# Design of the cross-polarization scattering diagnostic on the HL-2A tokamak

R.H. Tong, W.L. Zhong, J. Wen, Z.B. Shi, X.L. Zou, A.S. Liang, Z.C. Yang, K.Y. Fang, M. Jiang, P.W. Shi, et al.

► To cite this version:

R.H. Tong, W.L. Zhong, J. Wen, Z.B. Shi, X.L. Zou, et al. Design of the cross-polarization scattering diagnostic on the HL-2A tokamak. Journal of Instrumentation, 2022, 17 (02), pp.C02025. 10.1088/1748-0221/17/02/c02025. cea-04790847

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Distributed under a Creative Commons Attribution 4.0 International License
4TH EUROPEAN CONFERENCE ON PLASMA DIAGNOSTICS (ECPD2021)

7–11 JUNE, 2021

ONLINE

# Design of the cross-polarization scattering diagnostic on the HL-2A tokamak

R.H. Tong,ª W.L. Zhong,ª,\* J. Wen,ª Z.B. Shi,ª X.L. Zou,ᵇ A.S. Liang,ª Z.C. Yang,ª K.Y. Fang,ª M. Jiang,ª P.W. Shi,ª X. Yu,ª Y. Zhou,ª M. Xuª and the HL-2A teamª

E-mail: zhongwl@swip.ac.cn

**ABSTRACT:** A new cross-polarization scattering (CPS) diagnostic has been developed on HL-2A, which aims to measure the local magnetic fluctuation inside the plasma. It is based on the scattering of an incident microwave beam into the perpendicular polarization by magnetic fluctuations. The CPS diagnostic has been designed in the *Q*-band (33–50 GHz), which consists of the electronic system, quasi-optical, and polarization rejector. The ray-tracing code is used to simulate the propagation of the probe and scattered rays. To test the performance of the quasi-optical system, a 3D test platform is built and detailed test results are shown. Two methods are developed for polarization rejector on HL-2A: wire grid polarizer and dual-polarized horn antenna (DPHA). The laboratory test result shows that the polarization rejection of both methods is better than 30 dB, which meets the needs for magnetic fluctuation detection. In the future, the CPS diagnosis will be used to study the electromagnetic turbulence behavior in the high-performance plasma of the HL-2A tokamak.

**KEYWORDS:** Plasma diagnostics — interferometry, spectroscopy and imaging; Nuclear instruments and methods for hot plasma diagnostics

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<footnote>ªSouthwestern Institute of Physics, P.O. Box 432, Chengdu 610041, People's Republic of China</footnote>

<footnote>ᵇCEA, IRFM, 13108 Saint-Paul-lez-Durance, France</footnote>

<footnote>*Corresponding author.</footnote>
# Contents

<table><tr><td>1</td><td>Introduction</td><td>1</td></tr><tr><td>2</td><td>CPS diagnostic on HL-2A</td><td>2</td></tr><tr><td>3</td><td>Quasi-optical system</td><td>3</td></tr><tr><td>4</td><td>Polarization rejector</td><td>4</td></tr><tr><td>5</td><td>Summary</td><td>6</td></tr></table>

## 1 Introduction

The improved physical model of confinement in plasma, especially the understanding of the high-confinement operation, is particularly important in a tokamak. The gyrokinetic theories predict that as the plasma $\beta$ (the ratio of volume-averaged plasma pressure $\langle p \rangle$ to magnetic pressure $B^2/2\mu_0$) increased, the electromagnetic turbulence can be dominant and drive significant anomalous transport [1]. It is found that magnetic turbulence can influence plasma confinement [2], pedestal dynamics [3], and edge-localized modes evolution [4] in experiments. In addition, the nonlinear gyro-kinetic simulation indicates that the electromagnetic turbulence, i.e. the micro-tearing modes (MTMs), is uniquely able to drive the abnormal electron transport in DIII-D high bootstrap current scenario [5, 6], which is a candidate scenario for future tokamak reactors.

