Effect of N_2 and O_2 on OH radical production in an atmospheric helium microwave plasma jet

UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X)(0–0)band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source.The effect of the addition of molecular gases N_2 and O_2 to He plasma jets on OH generation was studied.Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species.Stark broadening of the hydrogen Balmer emission line(H_β)was used to estimate the electron density nein the jets.For both He/N_2 and He/O_2 jets, newas estimated to be on the order of 10~(15)cm~(-3).The effects of plasma power and gas flow rate were also studied.With increase in N_2 and O_2 flow rates, netended to decrease.Gas temperature in the He/O_2 plasma jets was elevated compared to the temperatures in the pure He and He/N_2 plasma jets.The highest OH densities in the He/N_2 and He/O_2 plasma jets were determined to be 1.0?×10~(16)molecules/cm~3 at x?=?4 mm(from the jet orifice)and 1.8?×?10~(16)molecules/cm~3 at x=3 mm, respectively.Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways, respectively, for OH formation within the jet column and in the downstream and far downstream regions.The presence of strong emissions of the N_2~+ bands in both He/N_2 and He/O_2 plasma jets, as against the absence of the N_2~+ emissions in the Ar plasma jets, suggests that the Penning ionization process is a key reaction channel leading to the formation of N_2~+ in these He plasma jets.     

Experimental investigation on the evolution of plasma properties in the discharge channel of a pulsed plasma thruster

Pulsed plasma thrusters(PPTs) are an attractive form of micro-thrusters due to advantages such as their compactness and lightweight design compared to other electric propulsion systems.Experimental investigations on their plasma properties are beneficial in clarifying the complex process of plasma evolution during the micro-second pulse discharge of a PPT. In this work, the multi-dimensional evolutions of the light intensity of the PPT plasma with wavelength, time, and position were identified. The plasma pressure was obtained using an iterative process with composition calculations. The results show that significant ion recombination occurred in the discharge channel since the line intensities of CII, CIII, CIV, and FII decreased and those of CI and FI increased as the plasma moved downstream. At the center of the discharge channel, the electron temperature and electron density were in the order of 10 000 K and 10~(17) cm~(-3),respectively. These had maximum values of 13 750 K and 2.3?×?10~(17) cm~(-3) and the maximum temperature occurred during the first half-cycle while the maximum number density was measured during the second half-cycle. The estimated plasma pressure was in the order of 10~5 Pa and exhibited a maximum value of 2.69?×?10~5 Pa.     

Investigation on the characteristics of an atmospheric-pressure microplasma plume confined inside a long capillary tube

An atmospheric-pressure microplasma plume of diameter 10 μm is generated inside a long tube. The length of the microplasma plume reaches as much as 2 cm. First, with the assistance of an air dielectric barrier discharge (DBD), the ignition voltage of the microplasma decreases from 40 kV to 23.6 kV. Second, although the current density reaches as high as (1.2−7.6)×10⁴ A cm ⁻² , comparable to the current density in transient spark discharge, the microplasma plume is non- thermal. Third, it is interesting to observe that the amplitude of the discharge current in a positive cycle of applied voltage is much lower than that in a negative cycle of applied voltage. Fourth, the electron density measured by the Stark broadening of Ar spectral line 696.5nm reaches as high as 3×10¹⁶ cm⁻³ , which yields a conductivity of the microplasma column of around 48 S m⁻¹ . In addition, the propagation velocity of the microplasma plume, obtained from light signals at different axial positions, ranges from 1×10⁵ m s ⁻¹ to 5×10 ⁵ m s⁻¹ . A detailed analysis reveals that the surface charges deposited on the inner wall exert significant influence on the discharge behavior of the microplasma.     

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.     

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.     

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.     

