Research on the real-time signal conditioning method for dispersion interferometer in HL-2M

A new CO₂ laser dispersion interferometer has been developed in the HL-2M tokamak to measure the electron density. In order to meet the needs of high-precision measurement, a data acquisition system with real-time signal conditioning (RSC) method is proposed. It can eliminate part of the impacts of environmental factors, such as mechanical vibration, light path changes, and plasma refraction effect during experiments. In harsh environments, the system can measure the line-integrated density with a high precision of 2 × 10¹⁸ m⁻² with the RSC method. The system has been tested in a recent HL-2A experimental campaign, and the results show that the RSC method plays an important role in the plasma electron density measurement.     

Improved density measurement by FIR laser interferometer on EAST tokamak

A three-channel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer is in operation since 2010 to measure the line averaged electron density on experimental advanced superconducting tokamak (EAST). The HCN laser signal is improved by means of a new schottky barrier diode (SBD) detector. The improved measurement and latest experiment results of the three-channel FIR laser interferometer on EAST tokamak are reported.     

Far-Infrared Laser Diagnostics in EAST

Structure, improvements and experiment results of a vertical three-channel far- in- frared (FIR) hydrogen cyanide (HCN) laser interferometer, operated routinely in EAST to measure the electron density profi, are presented. Moreover, a five-channel deuterium cyanide (DCN) laser interferometer was developed successfully. Some key issues confronted in development, including the economization of working gas and the solution to atmospheric absorption, are resolved and described in detail.     

Design of Q-band FMCW reflectometry for electron density profile measurement on the Joint TEXT tokamak

The Q-band(33-50 GHz) fast sweep frequency modulated continuous wave(FMCW)reflectometry has been recently developed for electron density profile measurement on the Joint TEXT tokamak.It operates in ordinary mode(O-mode) with a 20 μs sweeping period,covering the density range from 1 × 10~(19) m~(-3) to 3 × 10~(19) m~(-3).On the bench test,a Yttrium Iron Garnet(YIG) filter is used for the dynamic calibration of the voltage controlled oscillator(VCO) to obtain a linear frequency sweep.Besides,the use of a power combiner helps to improve the sideband suppression level of the single side-band modulator(SSBM).The reconstructed density profiles are presented,which demonstrate the capability of the reflectometry.     

Radiation shielding design of the CFETR polarimeter interferometer and CO2 dispersion interferometer

A three-wave based laser polarimeter/interferometer and a CO₂ laser dispersion interferometer are used to determine the electron and current density profiles on a Chinese fusion engineering test reactor (CFETR). Radiation shielding is designed for the combination of polarimeter/interferometer and CO₂ dispersion interferometer. Furthermore, neutronics models of the two systems are developed based on the engineering-integrated design of CFETR polarimeter/interferometer and CO₂ dispersion interferometer and the major material components of CFETR. The polarimeter/interferometer and CO₂ dispersion interferometer's neutron and photon transport simulations were performed using the Monte Carlo neutral transport code to determine the energy deposition and neutron energy spectrum of the optical mirrors. The energy depositions of the first mirrors on the polarimeter/interferometer are reduced by three orders with the whole shielding. Since the mirrors of CO₂ dispersion interferometer are very close to the diagnostic first wall, shielding space is limited and the CO₂ dispersion interferometer energy deposition is higher than that of the polarimeter/interferometer. The dose rate after shutdown 10⁶ s in the back-drawer structure has been estimated to be 83 μSv h⁻¹ when the radiation shield is filled in the diagnostic shielding modules, which is below the design threshold of 100 μSv h⁻¹. Radiation shielding design plays a key role in successfully applying polarimeter/interferometer and CO₂ dispersive interferometer in CFETR.     

