Improvement of the efficiency of a glow discharge-based ion emitter with oscillating electrons

Ion emission from the plasma of a low-pressure (≈5×10⁻² Pa) glow discharge with electrons oscillating in a weak (≈1 mT) magnetic field is studied in relation to the cold hollow cathode geometry. A hollow conic cathode used in the electrode system of a cylindrical inverted magnetron not only improves the extraction of plasma ions to ≈20% of the discharge current but also provides the near-uniform spatial distribution of the ion emission current density. The reason is the specific oscillations of electrons accelerated in the cathode sheath. They drift in the azimuth direction along a closed orbit and simultaneously move along the magnetic field toward the emitting surface of the plasma. A plasma emitter with a current density of ≈1 mA/cm² over an area of ≈100 cm² designed for an ion source with an operating voltage of several tens of kilovolts is described.     

Gas-filled electron diode based on a glow discharge

Stable ignition and sustention of a pulsed discharge with a current of up to 180 A and duration of 12 μs at a pressure of 10⁻¹–10⁻² Pa are achieved in a glow-discharge plasma cathode with the help of an auxiliary initiating discharge. An electron emission current density of up to 100 A/cm² and accelerating voltageof 15 kV are obtained in a gas-filled diode based on this type of a plasma cathode. An electron beam witha neutralized space charge can be transported almost without losses in a weak axial magnetic field alonga plasma channel formed due to the gas ionization by the accelerated electrons over a distance of up to 30 cm.     

Effect of cathode surface state on the characteristics of pulsed hollow-cathode glow discharges

A study is made of the effect of pulse repetition rate (0.1−10³ s⁻¹) and average discharge current (0–1 A) on the breakdown delay time and burning voltage of low-pressure glow discharges (p<0.1 Pa) in an electrode system of the reverse magnetron type with a large cathode surface area (≈10³ cm²). It is shown that increasing the repetition rate leads to a many-fold reduction in the statistical spread in the delay time and in the discharge formation time, while the average discharge current has a significant effect on the burning voltage. The mechanism for the observed phenomena is interpreted qualitatively in terms of the presence of thin dielectric films on the cathode surface. Zh. Tekh. Fiz. 69, 20–24 (May 1999)     

Characteristics of an ion source with a plasma cathode and a multipole magnetic system for confining fast electrons

Parameters and ion-emission characteristics of the plasma generated in the anode stage of an ion source with a hollow glow-discharge plasma cathode are studied. To decrease the minimum operating gas pressure to 5×10³ Pa, a multipole magnetic system was installed on the surface of the hollow cathode and the peripheral magnetic field was enhanced in the anode stage of the source. The effect of the gas pressure, the plasma-cathode current, and the voltage between the electrodes of the anode stage on the value of the ion current extracted from the plasma is investigated. It is found that the size of the exit aperture of the hollow cathode substantially affects the efficiency of ion extraction. The potential (1–5 V) and the electron temperature (1–8 eV) of the anode-stage plasma are measured by the probe method. The conditions are determined that ensure the maximum ion-emission current from the plasma at low gas pressures.     

Probe measurements in a discharge with liquid nonmetallic electrodes in air at atmospheric pressure

Discharges with liquid nonmetallic electrodes of much interest for applications are investigated. It is found that a dc discharge between two streams of tap water in air at atmospheric pressure is stable at a currentof 40≤I≤100 mA. The discharge exists in the diffuse (volume) form with a relatively low current density (∼0.2 A/cm²) and a high (above one kilovolt) voltage drop across the air gap (∼1 cm) between the water electrodes. The current density and voltage depend only slightly on the discharge current. Probe measurements show that three regions can be distinguished in the discharge: two electrode regions (1–2 mm in length) and a discharge column with a constant electric field of ≈0.8 kV/cm (i.e., E/N≈20 Td, because the gas in the discharge is heated up to 1500–2000 K). The average electric field strength near the electrodes is E≈2–3×10³ V/cm (E/N≈60–80 Td). The charged particle density in the column is n ∼ 10¹² cm⁻³. The probe measurements of n agree with the previous microwave absorption measurements. The water vapor concentration in the column is also estimated from probe measurements.     