High Density Plasma Heating by EC-Waves Injected from the High-Field Side for Mode Conversion to Electron Bernstein Waves in LHD

To realize an excitation of electron Bernstein waves (EBW) via mode conversion from X-mode waves injected from the high magnetic field side (HFS), new inner-vessel mirrors were installed close to a helical coil in the large helical device (LHD). 77 GHz electron cyclotron (EC) wave beams injected from an existing EC-wave injection system toward the new mirror are reflected on the mirror so that the beams are injected to plasmas from HFS. Evident increases in the electron temperature at the plasma core region and the plasma stored energy were observed by the HFS beam injection to the plasmas with the line-average electron density of 7.5×10 19 m 3 , which is slightly higher than the plasma cut-off density of 77 GHz EC-waves, 7.35×10 19 m 3 . The heating efficiency evaluated from the changes in the time derivative of the plasma stored energy reached ～70%. Although so far it is not clear which is the main cause of the heating effect, the mode-converted EBW or the X-mode wave itself injected from the HFS, an effective heating of high-density plasma over the plasma cut-off of EC-wave was successfully demonstrated.     

Charge collection and noise analysis of heavily irradiated silicondetectors

Measurements performed on high-resistivity silicon detectors irradiated with proton and neutron fluences, up to 3.5×10¹⁴ p/cm², and 4.0×10¹⁵ n/cm² respectively, are presented. The charge collection efficiency (CCE) and the output noise of the devices have been measured to carry out a detector performance study after irradiation. The CCE is found to slowly decrease for fluences increasing up to approximately 1.8×10¹⁴ p/cm². For higher particle fluences, the device inefficiency increases rapidly because full depletion could not be reached (up to 75% for the highest fluence: 4×10¹⁵ n/cm ²). A complete analysis of the noise of the irradiated devices has been carried out assuming a simple model which correlates the main noise sources to the fluence and the leakage current. A linear dependence of the square of the noise amplitude on the fluence has been observed: a value of the leakage current damage constant has been found to be in good agreement with the values reported in literature, obtained with current-voltage (IV) analysis. An extension of the noise analysis is carried out considering the detectors irradiated with very high fluences, up to 4×10¹⁵ n/cm²     

Pulsed high-current discharge in water: adiabatic model of expanding plasma channel and acoustic wave

This paper presents the results of a theoretical and experimental study of the use of a pulsed discharge in water to obtain a strong acoustic wave in a liquid medium. A discharge with a current amplitude of 10 kA, a duration of 400 ns, and an amplitude pulsed power of 280 MW in water at atmospheric pressure created an expanding acoustic wave with an amplitude of more than 100 MPa. To describe the formation of the discharge channel, an isothermal plasma model has been developed, which made it possible to calculate both the expansion dynamics of a high-current channel and the strong acoustic wave generated by it. Our calculations show that the number density of plasma in the channel reaches 10²⁰ cm^–3, while the degree of water vapor ionization is about 10%, and the channel wall extends with a velocity of 500 m s−1. The calculations for the acoustic wave are in good agreement with measurements.     

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.     

Investigation of electron cyclotron current drive efficiency on the J-TEXT tokamak

Electron cyclotron current drive (ECCD) efficiency research is of great importance for the neoclassical tearing mode (NTM) stabilization. Improving ECCD efficiency is beneficial for the NTM stabilization and the ECCD power threshold reduction. ECCD efficiency has been investigated on the J-TEXT tokamak. The electron cyclotron wave (ECW) power scan was performed to obtain the current drive efficiency. The current drive efficiency is derived to be approximately η₀ = (0.06–0.16) × 10¹⁹ A m⁻² W⁻¹ on the J-TEXT tokamak. The effect of the residual toroidal electric field has been included in the determination of the current drive efficiency, which will enhance the ECCD efficiency. At the plasma current of I_p = 100 kA and electron density of n_e = 1.5 × 10¹⁹ m⁻³, the ratio of Spitzer conductivity between omhic (OH) and ECCD phases is considered and the experimental data have been corrected. The correction results show that the current drive efficiency η1 caused by the fast electron hot conductivity decreases by approximately 79%. It can be estimated that the driven current is approximately 24 kA at 300 kW ECW power.     