The magnetic fluctuation $\tilde{B}$ measurement can provide experimental verification of the theory and model of magnetic turbulence [7], especially in high beta plasma. In tokamak plasma, the magnetic fluctuation is typically measured by magnetic coils [3], motional stark effect (MSE) [8], polarimetry [4, 9], etc. But they are generally limited to low wavenumbers of turbulence measured. Or it can be estimated indirectly by analyzing runaway electron transport [10–12]. Cross polarization scattering (CPS) is an internal magnetic fluctuation diagnostic based on the microwave technique. It is a local, internal measurement due to the swelling effect near the cut-off layer [13]. The advantages of CPS include flexible spatial and wavenumber resolution, and the non-perturbing nature. The basic idea of the CPS is the incident microwave will be scattered into the orthogonal polarization to that of the incident polarization by local magnetic fluctuation. In a tokamak, two eigenmodes exist when the wave propagation is perpendicular to $\mathbf{B}$: the ordinary mode ($O$) with polarization parallel to $\mathbf{B}$, and the extraordinary mode ($X$) with polarization perpendicular to $\mathbf{B}$. So the CPS processes can be described as: $X_i + \tilde{B} \rightarrow O_s$ or $O_i + \tilde{B} \rightarrow X_s$, where the subscript indices i and s refer to the incident and scattered waves respectively. It is opposed to scattering to the same polarization by electron density fluctuations $\tilde{n}$: $X_i + \tilde{n} \rightarrow X_s$ or $O_i + \tilde{n} \rightarrow O_s$, which is typically used to measure density fluctuations and the propagation velocity of turbulence, i.e. Doppler backscattering (DBS) diagnostic. For DBS, a tilt angle of the microwave beam is launched relative
to the normal of the cutoff layer and backscattered radiation is detected [14, 15]. The CPS diagnostic was originally demonstrated on Tore Supra tokamak [16], where the effect of magnetic fluctuations on electron heat transport was studied. After that, the CPS was developed on GAMMA-10 tandem mirror [17], MAST spherical tokamak [18] and DIII-D tokamak [19–21].

HL-2A is a medium-sized conventional tokamak (major radius $R_0$ = 1.65 m and minor radius $a$ = 0.40 m), which performs with plasma current $I_p$ ~ 100–450 kA, centre line-average electron density $n_e$ ~ (0.2–5.5) × 10¹⁹ m⁻³ and toroidal magnetic field $B_t$ ~ 1.0–2.7 T. The electron and ion temperatures are up to $T_e$ = 5 keV and $T_i$ = 3.5 keV, respectively. Recently, a high beta, high confinement discharge has been achieved on HL-2A [22]. Besides, a broadband (frequency of 50–150 kHz) electromagnetic turbulence has been observed by magnetic coils in the HL-2A H-mode plasmas [3, 23]. To further study the behaviors of magnetic turbulence, which may play an important role in the achievement of high $\beta$ plasma and the evolution of the pedestal, a CPS system is developed on HL-2A recently. In the following section, we first introduced the detailed setup of the CPS diagnostic on HL-2A (section 2) followed by the design and test results for the quasi-optical (section 3) and polarization rejection system (section 4). Finally, a summary is presented in section 5.

## 2 CPS diagnostic on HL-2A

To measure the magnetic turbulence in the pedestal region for typical HL-2A discharge. The CPS is designed based upon Q-band (33–50 GHz) with X mode launched wave. The electronic system is as same as the eight-channel Q-band DBS as described in ref. [15]. It is based on the Multiplexer-based Frequency Array Source (MFAS) technique. The output power for 8 frequencies is >15 dBm with flatness <5 dB. A 40.5 GHz local oscillation is used, which results in the intermediate frequency varies from 0.5 to 7.5 GHz. Figure 1 shows the configuration of the CPS diagnostic developed on HL-2A. It mainly consists of 3 parts: (1) quasi-optical system; (2) launch/receive system with polarization rejector; (3) electronic system for launching/receiving the microwave signals. The X-mode polarization probe beam was selected by the polarization rejector and launched from the low field side midplane through the quasi-optical system. Then the O-mode beam will be scattered by the magnetic fluctuations near the cut-off layer.

<figure><img src="image_3.png" /><figcaption>Figure 1. CPS diagram on HL-2A.</figcaption></figure>
As the X- and O-mode waves possess different wavenumbers along the incident ray path, the local incident and scattered wavenumbers will be different for the CPS scattering process. Hence, the probe angle, probe frequency and plasma parameters will determine the probed wavenumber of $\tilde{B}$. Figure 2 shows ray tracing calculation for an X-mode probe beam and O-mode CPS receive beam with frequency 48 GHz. The plasma used is a 1.3 T, lower single null, HL-2A H-mode. To be received by the CPS diagnostic, the wavevector ($k_s = k_i + k_{\tilde{B}}$) and frequency ($\omega_s = \omega_i + \omega_{\tilde{B}}$) matching conditions should be satisfied, where $k_i$ and $k_s$ are the incident and scattered wavevector, respectively, and $k_{\tilde{B}}$ is the wavevector of the magnetic fluctuation (similarly for the frequencies $\omega_s$, $\omega_i$ and $\omega_{\tilde{B}}$). It should be noted that although the electric swelling effect can cause the most contribution of scattering near the cut-off layer [13], the multiple scattering centres may exist at any position along the entire ray path as reported in the reference [24].

