Predicted emittance and brightness of the pseudospark electron-beam

The emittance and brightness of the electron beam generated during the hollow cathode phase of pseudospark operation are calculated using the two-dimensional hybrid fluid-particle model previously developed to study the time and space development of the plasma in a pseudospark discharge. Two distinct energy components exist in the electron beam; a high-energy component with an energy equivalent to the full discharge voltage and another, broad, low-energy component. In the 100 ns following breakdown and for the conditions of the calculations, the emittance of the high energy component decreases by an order of magnitude and the brightness of the high energy component reaches almost 10¹⁰ A/m² rad². This work demonstrates the feasibility of using the model to guide the optimization of the pseudospark electron beam properties and shows that the optimum beam properties are achieved after the plasma has filled the hollow cathode and begun to expand radially in the main gap     

Pseudospark produced pulsed electron beam for material processing

An intense pulsed electron beam produced by a pseudospark discharge is used for material processing. The electron beam propagates in a self-focused manner in the background gas. Hardly 12 ns after the beginning of the discharge the fraction of space charge neutralization is about 96%. To sustain the neutralization effect high energy electrons (E <500 keV) are accelerated in radial direction at the beam head, due to strong electric field gradients. At current maximum the beam pinches due to its own magnetic field. At peak current of 400 A and charging voltage up to 16 kV power density reaches 10⁹ W/cm ² on the target surface. Some results of copper thin films are presented. Due to the high expansion velocity of 10⁴ m/s of the ablated target material a copper-matrix has been masked     

Study of field-induced explosive electron emission mechanisms ofpseudospark superemission cathodes

Various mechanisms of electron emission, including the field, field-enhanced thermionic, and explosive electron emissions from pseudospark cathodes, are discussed and compared. The mechanism of the field-induced explosive electron emission due to microstructure on the cathode surface is considered to be more likely the pseudospark superemissive mechanism. A high-mean electric field up to 3-5 MV/cm on the cathode surface in the end of hollow cathode phase is enough to initiate the mechanism. The cathode spot initiation delay time (<10 ns) and explosive emission threshold current (~10⁸ A/cm² ) prior to the high current conducting phase are given by solving the initial value problem of the one-dimensional heat conduction equation, thus explaining the existing experimental data of the pseudospark cathode superemission. In the case of multigap discharge, the above mechanism occurs on nearly all cathode and interelectrode surfaces. Experimental evidence in single- and multigap pseudospark discharges supports the suggested explanation     

Self-sustained self-sputtering: a possible mechanism for thesuperdense glow phase of a pseudospark

Self-sustained self-sputtering occurring during high current pseudospark operation (≈10⁴ A/cm², I>10³ A) is shown to be a possible mechanism for the superdense glow. The mean-free-path for ionization of cathode material sputtered in the low-current hollow-cathode phase can be shorter than the cathode-anode gap distance, and ionized atoms can return to the cathode surface, self-sputtering with a yield greater than one. The self-sputtered cathode atoms become ionized in the beam of electrons accelerated in the cathode sheath. A large fraction of the discharge current at the cathode surface can be carried uniformly over the surface by ions and a very high electron emission density is not required to maintain the high current     

A two-component model for the electron distribution function in ahigh-current pseudospark or back-lighted thyratron

Temperature, energy, and densities of two electron distribution function components, including an isotropic bulk part and an anisotropic beam, are analyzed for a hydrogen pseudospark and/or back-lighted thyratron switch plasma with a peak electron density of 1-3×10¹⁵ cm^-3 and peak current density of ≈10⁴ A/cm². Estimates of a very small cathode-fall width during the conduction phase and high electric field strengths lead to the injection of an electron beam with energies ⩾100 eV and density of 10¹³-10¹⁴ cm^-3 into a Maxwellian bulk plasma. Collisional and radiative processes of monoenergetic beam electrons, bulk plasma electrons and ions, and atomic hydrogen are modeled by a set of rate equations, and line intensity ratios are compared with measurements. Under these high-current conditions, for an initial density n_H2=10¹⁶ cm^-3 and electron temperature of 0.8-1 eV, the estimated beam density is ≈10¹³ -10¹⁴ cm^-3. These results suggest the possibility of producing in a simple way a very high-density electron beam     

Importance of high local cathode spot pressure on the attachment ofthermal arcs on cold cathodes

