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.     

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.     

PIC simulation of plasma properties in the discharge channel of a pulsed plasma thruster with flared electrodes

Plasma in the discharge channel of a pulsed plasma thruster (PPT) with flared electrodes is simulated by a self-developed two-dimensional code. The fully particle-in-cell method with Monte Carlo collision is employed to model the particle movement and collisions and investigate the plasma properties and acceleration process. Temporal and spatial variations of the electron density distribution and the ion velocity between electrodes are calculated and analyzed in detail. The computational results of the electron number density, which is in the order of 10²³ m⁻³, show good agreements with experimental results of a PPT named ADD SIMP-LEX. The ion velocity distributions along the center line of the channel lead to a comprehensive understanding of ions accelerated by electromagnetic field. The electron distributions of PPT with discharge voltages varying from 1300 to 2000 V are compared. The diffusion of electrons presents strong dependency on discharge voltage and implies higher degree of ionization for higher voltage.     

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.     

A low power 50 Hz argon plasma for surface modification of polytetrafluoroethylene

The characteristics of a low power 50 Hz argon plasma for surface treatment of polytetrafluoroethylene (PTFE) film is presented in this article. The current–voltage behavior of the discharge and time-varying intensity of the discharge showed that a DC glow discharge was generated in reversed polarity at every half-cycle. At discharge power between 0.5 and 1 W, the measured electron temperature and density were 2–3 eV and ∼10⁸ cm⁻³, respectively. The optical emission spectrum of the argon plasma showed presence of some 'impurity species' such as OH, N₂ and H, which presumably originated from the residual air in the discharge chamber. On exposure of PTFE films to the argon glow plasma at pressure 120 Pa and discharge power 0.5 to 1 W, the water contact angle reduced by 4% to 20% from the original 114° at pristine condition, which confirms improvement of its surface wettability. The increase in wettability was attributed to incorporation of oxygen-containing functional groups on the treated surface and concomitant reduction in fluorine as revealed by the XPS analysis and increase in surface roughness analyzed from the atomic force micrographs. Ageing upon storage in ambient air showed retention of the induced increase in surface wettability.     

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.     

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.     

Spectral Characteristics of Laser-Induced Graphite Plasma in Ambient Air

An experimental setup of laser-induced graphite plasma was built and the spectral characteristics and properties of graphite plasma were studied. From the temporal behavior of graphite plasma, the duration of CN partials(B~2∑~+→X~2∑~+) emission was two times longer than that of atomic carbon, and all intensities reached the maximum during the early stage from0.2 μs to 0.8 μs. The electron temperature decreased from 11807 K to 8755 K, the vibration temperature decreased from 8973 K to 6472 K, and the rotational temperature decreased from7288 K to 4491 K with the delay time, respectively. The effect of the laser energy was also studied, and it was found that the thresholds and spectral characteristics of CN molecular and C atomic spectroscopy presented great differences. At lower laser energies, the electron excited temperature, the electron density, the vibrational temperature and rotational temperature of CN partials increased rapidly. At higher laser energies, the increasing of electron excited temperature and electron density slow down, and the vibrational temperature and rotational temperature even trend to saturation due to plasma shielding and dissociation of CN molecules. The relationship among the three kinds of temperatures was T_(elec)T_(vib)T_(rot) at the same time. The electron density of the graphite plasma was in the order of 10~(17)cm~(-3) and 10~(18)cm~(-3).     

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.     

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.     

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.     

Numerical simulation of atmospheric pulse-modulated radio-frequency glow discharge ignition characteristics assisted by a pulsed discharge

A one-dimensional self-consistent fluid numerical model was developed to study the ignition characteristics of a pulse-modulated(PM) radio-frequency(RF) glow discharge in atmospheric helium assisted by a sub-microsecond voltage excited pulsed discharge. The temporal evolution of discharge current density and electron density during PM RF discharge burst was investigated to demonstrate the discharge ignition characteristics with or without the pulsed discharge. Under the assistance of pulsed discharge, the electron density in RF discharge burst reaches the magnitude of 1.87 × 10~(17) m~(-3) within 10 RF cycles, accompanied by the formation of sheath structure. It proposes that the pulsed discharge plays an important role in the ignition of PM RF discharge burst. Furthermore, the dynamics of PM RF glow discharge are demonstrated by the spatiotemporal evolution of the electron density with and without pulsed discharge. The spatial profiles of electron density, electron energy and electric field at specific time instants are given to explain the assistive role of the pulsed discharge on PM RF discharge ignition.     

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%.     

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.     

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.     

A relativistic canonical symplectic particlein-cell method for energetic plasma analysis

A relativistic canonical symplectic particle-in-cell(RCSPIC) method for simulating energetic plasma processes is established. By use of the Hamiltonian for the relativistic Vlasov–Maxwell system, we obtain a discrete relativistic canonical Hamiltonian dynamical system, based on which the RCSPIC method is constructed by applying the symplectic temporal discrete method.Through a 10~6-step numerical test, the RCSPIC method is proven to possess long-term energy stability. The ability to calculate energetic plasma processes is shown by simulations of the reflection processes of a high-energy laser(1?×?10~(20) W cm~(-2)) on the plasma edge.     

Plasma density measurement and downstream etching of silicon and silicon oxide in Ar/NF3 mixture remote plasma source

In this study, plasma density measurements were performed near the plume region of the remote plasma source (RPS) in Ar/ NF₃ gas mixtures using a microwave cutoff probe. The measured plasma density is in the range of 10 ¹⁰ –10 ¹¹ cm ⁻³ in the discharge conditions with RPS powers of 2–4 kW and gas pressures of 0.87–4 Torr. The plasma density decreased with increasing gas pressures and RPS powers under various Ar/ NF₃ mixing ratios. This decrease in the plasma density measured at the fixed measurement position (plume region) can be understood by the reduction of the electron energy relaxation length with increases in the gas pressures and mixing ratio of NF₃/(Ar / NF₃). We also performed downstream etching of silicon and silicon oxide films in this system. The etch rate of the silicon films significantly increases while the silicon oxide is slightly etched with the gas pressures and powers. It was also found that the etch rate strongly depends on the wafer position on the processing chamber electrode, and that the etch selectivity reached 96–131 in the discharge conditions of RF powers (3730–4180 W) and gas pressures (3.6–4 Torr).     

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.     

Development and performance characterization of a compact plasma focus based portable fast neutron generator

The design details and performance characterization results of a newly developed plasma focus based compact and portable system (0.5 m × 0.5 m × 1.2 m, weighing ≈100 kg) that produces an average neutron yield of ~2 × 10⁸ neutrons/shot (of fast D-D neutrons with typical energy ~2.45 MeV) at ~1.8 kJ energy discharge are reported. From the detailed analysis of the experimental characterization and simulation results of this system, it has been conclusively revealed that specifically in plasma focus devices with larger static inductance: (i) pinch current is a reliable and more valid neutron yield scaling parameter than peak current, (ii) the ratio of pinch/peak current improves as static inductance of the system reduces, (iii) the benign role of the higher static/pinch inductance ratio enables the supply of inductively stored energy in densely pinched plasma with a larger time constant and it is well depicted by the extended dip observed in the discharge current trace, (iv) there is the need to redefine existing index values of the pinch (Ipinch ^4.7) and peak (Ipeak ^3.9) currents in neutron yield scaling equations to higher values.