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Ray tracing result with X-mode launch and O-mode received at 48 GHz on HL-2A H-mode plasma.</figcaption></figure>

### 3 Quasi-optical system

The quasi-optical system consists of a spherical lens, a parabolic mirror and a plane mirror. The mechanical and optical designs are shown in figures 3(a) and (b). The design goal was to form a high-quality Gaussian beam near the edge of HL-2A plasmas for the range of frequencies applicable to Q-band operation. To probe a range of wavenumber values with the CPS system, an adjustable launch angle is necessary. With the help of feedthrough, the launched angle of the beam can be changed from -10° to +10° in the vertical direction. A positive angle indicates the wave is launched upward. A higher mirror angular range is limited by the size of the vacuum window. To test the performance of the quasi-optical system, a 3D test platform has been developed as shown in figure 3(c). The microwave is launched by the horn antenna and passes through the quasi-optical system. The power distribution in the X-Y-Z direction can be measured through the 3D displacement platform. The high-quality Gaussian beams can be observed for different frequencies in the Y-Z plane as shown in figure 3(c).
<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) The mechanical design. (1) Spherical lens; (2) parabolic mirror; (3) plane mirror. (4) Feedthrough for mirror angle control; (5) vacuum chamber. (b) Gaussian optical simulation results of the quasi-optical system. (c) The 3D test platform for the quasi-optical system: (1) CPS microwave source; (2) launch horn antenna; (3) quasi-optical system; (4) receive horn; (5) 3D displacement platform; (6) position controller; (7) spectrum analyzer; (8) control computer. The *X*, *Y* and *Z* direction corresponds to the radial, toroidal and vertical direction of plasma, respectively. The typical Gaussian beam measured in the *Y-Z* direction at *f* = 36 GHz is shown in (c).</figcaption></figure>

Figure 4 shows the relationship between the launch angle and the mirror position. Here the mirror position is defined as the stroke depth of the feedthrough, which is used to control the angle of the flat mirror. When the launch angle is out of range (±10°), the Gaussian beam will be destroyed due to the reflection of the vacuum chamber (not shown). The Gaussian beam radii distributions along the beam propagation direction are shown in figure 4(b) for 8 frequencies. By changing the distance between the launch antenna (WR-22 standard gain waveguide horn antenna in the test) and the spherical lens (L), the Gaussian beam distribution will be changed. At L = 225 mm, the beam radius is ~40 mm at the plasma edge, and the divergence half-angle of the Gaussian beam is about 4.4 degrees. A near-parallel incident beam near the cut-off layer is considered to be better for signal detection.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a) The relationship between the launch angle and mirror position. A positive angle indicates the wave is launched upward. (b) The Gaussian beam radius along with the beam propagation (*x*) direction. *L* is the distance between the launch antenna and the spherical lens. The shading indicates the range of 8 frequencies and the solid line is the mean value. Locations of various plasma radii are indicated by vertical red lines. The data are laboratory test results.</figcaption></figure>
## 4 Polarization rejector