Analysis of a hollow-cathode reflex discharge in a hydrocarbon flow

Variation of parameters of a reflex discharge with a hollow cathode operating continuously in propane with a flow rate of 1.3–5.6 (m³ mPa)/s and a discharge current of 0.1–0.4 A is analyzed. It is shown that for a hydrocarbon flow rate of 2.4 (m³ mPa)/s and higher, an increase in the discharge voltage takes place after a time interval depending on the discharge current and gas pressure; this is explained by the formation of coating of the dissociation products of hydrocarbon molecules on the electrodes of the discharge chamber. An increase in the thickness of the carbon coating of the cathodes with time and their charging with ions lead to electric breakdown of coatings and the formation of cathode spots. The oscillograms of the discharge current and voltage indicate a short-term transformation of the glow discharge into the arc discharge. The energy spectra of ions emerging from the discharge are measured, and the effect of the discharge current and the gas flow rate on the energy spread of ions is analyzed. The operation time of the discharge in hydrocarbon after which the cleaning of the discharge chamber is required is determined. The possibility of using an ion source based on the reflex discharge with a hollow cathode for technological purposes is established.     

The Investigation of Copper-Chromium Contacts in Vacuum Interrpters Subjected to an Axial Magnetic Field

The characteristics of a vacuum arc between CuCr contacts under an axial magnetic field have been investigated. Test samples were made of CuCr contacts, and the arcing voltage was measured. The arcing voltage of the CuCr contacts under an axial magnetic field is lower than that of pure copper contacts by 10-20 V. From the measurement of the post-arc current and interruption test results, it was found that the arc concentrates and a part of the electrode melts at a rather low current, but that the electrode melting does not affect the interrupting capability. Also, the insulation characteristics were measured. With respect to voltage conditioning, high-current conditioning improved the breakdown voltage by 50 percent for a 20-mm gap and by 100 percent for a 3-mm gap. These test results show that CuCr contacts, used with axial magnetic fields, are promising for use in high-voltage and high-power vacuum interrupters.     

Influence of the cathode and anode jets on the properties of a high-current electric arc

A study is made of the effects related to the formation of electrode jets in discharges in hydrogen and air at a current of 10⁵–10⁶ A, a current growth rate of 10¹⁰ A/s, an initial pressure of 0.1–4.0 MPa, and a discharge gap length of 5–40 mm. After secondary breakdown, jets are observed in a semitransparent discharge channel expanding with a velocity of (4–7)×10² m/s. The formation of shock waves in the interaction of the jets with the ambient gas and the opposite electrode is observed by the shadowgraphy method. Seventy microseconds after the beginning of the discharge, the pressure of the metal vapor plasma near the end of the tungsten cathode amounts to 177 MPa. The brightness temperature in this case is T=59×10³ K, the average ion charge number is [`(m)] = 3.1\overline m = 3.1 , and the metal vapor density is n=5.3×10<sup>19</sup> cm<sup>−3</sup>. After 90 μs, the average ion charge number and the metal vapor density near the anode end are [`(m)] = 2.6\overline m = 2.6 and n=7.4×10¹⁹ cm⁻³, respectively. Based on the experimental data, possible reasons for the abnormally high values of the total voltage drop near the electrodes (up to ∼1 kV) are discussed.     

Study of directed ion velocities in a vacuum arc by an emission method

Directed ion velocities in a vacuum arc discharge plasma are measured on the basis of a study of the ion emission current response to a rapid change of arc current. It is shown that these velocities are about 10⁶ cm/s, are determined by the cathode material, and are almost independent of the ion charge number. Applying a magnetic field results in an increase in the directed ion velocity. As the gas pressure increases, the directed ion velocity decreases; this is the only case where the directed velocities are observed to depend on the ion charge number.     

