EX/P- Influence of ECR Heating on NBI-driven Alfvén Eigenmodes in the TJ-II Stellarator Á. Cappa, F. Castejón, T. Estrada, J.M. Fontdecaba, M. Liniers and E. Ascasíbar Laboratorio Nacional de Fusión CIEMAT, Madrid, Spain Corresponding Author: alvaro.cappa@ciemat.es Abstract: NBI driven Alfvén modes are found in several magnetic configurations of the TJ-II stellarator, allowing us to test any control knob able to suppress, or at least modify, the Alfvén activity of the device. In particular, the paper describes the effect of electron cyclotron resonance heating (ECRH) on the Alfvénic oscillations when different EC beam launching parameters are tested. The observed modes may exhibit steady or chirping behavior, as well as a strong reduction in amplitude, depending on the launching direction of each beam. Introduction Controlling the amplitude of Alfvén eigenmodes (AE s) in fusion plasmas is an open issue with paramount relevance for ITER and beyond, because the fast ion losses associated to these modes might be deleterious for plasma performance as well as destructive for the plasma facing components. The stabilization of the reversed shear Alfvén eigenmodes (RSAE s) using ECRH has been analyzed in detail in the DIII D tokamak [, ] and recently, first experiments in TJ-II [] have also demonstrated the ability of ECRH to mitigate and modify the amplitude and nature of the fast ion driven modes. The initial experiments carried out in TJ II explored the impact of ECRH within a limited range of launching configurations. The experiments presented in this paper describe in detail the dependence of the AE s properties on the power deposition location of both beams, the NBI parameters and the magnetic configuration. Previous experiments, documented in [] and related to density dependence or mode suppression, were repeated looking for the reproducibility of the results. TJ II stellarator and its heating systems TJ II stellarator is a four-period heliac-type stellarator with low magnetic shear and good MHD stability properties provided by magnetic well. The major and averaged plasma radius of the device are R =. m and a. m respectively. The central magnetic field strength is.9 T (at the toroidal locations of ECRH injection) and a wide range
EX/P- of central rotational transform.9 ι()/π. is achievable. The NBI heating system which consists of two injectors providing co and counter sub-alfvénic H beams (v beam /v A.. for line averaged densities around. 9 cm ) with currents up to A and with a maximum acceleration voltage of kv provide up to 7 kw portthrough power per injector. The experiments described in this paper were carried out using only the co-direction injector. Two. GHz gyrotrons, delivering up to kw of EC power each, create and heat the plasma using second harmonic X mode. The power of each gyrotron is guided towards the vacuum vessel by its corresponding quasioptical transmission line and two internal steerable mirrors allow us to scan the plasma toroidally and poloidally. The lines are located in stellarator symmetric positions enabling EC driven current compensation with oblique injection. The launching geometry of the two beams. T ECH Φ=9..9 T z(cm) - - -.9 T - -.9 T - - r(cm) z(cm).9 T ρ =. ρ =. Φ=. -. T ρ =. ρ =. ECH - r(cm) FIG. : Launching directions of both ECH beams. The medium iota configuration and typical ECRH plasma profiles have been used for the ray tracing calculation. (ECH and ECH) and the several ECH off-axis positions used in the experiments are shown in figure. In general, unless otherwise stated, the launching direction of the first beam (ρ ECH =.) was kept constant. For a given launching direction given by some ρ, the EC beam points to the plasma so that, neglecting refraction effects, it crosses the EC cold resonance layer (.9 T) at this particular ρ, and perpendicularly to the magnetic fielddirection (N = ). Thefigureshowstheresult ofaraytracingcalculation performed with the TRUBA code ( rays were used). Experimental observations. NBI Plasmas Figure shows a typical NBI discharge in which several MHD modes appear during the NBI phase (t ms). In this case ECRH is only used to build the NBI target plasma. Figureshowsthespectrogramofthemagneticfieldfluctuations B(t)(mainly
