Coda, S.

Duval, B.

Goodman, T.

Klimanov, I.

Laqua, H.

Martin, Y.

Mück, A.

Pochelon, A.

Porte, L.

English

MPG.PuRe

O-X-B Mode Conversion in the TCV TokamakA.Mück1, H.P.Laqua2, S.Coda1, B.P.Duval1, T.P.Goodman1, I.Klimanov1, Y.Martin1,A.Pochelon1, L.Porte11 Association EURATOM- Conféd́eration Suisse, EPFL SB CRPP, CH-1015 Lausanne2 Max-Planck-Institut f̈ur Plasmaphysik, Assoziation EURATOM, D-17491 GreifswaldIn high density tokamak plasmas, the accessibility of the plasma to electron cyclotron resonanceheating (ECRH) is limited by so-called wave cut-offs. ECRH in extraordinary mode (X-mode)can not propagate to the plasma centre and heating in ordinary mode (O-mode) is prevented bythe plasma cut-off atω= ωpe. At a particular launching angle, an O-mode wave converts toX-mode at the plasma cut-off. After this first mode conversion,the wave propagates back towardsthe plasma edge until it encounters the upper hybrid layer. There, a second mode conversionfrom X-wave into a Bernstein (B) wave, known as the O-X-B conversion, occurs. The Bernsteinwave can propagate now to the plasma centre and may be absorbed at harmonics of the cyclotronresonance [1],[2].In TCV, a high density H-mode scenario with up tone≈ 2·1020m−3 was developed to place theplasma ECH cut-off within the steep density gradient regionat the H-mode discharge edge. Theangular window of the O-X conversion is calculated with the power transmission functionT [3]T(Nx,Nz) = exp(−πk0L√12α·[2(1+1α(Nz,opt−Nz)2+N2x])(1)which depends on the density scale lengthL and the refraction indicesNx andNz. Only for theoptimumNz, equivalent to an optimum angle, can the O-mode wave be completely converted toX-mode.In figure 1a), the main characteristics of such a H-mode discharge, as used in the reported exper-iments, is shown. High densities were achieved in non-station ry, but reproducible, discharges.Modulated O-mode ECRH at the second harmonic at 82.7 GHz andPECRH≈ 470 kW was in-jected with a low duty cycle to avoid ELM triggering. Poloidal and toroidal ECRH launcherangle scans were then performed on a shot by shot basis. In figure 1b), the change in the strayECRH radiation signal of a midplane launcher from a diode located in sector 14 is shown. Thesame tendency in the signal is observed with a second diode ins ctor 9 and a sniffer probe insector 2 (ECH is injected from sector 6 of TCV’s 16 sectors). Aclear dependence on the ECRHlauncher angle is observed, shown in further detail in figure2. In figure 2a) dots show the scanpositions for a midplane launcher L4 for the injection anglecalculated with the ART code [4].In the figures 2b) and 2c) the stray radiation signal for the poloidal and toroidal angle scan aredrawn separately as a function of the ECRH launcher angle. Inthese scans, as well as in scanswith an upper lateral launcher, the angular dependence of the O-X conversion is clearly visible.The minimum in the stray radiation intensity, assumed to be the maximum of the ECRH power32nd EPS Conference on Plasma Phys. Tarragona, 27 June - 1 July 2005 ECA Vol.29C, P-4.110  (2005)time [s]stray radiation/a.u.[]Hα(22°,-110°)(25°,-110°)(31°,-110°)(34°,-110°)(37°,-110°)a)b)D [a.u.]aStray radiation Diode X2_2[a.u.]ne on axisz-axisBphi nom =R*Bphi/0.88[T]IP [kA] #290140.20.1time [s]Figure 1: a) Characteristics of developed high density H-mode discharge. b) ECRH stray radi-ation for different launching angles of the midplane launcher L4, showing an angle dependentbehaviour and Dα signal, showing the influence of ELMs.absorption, varies of about 2o from its predicted position, as shown in figure 2a).Heating experiments were performed with a fixed poloidal launcher angle of 31o and toroidalangle of -107o with a higher duty cycle. The correlation between the soft X-ray signal (DMPX)and the stray radiation signal was used to determine the deposition location. An anti-correlationpolynomial regression, launcher 4 polynomial regression, launcher 4a) b) c)Figure 2: a) ART simulation of angular window as transmission in percent. b) Poloidal anglescan with midplane launcher L4. c) Toroidal angle scan with L4.with the stray radiation signalSstray is expected from the inverse dependence of the