Development of the frequency scanning reflectometry for the registration of Alfvén wave resonances in the TCABR tokamak L. F. Ruchko, R. M. O. Galvão, A. G. Elfimov, J. I. Elizondo, and E. Sanada Instituto de Física, Universidade de São Paulo,05315-970, São Paulo, BRASIL M. Manso, and A. Silva Associacão EURATOM/IST Centro de Fusão Nuclear, Instituto Superior Técnico, Av. Rovisco Pais 1,1049-001, Lisbon, Portugal Abstract The progress of the experimental studies on the application of the frequency scanning microwave reflectometry to the analysis of the high frequency (HF) density perturbations, which are driven by the local Alfvén wave (AW) resonances, in the tokamak TCABR is presented. As it was shown in the previous TCABR experiments [1], these AW driven density perturbations can be registered by the microwave scanning reflectometer technique. These studies are focused on the demonstration of the possibility to use the active excitation of the AW spatially localized plasma density perturbations for the determination of the safety factor q-value profile q (r) and effective ion mass M eff in the tokamak diagnostics. Physics motivations are following: Possibility to excite localized Alfvén wave resonances close to the magnetic surfaces in the predefined plasma regions by external antennae; Amplitude of the AW driven density oscillations is directly connected with the local AW power deposition P; Sensitivity of the reflected microwave signal to the plasma density oscillations in the cutoff layer is high; Narrow band amplification and synchronous detection of the reflected signal at the AW generator frequency increases the selectivity. In this paper we present the experimental results, which were obtained with the existing low frequency reflectometer system, in which the improvements in the data acquisition system were made, and present the new reflectometer system with wider frequency range of the operation, which will be installed soon and which ensures the
possibility to investigate the AW driven plasma density oscillations at the higher plasma densities. Low frequency range of the reflectometer f = 16-26.5 GHz, which was used in the experiments in the TCABR tokamak, gives the possibility to study AW power deposition only at the plasma periphery in the low density regimes of the TCABR tokamak. Experiments have shown that intensive density oscillations at the AW frequency are detected by the reflectometer close to the plasma boundary. 1. Introduction Nowadays the microwave reflectometry is routinely used for studying the structure of magneto-hydrodynamic and turbulent fluctuations in fusion plasmas due to its sensitivity to plasma density oscillations in the cutoff layer. New wide possibilities of this diagnostic technique can be realized by integrating it with AW excitation of the density oscillations by external antennae. In this case coherent plasma density oscillations are excited in predefined plasma regions, and their registration by the reflectometry technique can be used for diagnostic purposes. The magnitude of plasma density oscillations in the course of the AW excitation has maximum value in the vicinity of the AW resonance layer. Its position can be found from the equation 2 2 2 2 // ( ) ( )(1 = k ra C A ra 2 ). (1) k ( ) N 1 + M, CA is Alfvén velocity, ci is the effective where // r = ( ) R Nq( r) cyclotron frequency taking into account impurities. The numerical calculations show that the density fluctuations are concentrated within few ion gyroradii of the resonance layer. The localization of this zone depends both on the frequency and wave numbers M, N of the excited wave, and on the local plasma parameters n e (r), q (r), M eff. In the case of the O-mode reflectometer the AW dispersion in the plasma region, where the reflectometer signal is reflected, can be simplified [1]. ci N R M 1 + Nq( r A ) me m M i eff = c ce 1 2 2 ci (2)
It is seen that this expression does not depend on the plasma density explicitly, but only implicitly through q ( r A ). It means that AW parameters M, N, and reflectometer frequency directly determine q(r) and M eff. This fact helps us to identify the excited mode numbers M, N and to find the q(r) values. For example, if plasma density increases in the course of discharge, the AW resonance zone moves to the plasma boundary, where q r ) is larger. In this case, if AW modes with M N > 0 ( A are excited, the reflectometer will detect the increased AW modulation of the reflected signal at lower frequencies. This effect is not present for AW modes with M=0, which can be excited due to the toroidal effects. 