A. Shashurin

Laboratory Modeling of the Plasma Layer at Hypersonic Flight

A simple approach to modeling the plasma layer similar to that appearing in the vicinity of a hypersonic vehicle is demonstrated in a laboratory experiment. This approach is based on the use of a hypersonic jet from a cathodic arc plasma. Another critical element of this laboratory experiment is a blunt body made from a fairly thin foil of refractory material. In experiments, this blunt body is heated by the plasma jet to a temperature sufficiently high to ensure evaporation of surface deposits produced by the metallic plasma jet. This process mimics reflection of gas flow from the hypersonic vehicle in a real flight. Two-dimensional distributions of the hypersonic plasma flow around the blunt body were measured using electrostatic Langmuir probes. Measured plasma density was typically 1012u2009u2009cm−3, which is close to the values measured for real hypersonic flight. The demonstrated laboratory experiment can be used to validate numerical codes for simulating hypersonic flight and to conduct ground-based tests...

Density and temperature distributions in the plasma expanding from an exploded wire in vacuum

The plasma expansion from an exploded wire with characteristic times of energy deposition in the wire of tens of microseconds was studied. The probe method was used to measure the plasma temperature and plasma density distributions. Tungsten wires 25, 50, 75, and 125u2002μm in diameter and a copper wire 100u2002μm in diameter were used. The waveforms of discharge voltage Ud, discharge current Id, and floating potential showed that Ud was close to a constant, while Id decreased, indicating that the wire resistance increased until plasma appeared. Immediately after the appearance of plasma, Id was observed to peak, while the voltage decreased stepwise from ∼110 to about 70 V. A relatively high electron temperature (about 12 eV) was observed in the expanding plasma even at r=2u2002cm from the wire axis. The plasma density was a maximum of 2×1013u2002cm−3 at r=2u2002cm, and it decreased with increasing r. For r<2u2002cm, unusual electrical parameters were observed, indicating the probe activity and a significantly increased plasma d...

Experimental study of plasma parameters in a vacuum arc with a hot refractory anode

The spatial distributions of plasma density, electron temperature, plasma potential and ion energy were measured during hot refractory anode vacuum arc (HRAVA) development as functions of arc current and inter-lectrode gap distance. Plasma density increased with arc time and saturated (≥1014u2009cm−3) in the developed HRAVA stage. While the ion energy in the anode plasma at the gap exit was relatively small, the ions were accelerated outside the gap in radially expanding plasma to ~15u2009eV at 19u2009cm from the electrode axis. The electron temperature decreased during anode plume development and was ≥1u2009eV in the developed HRAVA stage. Measured plasma parameters agreed well with previously developed theory.

Measurements of the anode temperature in a vacuum arc with an asymmetric hot refractory Mo anode

The temperature of an asymmetric anode was measured at three points in the anode body of a Hot Refractory Anode Vacuum Arc (HRAVA) using high temperature thermocouple probes placed near the front and rear surface. Different anode geometries with inclined front surfaces were used for arc currents in the range 125-225 A and for gaps with electrode separations of 5-18 mm. The measurements show that the transition period to the Hot Anode mode in the HRAVA decreased with arc current. Photographic study of the interelectrode region indicated that the plasma plume in the transition period expands from the anode toward the cathode and filled the gap. The plasma plume was asymmetrically distributed on the asymmetric anode surface, resulting in an asymmetric anode surface temperature distribution. Asymmetry of the anode surface temperature distribution increased for more asymmetric anodes and gap geometries. Molybdenum anode temperatures increased with arc current and decreased with electrode separation. The anode surface steady state temperature exceeded 2200/spl deg/K-2300/spl deg/K when the arc currents were larger than 145 A. The observed surface temperature variations were caused by fluctuations of the heat flux to the anode.

Measurement of Ion Flux as a Function of Background Gas Pressure in a Hot Refractory Anode Vacuum Arc

The fraction of the ion flux in the radially expanding plasma flux was obtained in hot refractory anode vacuum arc by measuring the ion current and the film deposition characteristics. Experiments were conducted with an arc current of 200 A, a molybdenum anode and an electrode separation of about 10 mm. It was found that the collected ion current in vacuum increased about twice from the beginning of the arc (t < 30 s) to the developed HRAVA stage (t > 50-60 s). The measured ion current in argon, nitrogen, and helium environments remained approximately constant with background gas pressure up to some critical pressure (15, 30, and 75 mTorr for argon, nitrogen and helium respectively), and then decreased with pressure eventually reaching zero. The ion fraction in total deposition flux was 0.6 in vacuum and decreased with nitrogen pressure except for the pressure range 10-100 mTorr where the local maximum about 0.8 was observed

