Hydrogen Cyanide Laser Interferometer Research Articles

A fringe jump counting method for the phase measurement in the HCN laser interferometer on EAST and its FPGA-based implementation

Electron density in fusion plasma is usually diagnosed using laser-aided interferometers. The phase difference signal obtained after phase demodulation is wrapped, which is also called a fringe jump. A method has been developed to unwrap the phase difference signal in real time using FPGA, specifically designed to handle fringe jumps in the hydrogen cyanide (HCN) laser interferometer on the EAST superconducting tokamak. This method is designed for a phase demodulator using the fast Fourier transform (FFT) method at the front end. The method is better adapted for hardware implementation compared to complex mathematical analysis algorithms, such as field programmable gate array (FPGA). It has been applied to process the phase measurement results of the HCN laser interferometer on EAST in real time. Electron density results show good confidence in the fringe jump unwrapping method. Further possible application in other laser interferometers, such as the POlarimeter-INTerferometer (POINT) system on EAST tokamak is also discussed.

Optimization of power feedback control system for HCN interferometer on EAST Tokamak

The Hydrogen Cyanide (HCN) interferometer stands as an indispensable diagnostic tool, designated to measure the line-integrated electron density on the Experimental Advanced Superconducting Tokamak (EAST), thereby offering essential density feedback signals for EAST operation. The HCN laser used in the interferometer is a continuous glow discharge gas laser. However, owing to variations in external ambient temperature, the HCN laser exhibits output power instability. The current power automatic feedback control system for the HCN laser interferometer exhibits low actuator adjustment precision, thereby limiting its ability to achieve maximum output power. Moreover, the system may cause a temporary power drop to zero during adjustment, while EAST operation requires the HCN laser interferometer to maintain high power continuously. In order to solve the issues, this paper developed an innovative actuator that ingeniously amalgamates the virtues of piezoelectric ceramic and stepper motor. This novel actuator, demonstrating a broad adjustment range and high precision, is intended to replace the present one in the HCN interferometer power control system. While the stepper motor executes coarse adjustment, the piezoelectric ceramic enables precise adjustment, thus enhancing the adjustment accuracy of the automatic power control system to a sub-micrometer level and an adjustment range greater than 5 mm. Accurate determination of the regulator's adjustment direction can significantly improve the efficiency of adjustment. The slope of the laser output power peak decreases as it approaches the maximum value, and increases as it moves away from the maximum value. Therefore, this system replaces the original threshold algorithm with a slope algorithm. By correctly judging the adjustment direction, the efficiency of adjustment is improved. Furthermore, this algorithm can keep the output power stable on a power peak. There will be no instances of zero power output during the adjustment, allowing the power to remain stable above the set threshold for a long time. Currently, this system has been successfully applied to the HCN interferometer, and testing has found that the system can maintain stability for at least 16 hours. Furthermore, even when the laser output power was intentionally reduced to below the set threshold, the system was able to respond quickly and adjust the output, rapidly restoring it to above the predefined threshold. Multiple tests have shown the strong robustness of the system, indicating it meets the stringent demands of the EAST's complex environment.

Development of a dual HCN laser interferometer on a small tokamak device

A dual hydrogen cyanide (HCN) laser interferometer system was designed and developed to measure the electron density of plasma with high temporal resolution. The system comprises two terahertz continuous wave (CW) discharge-pumped HCN lasers, each emitting at a frequency of 0.89 THz and with an output power of up to 100 mW. The intermediate frequency (IF) signal is obtained by adjusting the length of the resonant cavity. The temporal resolution of the interferometer can reach 1 μs when the intermediate frequency is at 1 MHz. The feasibility of the dual HCN laser interferometer system was verified by simulating the plasma using a wedge. Preliminary experimental results were obtained on a small tokamak device, a root mean square error of the dual HCN laser interferometer is approximately 1.5×1017 m-3 when the line-integrated electron density obtained by the dual HCN laser interferometer is about 1.5×1019 m-3.

Development of a combined interferometer using millimeter wave solid state source and a far infrared laser on ENN’s XuanLong-50 (EXL-50)

A millimeter wave solid state source—far infrared laser combined interferometer system (MFCI) consisting of a three-channel 890 GHz hydrogen cyanide (HCN) laser interferometer and a three-channel 340 GHz solid state source interferometer (SSI) is developed for real-time line-integrated electron density feedback and electron density profile of the EXL-50 spherical tokamak device. The interferometer system is a Mach–Zehnder type, with all probe-channels measured vertically, covering the plasma magnetic axis to the outermost closed magnetic plane. The HCN laser interferometer uses an HCN laser with a frequency of 890 GHz as a light source and modulates a 100 kHz beat signal by a rotating grating, giving a temporal resolution of 10 μs. The SSI uses two independent 340 GHz solid-state diode sources as the light source, the frequency of the two sources is adjustable, and the temporal resolution of SSI can reach 1 μs by setting the frequency difference of the two lasers at 1 MHz. The main optical path of the two interferometers is compactly installed on a set of double-layer optical platform directly below EXL-50. Dual optical path design using corner cube reflectors avoids the large support structures. Collinear the probe-beams of two wavelengths, then the phase error caused by vibration can be compensated. At present, the phase noise of the HCN Interferometer is 0.08 rad, corresponding to a line-integrated electron density of 0.88 × 1017 m−2, one channel of measuring result was obtained by the MFCI system, and the highest density measured is about 0.7 × 1019 m−2.