Inactivation of Staphylococcus aureus by the synergistic action of charged and reactive plasma particles

In this paper, a low-pressure capacitively coupled plasma discharge sustained in an argonoxygen mixture was studied in order to evaluate its properties in terms of inactivation of Staphylococcus aureus. The plasma parameters as electron temperature and plasma density were measured by the Langmuir probe (N_e ≈ 10¹⁵ m⁻³, T_e ≈ 1.5 eV), while the neutral atom density was in the range of 10²¹ m⁻³. In the plasma phase, oxygen radicals were taken as reference of the reactive species with antimicrobial activity, and oxygen spectral lines, over a range of plasma process parameters, were investigated by the optical emission spectroscopy. Optimal plasma conditions were found, and a count reduction of 4 log in a few minutes of the bacterium proves the potentiality of an industrial grade plasma reactor as a sterilization agent.     

Characteristics of plasma in a novel laser-assisted pulsed plasma thruster

A novel laser-assisted pulsed plasma thruster (LA-PPT) is proposed as an electric propulsion thruster, which separates laser ablation and electromagnetic acceleration. It aims for a higher specific impulse than that achieved with conventional LA-PPTs. Owing to the short-time discharge and the novel configuration, the physical mechanism of the discharge is unclear. Time and spatial-resolved optical emission spectroscopy was applied to investigate the variation in the plasma properties in the thruster discharge channel. The plasma species, electron temperature, and electron density were obtained and discussed. Our investigation revealed that there were H_α, H_β, H_γ, H_ε atoms, C I, C II, C III, C IV, Cl I, Cl II particles, and a small amount of CH, C₃, C₂, H₂ neutral molecular groups in the plasma. The electron temperature of the discharge channel of the thruster was within 0.6–4.9 eV, and the electron density was within (1.1–3.0) $\times $ 10¹⁸ cm⁻³, which shows that the optical emission spectroscopy method is to measure the electron excitation temperature and electron density in heavy particles. But the Langmuir probe method is to measure the temperature and density of free electrons. The use of laser instead of spark plug as the ignition mode significantly changed the plasma distribution in the discharge channel. Unlike the conventional PPT, which has high electron density near the thruster surface, LA-PPT showed relatively large electron density at the thruster outlet, which increased the thruster specific impulse. In addition, the change in the ignition mode enabled the electron density in the LA-PPT discharge channel to be higher than that in the conventional PPT. This proves that the ignition mode with laser replacing the spark plug effectively optimised the PPT performance.     

Bright γ-ray source with large orbital angular momentum from the laser near-critical-plasma interaction

We propose a new laser-plasma-based method to generate bright γ-rays carrying large orbital angular momentum by interacting a circularly polarized Laguerre–Gaussian laser pulse with a near-critical hydrogen plasma confined in an over-dense solid tube. In the first stage of the interaction, it is found via fully relativistic three-dimensional particle-in-cell simulations that high-energy helical electron beams with large orbital angular momentum are generated. In the second stage, this electron beam interacts with the laser pulse reflected from the plasma disc behind the solid tube, and helical γ beams are generated with the same topological structure as the electron beams. The results show that the electrons receive angular momentum from the drive laser, which can be further transferred to the γ photons during the interaction. The γ beam orbital angular momentum is strongly dependent on the laser topological charge l and laser intensity a0, which scales as ${L}_{\gamma }\propto {a}_{0}^{4}$. A short (duration of 5 fs) isolated helical γ beam with an angular momentum of −3.3 × 10⁻¹⁴ kg m² s⁻¹ is generated using the Laguerre–Gaussian laser pulse with l = 2. The peak brightness of the helical γ beam reaches 1.22 × 10²⁴ photons s⁻¹ mm⁻² mrad⁻² per 0.1% BW (at 10 MeV), and the laser-to-γ-ray angular momentum conversion rate is approximately 2.1%.     

