Microwave reflectometry for plasma density profile. measurements on HL-2A tokamak

Transcription

1 Microwave reflectometry for plasma density profile measurements on HL-A tokamak Xiao Weiwen, Liu Zetian, Ding Xuantong, Shi Zhongbin Southwestern Institute of Physics, Chengdu, , China Vladimir Zhuravlev Kurchatov Institute, Mosco, Russia Abstract: A modulated microwave reflectometry has been successfully developed on HL-A. It can be used to measure the plasma density profile with time-delay method. This microwave reflectometry has two frequency ranges( GHz, 40-60GHz) and it is suitable for measuring the plasma density from cm -3 to cm -3. Temporal resolution is 1ms and spatial resolution is about 1cm. This paper will present the basic principle of the microwave reflectometry, parameters calibration of the equipment and experiment result on HL-A tokamak Keywords: reflectometry, time-delay, density profile PACS: 5.70.Gw, m, 5.5.Gj Introduction Reflectiometry is now widely applied to measure electron density profile and electro density fluctuation in tokamak [1-3]. Usually, we can measure the linear average density of the plasma by the multi-channel HCN laser interferometer or microwave interferometer. To obtain the plasma density profile, we have to treat the data of the multi-channel interference with an Abel inversion. Because the channel is limited, it is difficult to obtain the density profile with high spatial resolution. High temporal and spatial resolution is very important to research the transport and the instability of the plasma. Sweeping microwave reflectometry can obtain the density profile with high temporal and spatial resolution. There are two ways to get the plasma density profile by the microwave reflectometry: one is homodyne (or heterodyne) interference type [4] and the other is time-delay type [5,6,7]. Homodyne interference type has high sensibility, but has low signal-to-noise ratio. Whereas, time delay type has lower sensibility than the homodyne interference type, but has high signal-to-noise ratio because using the very narrow filter. In this paper the time delay type microwave reflectometry has been presented. First, the basic principle of the microwave reflectometry is given in the second section. Then, the equipment and experiment arrangement is presented. Finally, the experiment result of the HL-A tokamak is described. 1.Basic principle The microwave reflectometry for plasma density profile measurement is based on the standard radar technique. As we know, electromagnetic wave transferred in plasma will be reflected from a cut-off layer, when plasma density is high enough. For an O-mode This work is supported by the Natural Science Foundation of China under contract number

2 ( ) wave, the cut-off layer only depends on the plasma density. The dispersion relation of the O-mode electromagnetic wave is 1 ω pe( r, t) N = (1 ) ω ( t) Here: pe and are plasma frequency and electromagnetic wave frequency, respectively. N is the refraction index of the electromagnetic wave propagating in the plasma. Therefore, the different cut -off layers will reflect the electromagnetic wave with the different frequency. Using a scanning microwave, which is modulated by the 00MHz frequency, the cut -off layers can be determined by measurement of the flight time of the modulated wave. The basic quantity measured by the time-delay type microwave reflectometry is the transmission time of the modulated microwave towards the critical density layer. In order to obtain the full electron density profile, the modulated microwave at many different carrier frequencies have to be launched into the plasma, either simultaneously or successively. The delay time of the modulated wave can be measured through the phase difference comparing with a reference signal. The relationship between the phase difference and the time delay is simply given by the equation: 1 ϕ τ = π f m () Where, f is the modulation frequency, τ is the time delay, ϕ is the phase m difference between the signal wave and the reference wave. Then, the transmission time of the modulated microwave can be expressed by the phase difference between the signal wave and the reference wave. The phase difference can be calculated as following: 1 ω( t) r ϖ (, ) π ϕ ( ) = ant pe r t t (1 ) dr 3 r c c ϖ ( t) Where frequency; ω is the plasma frequency, ω pe = nee / meε 0 ; ω (t) is the microwave pe r ant is the distance from the center of the plasma to the antenna; r c is the distance from the center of the plasma to the cut-off layer, ω0is the modulation frequency. In equation (3) r and ϕ (t) c radius r of the cut-off layers with the different microwave frequencies through iterative method. The relationship between the plasma density and the microwave frequency is: c

3 n e ε0 meω ( t) = 4 e In fact the relationship of the radius r c and the plasma density n e of the cut-off layers obtained from the equation (3) and (4), is the electron density profile n e ( r c )..Equipment and experiment arrangement In the HL-A tokamak two modulated time delay type reflectometer has been installed in the middle plane of the torus. Fig 1 shows the block diagram of the reflectometer. In the system, there are four main parts: a scanning microwave source, a wide band modulator, a microwave detector and a phase detector. The microwave sources are two back wave oscillator (BWO) systems with different frequency range. The out put power in the total frequency range is not less than 50mW. The microwave oscillators includes two wave bands (Ka band from 6.5 GHz to 40GHz, U band from 40 GHz to 60 GHz). In this experiment, the plasma density is less than cm -3, therefore it is enough to use Ka band to measure the density profile on HL-A tokamak. The frequency of the BWO is controlled by a triangle voltage from 0 to 8.5V. The time resolution depends on the scanning time of the control voltage. The scanning time is 1ms in present experiment, it will be decreased to less than 100 s. Fig. 1, the block diagram of the time-delay type reflectometer The microwave out-put from BWO is modulated by a modulator with 50MHz frequency produced by a quartz oscillator. To modulate the microwave with wide frequency ranges a wide band modulator has been designed. Its modulation rate is more