Measurement on electron density of high-power and large-volume ICP-heated wind tunnel with HCN laser interferometer

This paper explains the physical behavior of the electron density of high-power and large-volume plasma wind tunnel using a single channel hydrogen cyanide (HCN) laser interferometer. Based on the characteristics of inductively coupled plasma (ICP)-heated wind tunnel, the temperature and pressure distribution of the ICP-heated wind tunnel are obtained from numerical simulations, during which the influence of neutral particles is considered to calculate the accurate electron density. The typical electron density order of ICP-heated wind tunnel is 1018m−3. We discovered that there is a positive correlation between the electron density of argon plasma jet and mass flow rate, while that of air plasma jet decreases slightly. We also found that the peak of electron density appears when the argon is switched to air. Within the voltage range of 6–10 kV, the electron density of argon and air plasma increases slowly. However, when the voltage increases from 10 to 12 kV, the electron density of air plasma increases sharply with the mass flow rate of 15 g/s. Finally, the electron density of argon plasma is much higher than that of air plasma at the same mass flow rate and voltage.

Designand implementation of plasma electron density measurements based on FPGA with all-phase FFT for tokamak devices.

Plasma electron density is one of the most fundamental parameters when studying tokamak plasma physics, which is widely used in plasma control systems and plasma physics analyses. A hydrogen cyanide laser interferometer is generally applied to measure the plasma electron density in many tokamak devices. Therein, the plasma electron density is calculated by measuring the phase difference between the reference signal and the detector signal. This work provides a new way to realize real-time measurements of the plasma electron density with a phase comparator and processing system based on a field-programmable gate array chip. The system integrates a signal processing module, an all-phase fast Fourier transform (ap-FFT) module realized via matrix operations and phase comparisons, and a network communication module all in one board. This work concludes that the ap-FFT is robust and accurate for phase calculations compared with a windowing FFT. A data-reuse method and a phase shift method are proposed to improve the time resolution and phase range. The phase error is less than 0.1° and the time resolution is 0.025ms, which is better than hardware methods and traditional software methods. This system is highly flexible with reduced designcosts to meet the requirements of a tokamak, which can provide a valuable reference for other tokamak and phase difference comparisons.

Design of HCN laser based interferometer on the Space Plasma Environment Research Facility (SPERF)

A single-channel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer will be constructed aiming to provide the line integrated electron density on the Space Plasma Environment Research Facility (SPERF) . A continuous glow discharge HCN laser with a cavity length of 3 m is used as the laser source with a wavelength of 337 μm and an output power about 10 mW . The interferometer system is a Mach-Zehnder type. The temporal resolution of the system is 0.1 ms since the phase modulation is achieved by a rotating grating with a modulation frequency of 10 kHz. The beat signal is detected by AlGaN/GaN high electron mobility transistors (HEMT) detector. It has a noise equivalent power of 10 pW/Hz−1/2. Excellent port access (1500×150 mm) and high laser power would promise a profile measurement across the whole plasma section with good signal quality. The design and structure of the single-channel FIR HCN laser interferometer in SPERF is described in detail.

Strong braking of m = 1 impurity-induced ‘snake’ in EAST sawtoothing plasma

Strong braking of the m = 1 impurity-induced (e.g. molybdenum emission) density perturbation ‘snake’ was observed in EAST sawtoothing plasma. The shift of the peak positions of soft x-ray emissions due to the snake can be as high as 7 cm. Notable electron density fluctuations (δne/ne0 can be as high as 8%) originating from this m = 1 snake are measured by a hydrogen cyanide laser interferometer. Due to mode coupling, synchronous locking of the m = 2 tearing mode and m = 1 snakes is expected to happen when the width of the m = 2 island exceeds a critical value. A critical width value of m = 2 for mode locking is calculated. The forced reconnection in the q = 2 surface is induced by a toroidal sideband of the m = 1 mode.

Far-infrared laser diagnostics on the HT-6M tokamak

A multichannel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer was developed to measure plasma electron density profile on the HT-6M tokamak. The structure of the seven-channel FIR laser interferometer is described. The laser source used in the interferometer was a continuous-wave glow discharge HCN laser with a cavity length of 3.4 m and power output of about 100 mW at 337 μm. The detection sensitivity was 1/15 fringe with a temporal resolution of 0.1 ms. Experimental results were measured by the seven-channel FIR HCN laser interferometer with edge Ohmic heating, a pumping limiter, and ion cyclotron resonant heating on the HT-6M tokamak are reported.