Characteristics of GaAs buffered FET logic (BFL) MESFETs andinverters exposed to high-energy neutrons

A systematic investigation of the effects of high-energy neutrons on GaAs metal-semiconductor field-effect transistors (MESFETs) and buffered FET logic (BFL) gates has been carried out. Discrete transistors, inverters, and ring oscillators were characterized and modeled as a function of neutron fluence. Measurements were made of the threshold voltage shifts, the transconductance degradation, and saturation current degradation of GaAs depletion mode MESFETs, which comprise the BFL logic gates, irradiated with neutron fluences ranging from 5×10¹³ n/cm² to 2×10¹⁵ n/cm² (for particle energies above 10 keV). The threshold voltage was found to shift positively by 0.45 V, the transconductance decreased to 3%, and the saturation current to 1% of their unirradiated values at the highest neutron fluence (2×10¹⁵ n/cm²). The BFL inverter characteristics were measured and successfully simulated with SPICE using device parameters extracted from the neutron-damaged FETs. Ring oscillator measurements were made to determine the effects of high-energy neutrons on the frequency performance of BFL circuits. The ring oscillator frequency decreased to 9% of its unirradiated value at the highest neutron fluence     

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.     

Laser-driven pulsed emission and plasma formation of Cs3Sb photocathodes

The experimental progress of laser-driven Cs₃Sb photocathodes is reported. The cathodes prepared in an UHV system can be used to generate short-pulsed, high-brightness electron beams. Emission properties are tested under a 50–200 ns pulsed Xe⁺ laser illumination. The quantum efficiency in the range of 2–5.6% and current density of 108 A/cm² are obtained. A brightness of 1.85 × 10⁹ A/m² rad² is also measured. Mass analysis and other methods have been used for investigating the plasma formation when laser intensity rises above the “break-down” threshold. The current density increases rapidly during the plasma electron emission, but the pulse width of the emission is enlarged, and the brightness is limited. It is observed that the plasma is just composed of cesium and antimony atoms from the cathode rather than adsorbed residual gases.     

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.     

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%.     

Investigation of the radiation hardness on semiconductor devices using the ion micro-beam

Variation of the ion beam induced charge (IBIC) pulse heights due to ion irradiation was investigated on a Si pn diode and a 6H-SiC Schottky diode using a 2 Mev He⁺ micro-beam. Each diode was irradiated with a focused 2 MeV He⁺ micro-beam to a fluence in the range of 1×10⁹–1×10¹³ ions/cm². Charge pulse heights were analyzed as a function of the irradiation fluence. After a 2 MeV ion irradiation to the Si pn junction diode, the IBIC pulse height decreased by 15% at 9.2×10¹² ions/cm². For the SiC Schottky diode, with a fluence of 6.5×10¹² ions/cm², the IBIC pulse height decreased by 49%. Our results show that the IBIC method is applicable to evaluate irradiation damage of Si and SiC devices and has revealed differences in the radiation hardness of devices dependent on both structural and material.     

Radiation damage induced by 5 keV Si ion implantation in strained-Si/Si0.8Ge0.2

The damage distributions induced by ultra low energy ion implantation (5 keV Si⁺) in both strained-Si/Si_0.8Ge_0.2 and normal Si are measured using high-resolution RBS/channeling with a depth resolution better than 1 nm. Ion implantation was performed at room temperature over the fluence range from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². Our HRBS results show that the radiation damage induced in the strained Si is slightly larger than that in the normal Si at fluences from 1 × 10¹⁴ to 4 × 10¹⁴ ions/cm² while the amorphous width is almost the same in both strained and normal Si.     

Ion implantation doping and electrical conductivity enhancement of C60 films

Pristine C₆₀ films sublimed onto sheet mica were implanted with 20 keV K⁺ ions and I⁺ ions at doses of 1.0 × 10¹⁶/cm², 3.0 × 10¹⁶/cm² and 5.0 × 10¹⁶/cm², and with 20 keV Ar⁺ ions at a dose of 5.0 × 10¹⁶/cm². The distributions of dopants were studied using Rutherford backscattering spectrometry (RBS). The temperature dependence of sheet resistivity of the films was investigated applying a four-probe system. It was proposed that the conductivity enhancement of K⁺ implanted C₆₀ films was due to the implanted ions in the films, while for I⁺ implanted C₆₀ films, both implanted I⁺ ions and irradiation effects of the ions contributed to the enhancement of conductivity.