4 than 30% in all frequency range. The modulated microwave launches into the plasma through the emitting antenna and reflected microwave is received by another receiving antenna, which is very closed to the emitting antenna. The modulated microwave received by the antenna passes through the wave-guide into a microwave detector. The 50MHz output signal of the detector is mixed with the signal of the quartz oscillator (5 MHz). The MHz mixing signal carried the phase shift information is amplified by the limiting amplifier. Before amplifying, the MHz signal will be filtered by a narrow band filter to increase the signal to noise ratio. The quartz oscillator 1 with 50 MHz frequency is divided two beams: one is used to modulate the microwave source, the other will be mixed with the signal of the quartz oscillator to provide the reference signal. With the similar mixer 1, the limiting amplifier 1 and the filter 1, we can obtain another MHz mixing signal. The phase shift signal and the reference signal are together transmitted into the phase detector. To avoid the large error of the measurement during the jump of the phase shift, a Q-I technique have been used. According to the Q-I technique, the phase shift signal is shifted / firstly then two phase difference can be detected: One is the phase difference between the phase shift signal and the reference signal, which is called Q channel phase difference. The other is the phase difference between the phase shift signal with additional / and the reference signal, which is called I channel phase difference. Because the two phase differences have an additional /, so the jump of the phase shift do not appear at the same time. It is easy to treat the data from both phase difference. Before plasma density profile measurements, the time -delay calibration must be done to take out the original phase differences produced by the transmission lines, the electronics and so on. The calibration will be done on the device without discharge. The microwave beam emitted into the vacuum vessel will be reflected at the chamber wall. The total delay time t 1 of the reflected wave from the chamber wall can be measured. The radius of the limiter in HL-A is 40cm, so we assume that the maximal radius of the plasma r ant is 40cm. Therefore, the total delay time, which should be taken out, is t = t1, so the delay time of the measurement in the plasma τ ( plasma) is c given by the following: τ ( plasma) = τ ( ω ) t 5 Where, τ ( plasma ) is the transmission time in the plasma, τ (ω ) is the total microwave transmission time. t is the microwave transmission time in the wave-guide and other devices. The frequency accuracy is very important for the measurement, so the relationship of the volt and the frequency must be calibrated. The following list is the relationship of the controlled volt and the frequency of the BWO, which are measured by accurate

5 voltage-meter and microwave frequency-meter,respectively. volt freq Fig. is the curve of the voltage vs frequency fitted by the formula f = V V V frequency voltage Fig., the voltage vs frequency fitted by the formula f = V V V 3.Experiment result The original data of the microwave reflectometry on HL-A is shown in Fig.3 In the figure from up to down there are the amplitude of the reflected signal, the phase difference from I detector, the frequency scanning voltage and the phase difference from Q detector of shot 560. The time of sampling is 1 s. The amplitude of the reflected signal is obtained from the limiting amplify as shown in Fig.1. to monitor the reflected wave. Because the phase difference from I detector or Q detector does not jump, so only one phase difference is needed to obtain the density profile. According to the basic principle of the microwave reflectometry mentioned above, the plasma density profiles can be obtained from the frequency scanning voltage and the phase difference through an available software.

6 Fig. 3, The original data of the microwave reflectometry of shot.560 on HL-A (a) the amplitude of the reflected signal (b)the phase difference from I detector, (c)the frequency scanning voltage and (d)the phase difference from Q detector The plasma density profile treated from the above original data is shown in Fig 4. The density range is from 0.8x10 13 cm -3 to x10 13 cm -3 using the Ka band microwave source. The evolution of the plasma density profile inside the r=16cm has been shown with the 3-D figure. To check the results of the new diagnostic on HL-A some comparisons with other diagnostics have been done. The average density measured by 3 the HCN interferometer is more than cm from 00 ms to 700ms in shot 560. In general, the density of the core plasma in tokamak is about 3/ of the average density. So the measurement results of the microwave reflectometer is in agreement with the measurement results of the HCN interferometer. Fig. 4, the plasma density profile of the shot. 560 Summary On HL-A tokamak, The time-delay reflectometry has been developed successfully. Using the equipment, we can measure the plasma density profile from cm -3 to cm -3. Temporal resolution is less than 1ms and spatial resolution is about 1cm.

7 The results of the new time-delay reflectometry are in agreement with other diagnostics. The microwave reflectometry is also can be used to measure the density fluctuation with fixed frequency and to obtain the density fluctuation profiles with different frequencies. Acknowledgment I am grateful to Prof. Ding Xuantong, Liu Zetian and Vladimir Zhuravlev for their enthusiastic support and valuable discussions, and I would like to thank all colleagues concerned in SWIP. Reference 1. John Wesson, 1997,Tokamaks, Oxford university press, Inc., New York, second edition, 483:494. Manso M E 1993, Plasma phys. Control, Fusion B 35: Sanchez J, Estrada T and Hartfuss H J 199, Microwave reflectometry for fusion plasma diagnostics Proc. IAEA Tech. comm. Meeting(Abingdon, 199), p C Laviron, A J H Donne, M E Manso and J Sanchez, Reflectometry techniques for density profile measurements on fusion plasmas, 907: Zhuravlev V, Sandchez J and de la Luna E 1995 Controlled fusion and plasma physics Proc. nd Eur. Conf.(Bournemouth, 1995) vol 19C pt IV (Geneva :European Physical Society) p C Laviron, A J H Donne, M E Manso and J Sanchez, Reflectometry techniques for density profile measurements on fusion plasmas, 90:93 7. V.A. Zhuravlev, V.A. Vershkov, Sov. Journal of technical physics, v.57, 1987, p 858.