Contact temperature and erosion in high-current diffuse vacuum arcson axial magnetic field contacts

We have investigated the surface heating effects of drawn vacuum arcs for several industrial designs of axial magnetic field (AMF) contacts, using near infrared (IR) photography of the Cu-Cr arcing surfaces with an image-intensified charge-coupled device (CCD) camera and an IR pyrometer. This enables detailed contact temperature mapping immediately after a half-cycle of arc current. The very homogeneous temperature distribution observed at current zero stands in contrast to the visually nonhomogeneous high-current diffuse arc, which was studied in separately reported experiments using high-speed digital photography and arc voltage measurements. The peak temperature at current zero increased relatively linearly with the peak current I_P, and reached well beyond the melting range. We combine the temperature maps with a heating model to determine the thermal sheath thickness after arcing and its dependence on I_P. The results suggest that near the interruption limit of AMF contacts, the interaction of the stable high-current arc with the anode and cathode is dominated by processes induced by flowing liquid metal, which redistributes the heat input from the axially concentrated arc over most of the contact surface. Furthermore, the flow of liquid metal off the cathode and anode faces contributes to the overall contact erosion     

HL-2M装置5 MW中性束加热束线离子源放电室研制

为了给HL-2M装置建设一条5 MW中性束加热束线,开展了中性束加热用热阴极弧放电离子源放电室的研制。这条中性束束线包含4套80 kV/45 A/5 s离子源,放电室的设计指标为850 A/5 s。首先采用CST软件中的电磁工作室对特定几何结构的放电室会切磁场进行了模拟计算,得到了会切磁场分布,验证了会切磁场布局的合理性。针对放电室加工工艺和实验过程中局部拉弧等问题,对放电室结构进行了不断改进。放电室侧壁由40列会切磁体改为7圈环形磁体,阴极灯丝结构从灯丝板结构最终改为陶瓷可伐结构,并且在放电室和加速器之间增加了陶瓷屏蔽。在阴极板结构放电室和阴极陶瓷可伐结构放电室内都获得了正常的弧放电。最终定型的放电室采用周边7圈环形会切磁体和陶瓷可伐结构。在定型的放电室内达到了5 MW中性束束线离子源弧放电的指标。弧放电时间接近5 s,最大弧放电电流达到1 000 A。     

Thermionic cathode parameters versus pressure in a sustained argon arc

Experimental data on the influence of the argon pressure on the performance of a rodlike thoriated-tungsten cathode in a sustained arc discharge are reported for the first time. The integral parameters of the cathode, namely, the electron work function, temperature, current density, and specific surface area of erosion at currents of 20–150 A in the pressure range 2.66 × 10³?1.00 × 10⁵ Pa, are measured. Taken together, these parameters provide comprehensive information about the operating conditions of the activated cathode. A significant influence of the pressure on the specific erosion of the cathode is noted.     

Experimental observations of steady anodic vacuum arcs withthermionic cathodes

Stationary plasma discharges have been investigated in a high vacuum ambient (background gas pressure <10^-2 Pa), with an externally heated cathode and a consumable hot evaporating anode. With various anode materials like chromium or copper, and electrode separations between 0.5 and 3 mm, the nonself-sustained discharge operates with DC arc currents in the range of 220 A. The waveform of the arc voltage is strongly influenced by the magnetic field of the cathode heating current, and arc voltages between a minimum of 3 V and a maximum exceeding 100 V have been observed. The voltage-current characteristics (V_CC) and the influence of the electrode separation have been measured separately for the minimum and the maximum of the arc voltages and show a different behavior. The metal plasma expands into the ambient vacuum toward the walls of the vacuum vessel and offers a macroparticle free deposition source of thin films. The arc voltage can be varied by external manipulations of the arc discharge, and the mean ion energy of the expanding metal plasma shows a linear dependence of the mean arc voltage     

Model Experiments on Study of the First Wall Erosion Under the Action of Vacuum Arcs

With d. c. vacuum arc discharges the dependence of cathode erosion rate on the transverse magnetic field induction is non-linear. At first erosion rate rises with the magnetic induction and then drops. The maximum erosion rate value corresponds to the magnetic induction interval (50 – 115) · 10^?4 T. The range of the vacuum arc discharge maximum stability also corresponds to the same interval. When the magnetic induction rises from 2 · 10^?3 up to 1.1 · 10^?2 T (I = 50 A), the erosion products angular distribution monotoneously narrows and with further magnetic ficld enhancement it abruptly widens. Current rise results in increasing extension of erosion products angular distribution. Asymmetry of erosion products angular distribution does not depend on the direction of the magnetic induction vector and gradually vanishes when the magnetic induction rises up to 1 · 10^?2 T and higher.     