Microwave preionization and electron cyclotron resonance plasma current startup in the EXL-50 spherical tokamak

Preionization has been widely employed to create initial plasma and help the toroidal plasma current formation. This research focuses on implementing a simple, economical and practical electron cyclotron resonance (ECR) preionization technique on the newly constructed EXL-50 spherical tokamak, and evaluating the effectiveness on improving the plasma current startup. Two types ECR microwave preionization experiments for the plasma initialization without the central solenoid are reported: (1) 2.45 GHz microwave preionization and current startup with 2.45 GHz ECR source; (2) 2.45 GHz microwave preionization and current startup with 28 GHz ECR source. Application of the 2.45 GHz ECR microwave preionization to the experiments has contributed to (1) getting rid of the plasma breakdown delay; (2) the significant improvement of the discharge quality: the discharge is much longer and more stable while the driven plasma current is larger, compared to the discharge without preionization.     

Oxide Coated Cathode Plasma Source of Linear Magnetized Plasma Device

Plasma source is the most important part of the laboratory plasma platform for fundamental plasma experimental research. Barium oxide coated cathode plasma source is well recognized as an effective technique due to its high electron emission current. An indirectly heated oxide coated cathode plasma source has been constructed on a linear magnetized plasma device. The electron emission current density can reach 2 A/cm 2 to 6 A/cm ² in pulsed mode within pulse length 5–20 ms. A 10 cm diameter, 2 m long plasma column with density 10 ¹⁸ m ⁻³ to 10 ¹⁹ m ³ and electron temperature T_e ≈ 3–7 eV is produced. The spatial uniformity of the emission ability is less than 4% and the discharge reproducibility is better than 97%. With a wide range of the plasma parameters, this kind of plasma source provides great flexibility for many basic plasma investigations. The detail of construction and initial characterization of oxide coated cathode are described in this paper.     

Research on the method of dual-frequency microwave diagnosis of plasma for solving phase integer ambiguity

In this work,microwaves and terahertz waves have performed a dual-frequency combineddiagnosis in high-temperature,large-scale plasma.According to the attenuation and phase shift of electromagnetic waves in the plasma,the electron density and collision frequency of theplasma can be inversely calculated.However,when the plasma size is large and the electron density is high,the phase shift of the electromagnetic wave is large (multiple times 2π period).Due to the limitations of the test equipment,the true phase shift is difficult to test accurately or to recover reality.That is,there is a problem of phase integer ambiguity.In order to obtain a phase shift of less than 180°,a higher electromagnetic wave frequency (terahertz wave with 890 GHz)is used for diagnosis.However,the attenuation of the terahertz wave diagnosis is too small (less than 0.1 d B),only the electron density can be obtained,and the collision frequency cannot be accurately obtained.Therefore,a combined diagnosis was carried out by combining twofrequencies (microwave with 36 GHz,terahertz wave with 890 GHz) to obtain electron density and collision frequency.The diagnosis result shows that the electron density is in the range of(0.65–1.5)×10~(19)m~(-3),the collision frequency is in the range of 0.65–2 GHz,and the diagnostic accuracy is about 60%.     

Numerical study on the modulation of THz wave propagation by collisional microplasma photonic crystal

In order to demonstrate the modulation of terahertz wave propagation in atmospheric pressure microplasmas, in this work, the band structure and the transmission characteristics of a onedimensional collisional microplasma photonic crystal are investigated, using the transfer matrix method. For a lattice constant of 150 μm and a plasma width of 100 μm, three stopbands of microplasma photonic crystal are observed, in a frequency range of 0.1–5 THz. Firstly, an increase in gas pressure leads to a decrease in the central frequency of the stopband. When the gas pressure increases from 50.5 kPa to 202 kPa, the transmission coefficient of the THz wave first increases and then decreases at high frequency, where the wave frequency is much greater than both the plasma frequency and the collision frequency. Secondly, it is interesting to find that the central frequency and the bandwidth of the first THz stopband remain almost unchanged for electron densities of less than 10¹⁵ cm^–3, increasing significantly when the electron density increases up to 10¹⁶ cm^–3. A central frequency shift of 110 GHz, and a bandgap broadening of 200 GHz in the first stopband are observed. In addition, an atmospheric pressure microplasma with the electron density of 1 × 10¹⁵–6 × 10¹⁵ cm^–3 is recommended for the modulation of THz wave propagation by plasma photonic crystals.     