Heat flux to an asymmetric anode in a hot refractory anode vacuum arc

A 3D thermal hot refractory anode vacuum arc (HRAVA) anode problem was developed and numerically solved by the finite-difference method. The heat flux from the plasma to an asymmetric anode with a slanted front surface in a HRAVA was determined using a three-dimensional numerical model for the heat flow in the anode and anode temperature measurements. The effective anode voltage (defined as the total heat flux to the anode divided by the arc current) was ~7.0–7.5u2009V for the range of gaps and arc currents 5–18u2009mm and 125–225u2009A, respectively. The effective anode voltage increases slightly (~2–3%) with the arc current increasing from 125 to 225u2009A. The effective anode voltage decreases (~6–7%) with gap distance increasing from 5 to 18u2009mm. The heat flux to the asymmetric anode was strongly asymmetric with the maximum at the anode apex. The calculated heat flux density at the apex was up to 3 times higher than at the anti-apex.

Angular distribution of ion current in a vacuum arc with a refractory anode

The total ion current and its angular distribution were measured in the radially expanded plasma of a hot refractory anode vacuum arc (HRAVA). The ion current increased during the transition from the initial cathodic arc stage to the developed HRAVA stage. The total ion current in the HRAVA was found to be significantly higher (up to 50%) than that found in the conventional cathodic arc. On the initial cathodic arc stage the ion current angular distribution peak was shifted from the gap mid-plane to the anode direction due to the contribution of the cathode plasma jets. In contrast, in the developed HRAVA stage and with small gaps ( 20 mm), the developed HRAVA stage was more stable than the initial cathodic arc stage, due to the increased plasma density in the gap.

Anode Plasma Plume Development in a Vacuum Arc With a “Black-Body” Anode–Cathode Assembly

A new type of vacuum arc plasma source, the vacuum arc with a black body assembly (VABBA), was designed to produce a directed plasma jet. In the VABBA, cathode material is emitted into a closed vessel formed by the cathode and anode, re-evaporated from the anode, which is made from a refractory material, and heated by the arc, and a plasma jet is extracted through an aperture in the anode. Images of different stages of the jet development are presented.

Measurement of ion flux as a function of background gas pressure in a Hot Refractory Anode Vacuum Arc

The hot refractory anode vacuum arc (HRAVA) is a metallic plasma source in which plasma expands radially from the interelectrode gap and may deposit substrates circumferentially disposed around the electrode axis. The dependence of copper ion flux expanding from the HRAVA interelectrode gap was determined as a function of background gas pressure. The fraction of the ion flux in the radially expanding plasma flux was obtained by measuring the ion current and the film thickness. Experiments were conducted with arc currents of 145-250 A, a molybdenum anode, and an electrode separation of about 10 mm. The saturation ion current was measured with a circular flat probe with 10-mm diameter biased at -30 V with respect to the anode. It was found that the collected ion current in vacuum was almost constant during the first 30 s of the arc - ~2.5 mA/cm2 at a distance of 110 mm from the arc axis, with an arc current of 200 A, and increased to a steady-state value in the developed HRAVA (t > 40 s) of ~5.5 mA/cm2. The measured ion current in argon, nitrogen, and helium environments and the deposition rate in nitrogen remained approximately constant with background gas pressure up to some critical pressure and, then, decreased with pressure eventually reaching zero. The critical pressures were 2, 4, and 10 Pa for argon, nitrogen, and helium, respectively. The critical nitrogen pressure for the deposition rate was 2 Pa in contrast with 4 Pa for the ion current. The ion fraction in total deposition flux was 0.6 in vacuum and decreased with nitrogen pressure, except that a local maximum of ~0.8 was observed at ~13 Pa.

Total ion current fraction in a hot refractory anode vacuum arc

The total ion current in the radially expanded plasma of a hot refractory anode vacuum arc (HRAVA) was found to be larger than that measured in the conventional cathodic vacuum arc. The total ion current fraction fi increased during the transition from the initial cathode spot stage to the HRAVA stage. While the fi is independent of arc current in the range of 150–350A and other arcing conditions in conventional arcs, the fraction increased with arc current in this current range in the HRAVA. The ion current fraction was found to be 50% larger for a 350A HRAVA than the maximal value obtained previously in conventional cathodic arcs.