stray ra-diation on the absorbed ECRH power. To obtain such a correlation, he value of the strayradiation signal without absorptionSstray,max≈ 1.5 was subtracted. To set a value of zero be-tween the ECRH pulses, the signal was multiplied by the ECRH power time trace, leading toCcross= (Sstray,max−Sstray) ·PECRH. By calculating the cross-correlation of one ECRH pulseacross a sawtooth crash and within an ELM free time interval,clear localised deposition wasfound. Figure 3 shows the correlation for DMPX channels 22 (ρ ≈ −0.44) and 47 (ρ ≈ 0.58),32nd EPS 2005; A.Mück et al. : O-X-B Mode Conversion in the TCV Tokamak 2 of 4marked by a solid line. The ART calculated absorption is situated further in the plasma atρ ≈ 0.35 (channels 26, 42).The calculated position of the plasma cut-off isO-modecut-offO-modecut-offEBW depositionFigure 3: Deposition location of #29032at B= 1.33T for ECRH pulse withoutsawtooth crash.marked by a dashed line. the ECRH power is re-flected at the plasma cut-off and absorbed at the cy-clotron resonance after reflection at the torus wall,explaining the correlation outside the cut-off. Theamount of ECRH power absorbed is measured withthe diamagnetic loop (DML). Figure 4a) shows anaverage absorbed power of 267 kW±12% ( 60%)with a phase below 90o, confirming the reliability ofthe analysis together with figure 4b), which showsthat the ECRH modulation frequency was well fit-ted in the DML signal. If O-X conversion doesnot take place, the maximum stray level rises toSstray,max 1.5. In the best case, a stray level of 0.15is observed, indicating that an O-X coupling of up to 90% was achieved.The magnetic field dependence of the EBW deposition was investigated by increasing the torio-a) b)absorbed powerphaseFigure 4: a) Average deposited power and phase with iamagnetic loop. b) Fit of energy andcurrent with ECRH frequency.dal field fromB = 1.33T toB = 1.39T. In figures 5a) and 5b) the cross-correlation ofCcrosswiththe DMPX signal is shown for a sawtooth crash free and ELM freetim period. The depositionlocations of #29550 and #29565 are consistent with the ART simulation. No significant variationin the deposition location forB = 1.33 T andB = 1.39 T was however observed, as summarisedin the table below . A possible explanation could be small changes in the edge current profile,strongly influencing thek‖ component of the converted Bernstein wave, leading to a change in32nd EPS 2005; A.Mück et al. : O-X-B Mode Conversion in the TCV Tokamak 3 of 4O-modecut-offO-modecut-offEBW depositionO-modecut-offO-modecut-offEBW depositiona) b)Figure 5: Deposition location of a) #29550 and b) #29565 at B= 1.39T for ECRH pulse withoutELM and sawtooth crash.the wave propagation. These variations in the current may bein the range of the uncertainty inthe equilibrium reconstruction.Shot B field O-X conversion Absorption (ART) Absorption (experiment)#29032 1.33T ρ ∼ 0.69; ch 15, 50 ρ ∼ 0.35; ch 26, 42 ch 22, 44#29550 1.39T ρ ∼ 0.88; ch 9, 54 ρ ∼ 0.45; ch 23, 44 ch 22, 45#29565 1.39T ρ ∼ 0.82; ch 12, 54 ρ ∼ 0.45; ch 23, 44 ch 22, 44Conclusions:For the first time in a standard aspect ratio tokamak, localised EBW deposition is demonstratedin overdense plasmas resulting from the O-X-B mode conversion process. Absorption of 60%of the injected ECRH power has been shown by DML measurementsin heating experiments.In low power experiments, the stray radiation signal indicates an absorption even up to 90%.The angular dependence of the O-X conversion has been explord by scanning the ECRHlauncher angles. The observed variation of the stray radiation signal is well confirmed by ARTcalculations of the angular window.References[1] I.Bernstein, Phys. Rev. Lett.109, 10 (1958)[2] H.P.Laqua et al., Phys. Rev. Lett.78, 3467 (1997)[3] E. Mjolhus et al., Plasma Phys.31, 7 (1984)[4] F.Volpe, H.P.Laqua, Rev. Sci. Instrum.74, No. 3 (2003), pp. 1409-1413A. Mück is supported by a EURATOM fellowship.32nd EPS 2005; A.Mück et al. : O-X-B Mode Conversion in the TCV Tokamak 4 of 4

O-X-B Mode Conversion in the TCV Tokamak

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O-X-B Mode Conversion in the TCV Tokamak

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