Development of the HCN heterodyne collective scattering system on J-TEXT

A heterodyne collective scattering system has been designed and developed to investigate the turbulent transport of core plasma on J-TEXT. A dual-HCN laser which consists of two separately pumped HCN gas lasers at 337 μm has been developed as the laser source of the scattering system. The intermediate frequency (IF) is ∼1 MHz when there is a 4 μm cavity length difference and capable to maintain stability more than 5 h without manual operation. Detection channels at three different angles (2 ≤ k⊥ ≤ 12 cm−1) have been installed with Schottky barrier diode mixers of 893 GHz. The sampling frequency of the acquisition system is 6 MHz to observe low-frequency density fluctuations. Initial experimental results have been detected and more results can be expected in future experiments.     

Estimation of plasma density perturbation from dusty plasma injection by laser irradiation on tungsten target in DiPS

To investigate the interaction of dusty plasma with magnetized plasmas at divertor plasma simulator, radial profiles of plasma density(ne) and electron temperature were measured in terms of plasma discharge currents and magnetic flux intensity by using a fast scanning probes system with triple tips. Dusty plasma with dusts(a generation rate of 3 μg s~(-1) and a size of 1–10 μm)was produced via interactions between a high-power laser beam and a full tungsten target. As ne increases, the scale of the effects of dusty plasma injection on magnetized plasmas was decreased. Also, the duration of transient fluctuation was reduced. For numerical estimation of plasma density perturbation due to dusty plasma injection, the result was ~10% at a core region of the magnetized plasma with n_e of(2–5)×10~(11) cm~(-3) at steady state condition.     

Development of a helicon-wave excited plasma facility with high magnetic field for plasma-wall interactions studies

The high magnetic field helicon experiment system is a helicon wave plasma(HWP)source device in a high axial magnetic field(B_0)developed for plasma–wall interactions studies for fusion reactors.This HWP was realized at low pressure(5?×?10~(-3)?-?10 Pa)and a RF(radio frequency,13.56 MHz)power(maximum power of 2 k W)using an internal right helical antenna(5 cm in diameter by 18 cm long)with a maximum B_0of 6300 G.Ar HWP with electron density~10~(18)–10~(20)m~(-3)and electron temperature~4–7 e V was produced at high B_0 of 5100 G,with an RF power of 1500 W.Maximum Ar~+ion flux of 7.8?×?10~(23)m~(-2)s~(-1)with a bright blue core plasma was obtained at a high B_0 of 2700 G and an RF power of 1500 W without bias.Plasma energy and mass spectrometer studies indicate that Ar~+ion-beams of 40.1 eV are formed,which are supersonic(~3.1c_s).The effect of Ar HWP discharge cleaning on the wall conditioning are investigated by using the mass spectrometry.And the consequent plasma parameters will result in favorable wall conditioning with a removal rate of 1.1?×?10~(24)N_2/m~2 h.     

Bench test of interferometer measurement for the Keda Reconnection eXperiment device (KRX)

Motivated by the need of the electron density measurement for the Keda Reconnection eXperiment (KRX) facility which is under development, an interferometer system has been designed and tested in bench. The 320 GHz solid-state microwave source with 1 mm wavelength is used to fulfill the high phase difference measurement in such low temperature plasma device. The results of the bench test show that the phase difference is accurately measured. In contrast to tens of degrees of phase shift expected to be measured on the KRX, the system noise (∼1°) is low enough for the KRX diagnostics. In order to optimize the system for better performance, we utilize the Terasense sub-THz imaging system to adjust alignment. The interferometer system has also been calibrated via changing of the optical path length controlled by the piezo inertial motor. Simultaneously, high density polyethylene thin film is introduced successfully to change a tiny phase difference and test the sensitivity of the interferometer system.     