EX/P- the fluctuations in the poloidal component of B) measured cm away from plasma by a Mirnov coil located in sector C of the device. The overlayed white curve in figure P ECH,NBI (a.u.), δ B (a.u.), <n e > ( 9 m ) P ECH P ECH P NBI <n e > (ECE) SXR δ B 7 δn (a.u.) 7 9 V beam (kv) δn (a.u.) 7 I beam (A) (c) (d) FIG. : Heating sequence and time evolution of several plasma magnitudes. Spectrogram of the magnetic field fluctuations. Amplitude of the density fluctuations at the mode frequency measured by DR, for different NBI operating regimes (c), (d). is given by α/ n e (t), where n e (t) is the line average density obtained from microwaves interferometry and α is an unknown factor that depends on the dispersion relation of the mode. Here, we have chosen alpha (α = khzcm / ) so that the curve closely follows the frequency evolution of the initial mode and therefore, disregarding profile evolution, we may identify it as an Alfvén type mode. The value taken for α will be retained throughout all the paper for comparison purposes. For modes highly localized in frequency, the standard deviation of the FFT (δb) in a short time interval ( ms) is a useful indicator of the time evolution of the fluctuations amplitude B(t) (see figure ). The continuous change of the plasma density and temperature, as well as the evolution of the profiles during the NBI phase, produces a strong variation in the frequency and the amplitude of the detected modes. A scan of the NBI parameters and its influence on the mode amplitude appears in figures (c) and (d). The data represented in the figures
EX/P- are the density fluctuations at the mode frequency, obtained from Doppler reflectometry (DR) []. A beam energy scan ( kv) at an approximately constant current ( ± A, blue circles in figures (c) and (d)) and a current scan ( A) at fixed beam energy (.±. kv, red circles in figures (c) and (d)) were performed. The corresponding port-through power ranges between and kw. Beam current variations have a stronger influence on the density fluctuations, probably because the fast ion drive is more dependent on the number of particles resonant with the mode than on the small variations on the resonant condition produced by the changes in the beam energy.. NBI+ECRH Plasmas The impact of ECR heating on the NBI-driven Alfvén modes has been studied using several heating configurations. First experiments were carried out with ρ ECH =. and ρ ECH =. []. As it will be shown later, having ρ ECH =. produces a strong mode (a.u.), <n e > ( 9 m ) P ECH,NBI, δ B (a.u.) P ECH P ECH P NBI δ B. <n e > (ECE) SXR 9 7 FIG. : kw NBI shot with modulated ECRH. Frequency chirping is observed a few milliseconds after the ECH switch-on. Applying further EC power with ECH at t= ms strongly reduces the mode amplitude. chirping in the medium iota (ι) configuration. Figure illustrates the case in which kw of ECH, targeting the plasma at ρ ECH =., is powered on again only ms after the starting of the NBI phase. As a consequence, the NBI driven steady mode disappears, nothing is observed during a short period of time ( 7 ms) and then, a chirping mode with f khz emerges. Note that the ECRH induced pump-out helps control the density which otherwise would gradually increase due to the NBI particle fueling. Adding further power using low frequency modulation with kw of ECH at ρ ECH =. significantly attenuates the amplitude of the chirping mode as it is clearly shown by the time evolution of the fluctuations amplitude δb, that is represented in figure ). The line density is approximately constant during the transition from ECH to ECH+ECH (t ms).
EX/P- Searching for the conditions for which the chirping mode appears, several ECH off-axis positions were tested, while keeping a fixed direction with the ECH beam (ρ ECH =.). In principle, power injection was carried out with N thus avoiding EC current drive. Figure shows the type of shot that was used for this purpose. In this set of experiments, ECH is shutdown after the NBI injection and only ECH is maintained during the whole shot. Depending on the ECH beam launching direction, steady or chirping modes are observed. This behavior is illustrated in figures,, (c) and (d), where the spectrogram of the fluctuations is presented for shots with different ρ ECH. 9 P P ECH PNBI δ B <ne> Te (ECE). 9 m ) Te (a.u.), <ne> ( ECH PECH,NBI, δb (a.u.) SXR. 7 FIG. : Typical NBI+ECH (.) shot and B (t) spectrogram. 7 9 7 9 7 (c) 9 9 7 9 (d) FIG. : B (t) spectrogram for ρ ECH =.,.,. (c) and. (d).