The importance of having high local cathode spot pressures for the self-sustaining operation of a thermal arc plasma on a cold cathode is theoretically investigated. Applying a cathode sheath model to a Cu cathode, it is shown that cathode spot plasma pressures ranging 7.4-9.2 atm and 34.2-50 atm for electron temperatures of ~1 eV are needed to account for current densities of 10⁹ and 10¹⁰ A·m^-2, respectively. The study of the different contributions from the ions, the emission electrons, and the back-diffusing plasma electrons to the total current and heat transfer to the cathode spot has allowed us to show the following. 1) Due to the high metallic plasma densities, a strong heating of the cathode occurs and an important surface electric field is established at the cathode surface causing strong thermo-field emission of electrons. 2) Due to the presence of a high density of ions in the cathode vicinity, an important fraction of the total current is carried by the ions and the electron emission is enhanced. 3) The total current is only slightly reduced by the presence of back-diffusing plasma electrons in the cathode sheath. For a current density j_tot=10⁹ A·m^-2 , the current to the cathode surface is mainly transported by the ions (76-91% of j_tot while for a current density j_tot = 10¹⁰ A·m^-2, the thermo-field electrons become the main current carriers (61-72% of j_tot). It is shown that the cathode spot plasma parameters are those of a high pressure metallic gas where deviations from the ideal gas law and important lowering of the ionization potentials are observed     

Development of a high current pseudospark switch and measurement ofelectron density

The pseudospark, a low-pressure gas discharge in a special geometry, is suitable for high-current switching. A single-channel prototype is tested with a 3.3-μF capacitor bank, voltages up to 30 kV, and peak currents up to 120 kA. The electrical circuit, not comprising any load resistor causes weakly damped sinusoidal pulses of 5-μs duration at 90% current reversal. For lifetime tests, a switch with an alumina insulator and copper seals is used. Hydrogen is the working gas. Several electrode materials like molybdenum, tungsten, graphite and chromium-copper are tested. Optical investigations of the discharge and of plasma parameters are done with an O-ring sealed pseudospark switch. The light of the discharge is observed spectrally integrated with a streak camera. Spectral resolution is obtained by using a high-speed shutter in combination with a monochromator. The radial electron density is determined by measuring the Stark broadening of the Balmer H_β-line. At 60 kA a maximum electron density of about 2×10¹⁷cm^-3 is calculated     

The spatial and temporal development of pseudospark switch plasmas

Optical spectroscopy is used to investigate the spatial and temporal development of high-current pseudospark switch plasmas. At a peak discharge current of 12 kA in amplitude and a current reversal of 20%, the electron density is measured from Stark width broadening of the hydrogen Balmer beta line. The peak electron density of ~4×10¹⁵ cm^-3 is measured briefly after the current maximum. The discharge initially starts on the symmetry axis of the cathode hole. A cylindrical plasma column is observed, which is produced mainly by ionizing collisions of beam electrons formed in the hollow cathode during the early part of the discharge. This plasma column rapidly expands in the radial direction, until it contacts the edge of the cathode hole. The same behavior is found when the Balmer beta line intensity is evaluated rather than the line shape. Although statistically distributed, localized bursts of light are found occasionally, an axially symmetric, homogeneous light intensity distribution is always predominant, and the local arcing is merely superimposed on it. These results confirm that the discharge remains diffuse during most of the current pulse     

Spatial characteristics of nanosecond pulsed micro-discharges in atmospheric pressure He/H2O mixture by optical emission spectroscopy

Atmospheric pressure micro-discharges in helium gas with a mixture of 0.5%water vapor between two pin electrodes are generated with nanosecond overvoltage pulses.The temporal and spatial characteristics of the discharges are investigated by means of time-resolved imaging and optical emission spectroscopy with respect to the discharge morphology,gas temperature,electron density,and excited species.The evolution of micro-discharges is captured by intensified CCD camera and electrical properties.The gas temperature is diagnosed by a two-temperature fit to the ro-vibrational OH(A²Σ⁺–X^П(2),0–0)emission band and is found to remain low at 425 K during the discharge pulses.The profile of electron density performed by the Stark broadening of Ha 656.1-nm and He I 667.8-nm lines is uniform across the discharge gap at the initial of discharge and reaches as high as 10²³m^-3.The excited species of He,OH,and H show different spatio-temporal behaviors from each other by the measurement of their emission intensities,which are discussed qualitatively in regard of their plasma kinetics.     