常压下气体放电等离子体振荡的实验与理论研究

在实验上研究了高压交流电弧发生器电极间隙的气体放电及等离子体振荡, 观察到了气体放电过程中的纳秒脉冲.以电子的流体运动方程和麦克斯韦方程为理论基础, 利用δ函数来描述交变外电场作用下电极处的电子堆积现象,建立了常压下气体放电时等离子体在外电场中振荡的理论模型,通过Laplace变换求解出电极间的放电电压.理论与实验结果基本符合, 从而可估算出实验中等离子体的电子数密度为1.3× 10¹²/m³.     

Focusing of an intense neutralized proton beam

An intense low-energy (15–20 keV) proton beam is focused in two stages: ballistic focusing is followed by magnetic compression. The beam is formed by an MAIS wide-aperture source, in which a plasma is generated by many discharge elements via a discharge over the polyethylene surface. In the presence of an external magnetic field, the beam turns out to be overneutralized by electrons coming from the cathode grids of the source and from the target. The maximal focusing efficiency (>70%) is observed within 10 μs after the pulse has been applied if the target is negatively biased. The degree of beam compression in terms of cross-sectional area is 1.6×10³. The numerical simulation of the focusing agrees well with the results of measurement.     

Spectroscopic study of a brush cathode plasma in a magnetic field

The brush cathode helium discharge in the magnetic field has been operated stably at discharge currents larger than those without magnetic field. The diameter of the plasma column has been determined by the configuration of the magnetic field. The measurements of the spectral intensities of the recombination continuum followed by the 2³S-n³P series reveals that the electron density is 1·8 × 10¹³ cm^-3 and the electron temperature is 0·17 eV at a discharge current of 500 mA and a pressure of 0·9 torr for a magnetic flux density of 1·3 kG. The principal quantum number for line merging is 20.     

Emission properties of a plasma cathode based on a glow discharge for generating nanosecond electron beams

The emission properties of a plasma cathode based on a nanosecond pulsed glow discharge with currents of up to 200A at a pressure of 5×10⁻² Pa are studied experimentally. Stable ignition and burning of the discharge are ensured if the current in the auxiliary pulsed discharge is 25–30% of that in the main discharge and its pulse duration exceeds that of the main discharge by more than an order of magnitude. Emission current pulses from the cathode with amplitudes of up to 140A fully reproduce the discharge current and are determined by the transparency of the grid anode. Zh. Tekh. Fiz. 69, 62–65 (November 1999)     

Charge-state distribution of ions in a vacuum arc discharge plasma in a high magnetic field

It is shown that the fraction of multiply charged metal ions generated in a vacuum arc discharge plasma grows substantially in a high magnetic field. This effect was observed for more than 30 different cathode materials. A relation is established between growth of the mean charge of the ions and increases in the burning voltage of the arc. It is demonstrated that the burning voltage of the vacuum arc can be ultimately increased to 160 V. Zh. Tekh. Fiz. 68, 39–43 (May 1998)     

Penetration of a low-pressure reflex-discharge plasma into a hollow electrode

It is shown experimentally that the plasma of a hollow-cathode reflex discharge is characterized by a nonequilibrium electron velocity distribution. The parameters of the electron distribution, which is approximated by a superposition of two Maxwellian distributions with different temperatures, are estimated. The penetration of the discharge plasma into the hollow cathode at various cathode potentials and a gas pressure of ∼10^{\t− 2} Pa is studied. It is shown that the plasma parameters in the hollow electrode depend not only on the parameters of the reflex-discharge plasma, but also on the magnitude and configuration of the magnetic and electric fields in the plasma expansion region. It is shown that the plasma penetration can be accompanied by quasineutrality violation and the formation of space-charge double layers. Experiments confirm that the ion current from the nonequilibrium plasma exceeds the Bohm current.