It should be mentioned that the power level from $\tilde{B}$ is much lower than $\tilde{n}$ for typical tokamak parameters. The relative magnetic fluctuation level $|\tilde{B}/B|$ is from about $10^{-4}$ to $10^{-6}$ measured with typical tokamak parameter [4, 16]. So the expected relative fluctuation level $|\tilde{B}/B|^2/|\tilde{n}/n|^2 \sim 10^{-2}-10^{-5}$ [4, 16, 18, 25]. As mentioned in reference [13, 16], the scattered power from magnetic fluctuation $P_{CPS} \propto (\tilde{B}/B)^2$, and the scattered power from electron density fluctuation $P_{DBS} \propto (\tilde{n}/n)^2$. So one of the key points of CPS diagnostic is the signal scattered by $\tilde{n}$ must be rejected, i.e. the polarization of the launched and scattered microwave is the same. The isolation must be larger than 20 dB at least, which corresponds to $|\tilde{B}/B|^2/|\tilde{n}/n|^2 = 10^{-2}$. Considering the robustness in practical application, at least 30 dB should be guaranteed. To meet this requirement. The so-called Polarizing mirror effect with a cut-off layer to reject the signal from $\tilde{n}$ is applied on Tore Supra [16], and the wire grid polarizers are applied in DIII-D [21]. Two methods are developed for polarization rejector on HL-2A: wire grid polarizer and dual-polarized horn antenna (DPHA). Another key point is the contamination by polarization mode conversion or the mismatched launch/receive polarization with the plasma magnetic field pitch angle [16, 18, 21]. Since the power of the unwanted scattering component is proportional to $\sin^2(\delta)$, where $\delta$ is the mismatch angle. A polarization rejection of ~30 dB can still satisfy with $\delta = 2^\circ$ mismatch angle in theory. To minimize the mismatch angle, a motorized rotation stage is used to control the angle of the polarizer or DPHA, and match the polarization of the beam with magnetic field pitch angle with an angle resolution of 0.1 degree. The laboratory test results show that the polarization rejection of both methods is better than 30 dB, as shown below.

The first method is the wire grid polarizer. It is made of 10 micron diameter tungsten wire and the wire spacing is 25 microns. The wire grid polarizer will reflect incoming waves where the electric field is polarized parallel to the wires, and transmit radiation where the electric field is polarized orthogonal to the wires. The polarizer can be used up to 3 THz with a normalized transmission close to 1. The relationship between the power received by the rectangular horn antenna and the polarizer angle is shown in figure 5. When the polarizer angle is 0°, the launched wave (horizontal polarization) will be reflected by the polarizer, resulting in the min power being received by R1 (about -50 dBm) and the max power being received by R2 (about -10 dBm). When the polarizer angle is 90°, the wave will transmit, so the max power is received by R1 (-10 dBm). The test results show that the rejection of the polarizer is >40 dB. It should be mentioned that the actual rejection can be better because the test result is limited by the detector noise floor.

The second method is to use the dual-polarized horn antenna (DPHA), which is a three-port antenna, which can separate a circular or elliptical polarized wave into two linear, cross polarized waves. Compared with the wire grid polarizer, DPHA has the advantage of long working life, compact structure and less space occupation. To test the performance of DPHA, a linear polarization wave is launched towards DPHA with a rectangular horn antenna. And the power received by the two ports of DPHA with different angles is shown in figure 6. The test results show that the orthogonal polarization rejection of DPHA is >30 dB, which meets the needs of CPS diagnostic.
<figure><img src="image_7.png" /><figcaption>**Figure 5.** The power received by antenna (a) R1 and (b) R2 with different polarizer angles. The polarization direction for launch (L1) and receive (R1 & R2) rectangular horn antenna are all in horizontal directions. 0 degree of polarizer corresponds to the wave with horizontal polarization will be reflected. The data are laboratory test results.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 6.** The power received by (a) port1 and (b) port2 of DPHA with different DPHA angles. 0 degree corresponds to the polarization directions of the launch rectangular horn antenna and the port2 of DPHA are the same. The data are laboratory test results.</figcaption></figure>

**5 Summary**

In summary, a Q-band multi-channel cross-polarization scattering diagnostic has been developed on HL-2A. The 8 working frequencies are from 34 to 48 GHz, with a frequency interval of 2 GHz. The test and calibration of the quasi-optical system have been done with a 3D test platform. It is shown that the launch angle can be controlled from -10° to 10°, with the beam radius ~40 mm at the plasma edge. For polarization rejector, the cross isolation >40 dB has been achieved with wire grid polarizer and >30 dB with Dual-polarized horn antenna (DPHA) in laboratory tests. The CPS diagnostic has already been installed on HL-2A, and an experimental investigation of
electromagnetic turbulence will be carried out in the next HL-2A campaign. 3D ray-tracing and full-wave simulations will be the next steps for a better understanding of the CPS data. In addition, the toroidal angle launching capability will be evaluated in the future system upgrade to optimize high-k measurement [26] and improve signal quality.

## Acknowledgments

The author R.H. Tong would like to gratefully acknowledge the HL-2A team for the support of the study and Doctor Y.L. Zhu for useful discussions. This work was supported by the National Key R & D Program of China under Grant No. 2017YFE0301106 and the National Natural Science Foundation of China under Grant No. 11922503. It was partially supported within the framework of the cooperation between the French Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA) and the China National Nuclear Corporation (CNNC).

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