Pressure dependence of plasma parameters in medium-vacuum nitrogenarc discharge with the titanium cathode

The plasma properties of a medium-vacuum nitrogen arc discharge from a titanium cathode were studied. The arc chamber use was 400 mm in diameter and 600 mm in length. The cathode diameter and thickness were 64 and 25 mm, respectively. The experimental conditions are given as follows: pressure range=1×10^-3~2×10^-1 torr; N₂ gas flow rate=6 ml/min; arc current=50 A. Electric probe characteristics are measured as a function of pressure and distance from the cathode surface. The analytical results obtained show that the electron energy distribution takes 1-Mx at pressures above 1×10^-2 torr but 2-Mx at pressures under 4×10^-2 torr and that the electron density has a maximum value at a certain pressure. The Ti⁺, Ti⁺⁺, and N ⁺₂ ion spectral intensities are measured as a function of pressure and distance from the cathode surface. On comparison of these results and the electron density, the Ti⁺ spectral intensity turns out to be proportional to that of the electron density. This suggests that the major ion in the plasma volume is of the Ti⁺ species     

Correlation of current quenching and occurrence of metal vapor in apseudospark discharge

The quenching phenomenon, i.e., a sudden interrupt of the discharge current, was investigated in a pseudospark discharge with charging voltage of 2.5 kV, maximum current of 2 kA and discharge duration of 3 μs. The working gas was hydrogen at a pressure of 40 Pa. Concerning electrode material and geometric parameters, molybdenun electrodes were chosen with hole diameters of 5 mm; the electrode distance was 3 mm. In this parameter range, a temporal correlation of current quenching and the occurrence of metal vapor could be detected by means of time-resolved optical spectroscopy. With each current interruption a sudden increase of emission from neutral molybdenum atoms as well as an increase of cathode spot emission, which is spatially localized on the cathode, occurs. Also oxygen ions were observed which show a similar time-dependence, however with a significant delay of the order of 200 ns. The results are discussed in the scope of the mechanism proposed for quenching, i.e., ion depletion in the plasma boundary layer, and the mechanisms occurring in the high current phase of a pseudospark discharge     

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The electron densities in the atmospheric pressure helium plasma were calculated by means of electron drift velocity and the jet velocity respectively. The electron velocity and jet velocity can be calculated by means of helium plasma jet current measured by a dielectric probe and plasma discharge current signal measured by voltage probes. The results show that the estimated electron densities of the helium plasma jet calculated from electron drift velocity and the jet velocity are in the order of 10 ¹¹ cm ^-3 and they increase with applied voltage. There is a little fluctuation in the value of the electron density along the jet axis of the plasma. This result is the same as the measured electron density in atmospheric pressure helium non-thermal plasma jet by using a Rogowski coil and a Langmuir probe. This is in one order lower than the electron density measured by microwave antenna.     

利用介质阻挡探针法测量大气压氦等离子体射流电子密度

分别利用电子的漂移速度和等离子体的传播速度计算了大气压下氦等离子体射流的电子密度。     

Simulation study of a capacitively coupled plasma torch array

Using capacitively coupled electrical discharges, an array of three plasma torches powered by a single 60-Hz source are lit up simultaneously to produce a dense plasma in the open air. The discharge voltage and current of each torch is measured for three cases of one to three torches being lit up in the array. The results determine the ν-i characteristic of the discharge which indicates that the torch is operating in a diffuse are mode. The torch array is modeled by an equivalent circuit for simulating its operation. The simulation results of the discharge voltage and current of a torch are shown to agree well with those from the experimental measurements for the three cases. The lump circuit model is then used to carry out numerical simulations of the discharge for a broad parameter space of plasma species. By fitting the simulation results, a function giving the parametric dependence of the consumed average power density 〈P〉 on the normalized average electron density 〈n_e〉 maintained in the plasma is determined to be 〈P〉 48 〈n_e〉 ^1.9α_^0.4(W/cm³), where 〈n_e〉 is normalized to 10¹³cm^-3 and α_, the electron-ion recombination coefficient normalized to 10^-7 cm³·s^-1, is used as a variable parameter in the simulation     

High current electron beam generation in a vacuum transient hollowcathode discharge