Pulse-burst laser-based 10kHz Thomson scattering measurements

Thomson scattering(TS),as a popular and reliable diagnostic technique,has successfully measured electron temperatures and electron number densities of plasmas for many years.However,conventional TS techniques using Nd:YAG lasers operate only at tens of hertz.Here,we present the development of a high-repetition-rate TS instrument based on a high-speed,pulse-burst laser system to greatly increase the temporal resolution of measurements.Successful instrument prototype testing was carried out by collecting TS light from laboratory helium and argon plasmas at 10 kHz.Calibration of the instrument detection sensitivity using nitrogen/oxygen rotational Raman scattering signal is also presented.Quantitative electron number densities and electron temperatures of the plasma were acquired at 10 kHz,for stable plasma discharges as,respectively,～0.9 eV and ～5.37×10~(21)m~(-3) for the argon plasma,and ～1eV and ～6.5×1021 m~(-3) for the helium plasma.     

光化学法锂同位素分离中化学反应速率常数测量

氢气与锂蒸气的化学反应速率常数是激光化学法锂同位素分离过程的一个重要参数。本文以可调谐半导体激光器为工具,通过监测锂原子蒸气对光的吸收测量原子蒸气密度的相对变化,建立了一种测量该速率常数的方法。在光化学锂同位素分离的典型条件下测得化学反应速率常数为9.0×10^-22 m³/s。该数值对于未来锂同位素分离装置中激光照射时间和氢气密度等参数的选择具有重要指导意义。     

Spectral characteristic of laser-induced plasma in soil

The spectral characteristic of laser-induced plasma in soil was studied in this work, laser-induced breakdown spectroscopy was used to analyze the spectral characteristic of plasma under the condition of different time delays and irradiances. Moreover, the time evolution characteristics of plasma temperature and electron density were discussed. Within the time delay range of 0-5 μs,the spectral intensity of the characteristic lines of Si I: 288.158 nm, Ti I: 336.126 nm, Al I:394.400 nm and Fe I: 438.354 nm of the four main elements in two kinds of national standard soil decayed exponentially with time. The average lifetime of the spectral lines was nearly 1.56 μs. Under the condition of different time delays, the spectral intensity of Pb I: 405.78 nm in soil increased linearly with laser energy. However, the slope between the spectral intensity and laser energy decreased exponentially with the increase in time delay, from 4.91 to 0.99 during 0-5 μs. The plasma temperature was calculated by the Boltzmann plot method and the electron density was obtained by inversion of the full width at half maximum of the spectrum. The plasma temperature decreased from 8900 K to 7800 K and the electron density decreased from 1.5 × 10~(17) cm~(-3) to 7.8 × 10~(16) cm~(-3) in the range of 0-5 μs.     

High density γ-ray emission and dense positron production via multi-laser driven circular target

A diamond-like carbon circular target is proposed to improve γ-ray emission and pair production with a laser intensity of 8×10²² Wcm⁻² by using 2D particle-in-cell simulations with quantum electrodynamics. It is found that the circular target can enhance the density of γ-photons significantly more than a plane target, when two colliding circularly polarized lasers irradiate the target. By multi-laser irradiating the circular target, the optical trap of lasers can prevent the high energy electrons accelerated by laser radiation pressure from escaping. Hence, γ-photons with a high density of beyond 5000n_c are obtained through nonlinear Compton backscattering. Meanwhile, 2.7×10¹¹ positrons with an average energy of 230 MeV are achieved via the multiphoton Breit–Wheeler process. Such an ultrabright γ-ray source and dense positron source can be useful in many applications. The optimal target radius and laser mismatching deviation parameters are also discussed in detail.