2. Experimental setup The experiments were carried out in the tokamak TCABR (a=0.18m, R=0.61m, B = 1.1T). The basic parameters in this investigation were the following. Plasma current: Ip = 70-95 ka; edge safety factor: q(a) 3.1-4; line averaged plasma density: < n e >= ( 0.9 1.5) 10 19 m -3 ; working gas: hydrogen. In the basic regime of operation, the AW antenna straps in the same toroidal cross-sections are fed by currents with ( 0, π ) phasing so that they can excite mainly the modes M = ± 1; N = ±1, ± 2... The power absorbed by the plasma was limited to P ~ 40 kw. The registration of the AW driven density oscillations was carried out by a microwave reflectometer, which was developed in IST/CFN (Lisbon, Portugal), and operates at the frequency band of 16 to 25.5 GHz. Its scheme is shown in Fig.1. The reflectometer was adapted to the AW experiments in the TCABR. In order to register small signals in the AW frequency band, additional high sensitive selective amplifiers were constructed. The output signal from the diode (9) is preamplified and then is divided in into two parts and then is conditioned in two channels. One signal is amplified by the wide band amplifier (10) and then is directed to VME through the low pass filter. This signal is used for plasma radial profile reconstruction. Second part is amplified by the selective amplifier (11) with pass-band frequency f=(4.0±0.5) MHz, which corresponds to the excited AW frequency. Then this signal is rectified, time integrated with τ 5 s and acquired by the VME. The VME sampling frequency was RC µ f S =3 MHz.
Fig.1. Schematic representation of the reflectometer diagnostic scheme: 1- functional signal generator; 2- HTO oscillator f=8 12.4 GHz; 3 frequency doubler; 4,7 DC break; 5 - directional coupler; 6 reference pin; 8 microwave isolator; 9 microwave diode detector; 10 wide band amplifier; 11 narrow band selective amplifier; 12 data acquisition system. The data analysis includes the determination of the amplitude and spectral characteristics of these signals and their dependence on the variation of plasma current and line averaged plasma density <n(t)>. Output reflectometer signal Narrow band amplifier (f=f A ) and rectifier Frequency modulating signal τ VME (fs=3.0mhz) Profile n e (r,t) reconstruction VME (fs=3.0mhz) A(f refl,t) contour plot Fig.2 Block-scheme of the reflectometer data analysis. 3. Experimental results The registration of Alfvén wave resonances by microwave reflectometry was studied in the typical experimental conditions of the tokamak TCABR. Because of the low frequency band of the reflectometer 16-25.5 GHz the initial rather low value of the line averaged plasma density 19 n ( 0.9 1.2) 10 m 3 was chosen. The typical
traces of the plasma parameters are shown in Fig.2(a). The zoomed traces of the reflectometer output and of the frequency modulating voltage are shown in Fig.2(b). In the course of the reflectometer frequency sweeping the amplitude of the diode signal is increased when the microwave cutoff zone coincides with AW resonance zone. In this case the continuous sweeping of the reflectometer frequency will result in the repetitive Fig.3(a). Shot #21445. From the top to the bottom: plasma current, loop voltage, reflectometer modulating signal, output reflectometer signal at f=4 MHz, plasma density Fig3(b). Shot #21445. Expanded traces of the reflectometer output Vd and of the frequency modulating voltage Vm. enhancements of the diode signal for the certain reflectometer frequencies, which vary with the plasma parameters. In the following data analysis the output reflectometer signal at the Alfvén wave frequency f=4 MHz was transformed in the reflectometer frequency-time plane, and the amplitude of the diode signal was presented by the color, which is linearly mapped to the given colormap in Fig.4. It is seen that the chain of repetitive increases of the reflectometer signal occurs inside the time interval t=125-130 ms in the reflectometer frequency range f 20.3-21.8 GHz. They correspond to increase of density oscillations induced by AW absorption near the resonant surface. The calculations show that this surface can be identified as M= ± 1, N= m 2 Alfvén wave resonant surface. In this case after substitution the known values in the Eq.2 we can receive the expression for local value q(r AW ). The value r AW can be found after density profile reconstruction. 34 q( raw ) = (3) 67.8 2.41 f Aeff
1 2 3 Fig.4 The output signal from the reflectometer at the Alfvén wave frequency as function of the discharge time and of the reflectometer frequency These results have to be considered as illustrative example only. The reflectometer system, which was used in these experiments, has many shortcomings. The main of them are following: Narrow reflectometer frequency range f=16-25 GHz, which makes it possible to study only plasma periphery, Slow velocity of the frequency scanning; High sensitivity to the noise In order to overcome them the new reflectometer for TCABR was constructed in the Instituto Superior Técnico (Portugal). 4. Main characteristics of the new reflectometer for the TCABR tokamak Main objectives, which can be reached with the new reflectometer, are: - Measurement of the temporary evolution of the plasma radial profile over the whole plasma column (external side); - Measurement of the fast variations of the density profile (τ 100 µs). - Measurement of the density fluctuations; - Measurement of the density oscillations induced by Alfvén waves at the frequencies from 3 to 6 MHz. These objectives determine the following reflectometer characteristics.