Experimental observations are presented of prebreakdown electron beam generation in a transient hollow cathode discharge (THCD) in a vacuum. The discharge driver consists of a 400-kV maximum voltage, 25-nF Marx operated at 450-J stored energy coupled to a 120-ns, 1.5-Ω coaxial line. Electron beams with peak currents up to tens of kA are observed when a pulsed Nd:YAG laser is used to produce a plasma at the back of the cathode surface, inside the hollow cathode region (HCR). It is found that a plasma density of a few 10¹⁸ cm^-3 in a volume of a few mm³ is required to generate intense electron beams. Optimal conditions are determined by varying the position of the laser focal spot inside the HCR and the time delay between the laser and the applied voltage. The main features of the electron beams are similar to those observed in conventional THCD at pressures in the 10-200 mtorr range     

Plasma parameters within the cathode spot of laser-induced vacuumarcs: experimental and theoretical investigations

Cathode spot formation in laser-induced breakdown in vacuum was investigated by laser absorption photography with high spatial (0.5 μm) and temporal (100 ps) resolution. The discharge was initiated between Cu electrodes with a cathode-anode distance of 15-250 μm. The duration of pulsed discharges was 750 ns and dc discharges some milliseconds; the current was below 10 A. Picosecond momentary absorption photography yielded spatial-temporal density distributions in the ignition phase of the cathode spot. An absolute electron density >5×10²⁶ m^-3 in narrow plasma fragments with a diameter smaller than 5 μm was estimated. Mathematical modeling has satisfactorily explained the formation of the narrow plasma channel due to the bulk current self-focusing, as well as due to the generation of nonstationary emissive centers at the moving boundary of the expanding cathode spot plasma     

300kV的虚火花放电实验

 介绍了用脉冲线加速器作驱动电源产生高压虚火花放电的实验。脉冲线加速器由十级马克斯发生器和改装的脉冲形成线构成, 虚火花放电室中注入低气压氮气, 在300kV的放电电压下获得药10kA的高亮度电子束。     

氦氩射频容性放电发射光谱分析

光抽运亚稳态稀有气体激光器利用放电等离子体作为激光的增益介质.为掌握容性射频放电的放电参数对等离子体各项参数的影响的基本规律,利用等离子体发射光谱法研究了氦氩混合气体在不同装置、不同Ar组分、不同气压和不同射频注入功率下的等离子体参数.利用残留水蒸气产生的OH自由基A~2Σ~+→X~2Π的转动光谱分析获得气体温度;利用电子态光谱的玻尔兹曼做图法获得电子激发温度,利用Ar原子696.5 nm谱线的斯塔克展宽获得电子密度.结果表明:气体温度随气压增加略微上升,在一个大气压下改变组分和放电功率,气体温度变化不大;电子激发温度随总气压的下降而上升,且随着Ar组分的增加而略微下降;目前放电条件下的电子密度均在10~(15)cm~(-3)量级;长时间放电监测表明,残留的水蒸气会导致电子温度的下降,从而降低Ar亚稳态的产率.     

Characteristics of a vacuum spark triggered by the transient hollowcathode discharge electron beam

The discharge characteristics of a vacuum spark triggered by the transient hollow cathode discharge (THCD) electron beam is investigated over a wide variety of discharge conditions. Two systems of the vacuum spark device have been considered-the first system powered by eight 2700-pF doorknob capacitors charged to a voltage of 40 kV (input energy of 17.6 J); while the second system employs a single 1.85-μF Maxwell capacitor discharged at a voltage of 20 kV (input energy of 370 J). The operating pressure of these systems has been varied over the range of 10 ^-2 to 10^-5 mbar in order to examine the effect of the operating pressure on the plasma formation of the vacuum spark discharge. The effectiveness of plasma heating has been found to be significantly enhanced in the two vacuum spark systems studied here. In particular, the plasma of the 17.6 J system has been observed to be heated to a condition hot enough to emit in the X-ray region when the operating pressure is reduced from 10^-2 to 10^-5 mbar. Similarly, in the case of the 370 J system, hot spot formation is also observed to occur only at a low operating pressure of 10^-4 mbar     

Analysis of time-dependent characteristics for a pseudosparkdischarge in air

The time-dependent resistive voltage and resistance of a pseudospark discharge in air are obtained by solving the equivalent circuit equation using the measured values of the discharge current and breakdown voltage. Pulsed, underdamped discharge currents ranging from about 30 to 100 A are investigated experimentally. The gas pressures range from 0.005-0.1 torr. The discharges can be characterized by three phases: initial, quasi-stationary, and relaxation. The quasi-stationary phase occurs near the time of the current maximum. The variations of the resistive pseudospark voltage are similar to those observed in a spark discharge, which can explain the sparklike behavior of a pseudospark discharge. Details of the current-voltage characteristics and resistance are presented