he compared the efficiency in heating for monopole ohasing of the A2 antenna with 4, 2 and 1 strap. The worst efficiency was seen for 4 straps and the best with 1 strap. The 4 strap had the narrowest spectrum and the 1 strap the widest spectrum. T. Hellsten TG H&CDRF Dec 2008 Brussels

10. Typical discharge for FWCD with reversed shear NBI: 13MW LHCD: 2-2.5MW ICRH: 3-5.6MW Te =8keV Ti =12keV ne=2.21019m-3 I = 2MA B0 = 3.4T f = 37MHz T. Hellsten TG H&CDRF Dec 2008 Brussels

11. Evidence of degradation Similar plasmas at different ICRH powers Electron temperature anddensity Power deposition by directelectron heating #60663 (5.0MW full line+90°); #60664 (6.2MW -90o); #60664 (6.2mW dashed line –90°); #60665 (5.0MW +90o); #60667 (dotted line 3.0MW dipole). #60667 (dotted line 3.0MW dipole). T. Hellsten TG H&CDRF Dec 2008 Brussels

12. BeII #60664 -90 #60663 +90 #60673 dipole Evidence of rectified RF-sheath BeII Te • BeII line radiation intensity at the inner divertor for: #60663, #606644, #60673; • BeII line radiation for #60673 (different scale); • central electron temperature. T. Hellsten TG H&CDRF Dec 2008 Brussels

13. Evidence of lost power from energy balance. Losses of up to 50% of the total RF energy was seen when comparing the total energy injected into the vacuum vessel by the heating system and the energy radiated into the vacuum vessel (measured with the bolometers), energy transferred to the divertor measured with thermo couplers (discharge without NBI). T. Hellsten TG H&CDRF Dec 2008 Brussels

14. Are there other effects than RF-sheath effects important for the heating degradation? Mode conversion at the high field side; Losses of fast ions. What is the maximum value of the misalignment angel, for which losses are independent of the misalignment angle? Conduct experiments at JET with large losses to increase the data bank and better understand the cause of the losses. T. Hellsten TG H&CDRF Dec 2008 Brussels

15. Evidence of rectified RF-sheath #58682 with 4.2MW +90°H-minority heating at 51MHz;#60673 with 3.4MW dipole; #60675 with 5.6MW +90°; and #60676 with 5.7MW -90°. • BeII line radiation intensity at the inner divertor for: #60663, #606644, #60673; (b) BeII line radiation for #60673 (different scale); • (c) central electron temperature. T. Hellsten TG H&CDRF Dec 2008 Brussels