- Operation in 4 frequency bands: K, Ka, U and V. It corresponds to frequencies in the interval 18-72 GHz. - It can operate in the regimes O and X (ordinary and extraordinary), in agreement with the conditions of accessibility of the area of the plasma to be probed. - Density range: - (0,45-6,4) x 10 19 m -3 (O- mode); - (0-4,2) x 10 19 m -3 (X -mode). - Flexibility to operate at the fixed frequencies or in sweeping regime (chosen by the software). - In the sweeping regime the reflectometer will be used to determine the radial profile of plasma density in TCABR. - In fixed frequency regime the reflectometer can be used for turbulence studies and for correlation study of the density oscillations. - The band width of the detectors and of the amplifiers of IF (intermediate frequency) will be of the order of 20 MHz, making possible the study the density oscillations associated with the heating by Alfvén waves. The frequency bands refer to the following frequency ranges: K: 12,4-26,5 GHz Ka: 26,5-40,0 GHz U: 40,0-60,0 GHz V: 50,0-75,0 GHz The solution proposed in this project consists in joining the frequency bands in two groups (K, Ka) and (U, V), each one of the two groups will have an independent hardware, their own sources, detectors, wave-guides and antennas. In Fig. 5 it is shown one of the groups, which corresponds, for instance, to the bands K and Ka. This reflectometer is of the homodyne type. It is capable to sweep the whole frequency range (18-72 GHz) in 30-40 µs, with band width of the receiver 8 MHz, and can operate in regimes at fixed frequencies, in sweeping mode or in the combination of both during a tokamak shot. The choice of HTOs as generators of microwaves for the reflectometer doesn't present doubts: they are wide band devices, they allow very short times to sweep the band, it has good spectrum purity and high reliability. The diodes PIN are a cheap solution to toggle the system very quickly between the bands K and Ka and between the bands U and V. The frequency multipliers have to be active (they are more expensive) to provide enough power. The passive multipliers have very high losses.
Fig.5 Schematic representation of the new reflectometer diagnostic scheme of TCABR tokamak.
Conclusions Low frequency range of the used reflectometer f = 16-26.5 GHz gives the possibility to study AW power deposition only at the plasma periphery in the low density regimes of the TCABR tokamak; Experiments have shown that intensive density oscillations at the AW frequency are detected by the reflectometer close to the plasma boundary. The new reflectomer, which will be installed in the TCABR tokamak, can provide the investigations of AW driven plasma density oscillations in the inner plasma regions. A combination of sweeping the frequency of the AW excitation system with scanning of the reflectometer frequency may be a very powerful diagnostic tool to localize the AW power deposition and to find the effective mass number M eff and q- profiles in tokamaks. Acknowledgment This work has been supported by the State of São Paulo Research Foundation - FAPESP. References [1] L. F. Ruchko, R. M. O. Galvão, E. A. Lerche, A. G. Elfimov, V. Bellintani Jr, J. I. Elizondo, A. N. Fagundes, A. M. M. Fonseca, Yu.K. Kuznetsov, I. C. Nascimento, W. P. de Sá, E. Sanada, R. P.da Silva, R. Valencia, Rev. Sci. Instrum. 75, 655 (2004). [2] A.G. Elfimov, L.F. Ruchko, R.M.O. Galvão et al. Nuclear Fusion, v. 46, 722 (2006).