EX/P- Mode chirping appears for ρ ECH >. while a steady mode, similar to the one observed with only NBI heating, is present when ρ ECH <.. Next, in order to check the reproducibility of the mode attenuation result, ECH power was added, always targeting at ρ ECH =. and the mode amplitude was measured both in plasma with steady (ρ ECH =.) and chirping (ρ ECH =.) mode behavior. 7 δ B ( - T)........... <n e > ( 9 m - ) FIG. : Steady mode amplitude vs. line density with (green points) and without (red points) ECH. Mode chirping amplitude with (top) and without (bottom) ECH. P ECH,NBI,n e,i p, P ECRH P ( ) ECRH P NBI n L () I pl time(ms) n e ( 9 m - )... (kev). P ECH,NBI,n e,i p, P ECRH P ( ) ECRH P NBI n L () I pl.. time(ms).... ρ.... ρ FIG. 7: Plasma current reversal in shots with ECH at different launching directions,. (bottom) and. (top). Plasma profiles of both shots. If the mode is well localized in the (f,t) space, as it occurs in the steady mode conditions, the dependence of its amplitude (actually δb) on the line averaged density can be obtained. This result is shown in figure where data coming from several shots has been
7 EX/P- used. When ECH is added, the amplitude δb is half the value encountered when only ECH is operating. The effect on the chirping mode amplitude also becomes very clear as it is shown in figure. Together with the excitation of a chirping mode, a reversal in the total plasma current I p is observed. A comparison between shots obtained at different ρ ECH is presented in figure 7). Plasma current changes its sign when the second beam is moved from ρ ECH =. to ρ ECH =.. The absolute value of I p (t) barely changes. The density and temperature profiles, that are also presented in figure 7, show almost no variation in the gradient zone, where the contribution from the bootstrap current is expected to be larger and thus, the change of the plasma current can not be explained by modifications in the plasma profiles. low ι rho=. rho=.. medium ι rho=. rho=.. δ B ( - T)... <n e > ( 9 m - ) δ B ( - T)... <n e > ( 9 m - ) 9 9 9 9 9 9 9 9 9 9 δ B ( - T) high ι 7 rho=. rho=..... <n e > ( 9 m - ) ι/π.......... low medium high..........7. ρ.... P(W/cm ) (c) (d) FIG. : Chirping mode amplitude for low, medium and high ι (c). Power deposition and ι/π profiles. Low order rational values of ι are highlighted (d). The results presented so far have been obtained in a medium ι magnetic configuration. Two alternative configurations, with low and high ι, were also investigated in shots where only ECH was operated during the whole plasma duration. Although only three configurations have been tested, it would seem that for each configuration the stronger mode chirping occurs at different launching directions (see figures and (c)). In an attempt to understand this finding, the rotational transform profiles for each configuration, as well as the power deposition profiles for each ρ are presented in figure (d). The comparison of the results for low and medium ι suggests the ex-
EX/P- istence of a given ι value for which frequency chirping is maximum. The same reasoning would lead us to think that the high ι case would present maximum chirping for ρ ECH =.. However, for such launching directions, mode chirping never occurs. Finally, figure 9 shows the energy spectrum of the neutral flux measured by a -channel compact neutral particle analyzer (CNPA) in three different stages of the mode chirping shot represented in figure. As it is shown in the figure, ECRH power enhances the neutral flux for energies above one third the nominal beam energy consistently with an increase of the fast ion population during the NBI+ECRH phase. Conclusions Neutral flux (a.u.) t= ms (NBI) t= ms (NBI+ECH) t= ms (NBI+ECH+ECH) Energy (kev) FIG. 9: Neutral flux spectrum from CNPA measured in shot 9 (see figures and ). EC heating modifies strongly the character and amplitude of the NBI driven AE s. Frequency chirping appears for off-axis positions beyond ρ ECH =. and whether we consider a steady or a chirping mode, the application of the other gyrotron produce a strong reduction of the mode amplitude. The fact that opposite I p is obtained in otherwise very similar shots is not yet understood. The findings regarding mode chirping amplitude in three distinct magnetic configurations suggest that the maximum amplitude of the chirping mode generated by ECRH is linked to some particular value of ι/π. The experiments were performed assuming symmetrical behavior in respect to the two ECRH beams. This has still to be tested experimentally. Acknowledgements This work was partially funded by the EUROfusion Enabling Research programme under project number ER-WP-ER- and the Spanish Ministry of Economy and Competitiveness under contract number ENE-9-P. References [] M. A. Van Zeeland et al., Plasma Phys Control Fusion 9 () [] M. A. Van Zeeland at al., Nucl. Fusion 9 (9) [] K. Nagaoka et al., Nucl. Fusion 7 () [] T. Happel et al., Rev. Sci. Instrum. 7 (9)