Ya. Z. Slutsker

High‐power radio frequency generation in a hollow‐cathode discharge

High‐power rf oscillations are obtained in a resistive load installed in the external electrical circuit of a low‐pressure magnetic‐field‐free hollow‐cathode discharge. These intense rf oscillations have been shown to be due to a collisionless instability of the cathode sheath. A very simple device based on this effect is shown to serve as a convenient and powerful rf oscillator. Typical results first achieved without any optimization, for frequencies of 20–30 MHz and load resistances of several tens of ohms, are as follows: power of about 5 kW and efficiency ∼25%.

Broad-band polarization-independent absorption of electromagnetic waves by an overdense plasma

Surface plasmon-polaritons can be efficiently excited on a plasma-vacuum interface by an electromagnetic wave when a subwavelength diffraction grating is placed in front of the plasma boundary. The excitation efficiency depends strongly on the wave frequency (or plasma density, when the frequency is fixed) and polarization. We show both experimentally and theoretically that this sensitivity can be essentially suppressed. A nonzero angle of incidence and an axially symmetric diffraction grating ensure near-total absorption of the incident wave in a broad range of wave frequencies (or plasma densities, when the frequency is fixed). Direct detection of surface plasmon-polaritons has been achieved for the first time using a miniature antenna embedded in the plasma. A new absorption mechanism which is not associated with surface plasma wave excitation is revealed.

Cold Atmospheric Plasma, Created at the Tip of an Elongated Flexible Capillary Using Low Electric Current, Can Slow the Progression of Melanoma

Introduction Cold Atmospheric Plasma Jet (CAPJ), with ion temperature close to room temperature, has tremendous potential in biomedical engineering, and can potentially offer a therapeutic option that allows cancer cell elimination without damaging healthy tissue. We developed a hand-held flexible device for the delivery of CAPJ to the treatment site, with a modified high-frequency pulse generator operating at a RMS voltage of <1.2 kV and gas flow in the range 0.3–3 l/min. The aims of our study were to characterize the CAPJ emitted from the device, and to evaluate its efficacy in elimination of cancer cells in-vitro and in-vivo. Methods and Results The power delivered by CAPJ was measured on a floating or grounded copper target. The power did not drastically change over distances of 0–14 mm, and was not dependent on the targets resistance. Temperature of CAPJ-treated target was 23°-36° C, and was dependent on the voltage applied. Spectroscopy indicated that excited OH- radicals were abundant both on dry and wet targets, placed at different distances from the plasma gun. An in-vitro cell proliferation assay demonstrated that CAPJ treatment of 60 seconds resulted in significant reduction in proliferation of all cancer cell lines tested, and that CAPJ activated medium was toxic to cancer cells. In-vivo, we treated cutaneous melanoma tumors in nude mice. Tumor volume was significantly decreased in CAPJ-treated tumors relatively to controls, and high dose per fraction was more effective than low dose per fraction treatment. Importantly, pathologic examination revealed that normal skin was not harmed by CAPJ treatment. Conclusion This preliminary study demonstrates the efficacy of flexible CAPJ delivery system against melanoma progression both in-vitro and in-vivo. It is envisioned that adaptation of CAPJ technology for different kinds of neoplasms use may provide a new modality for the treatment of solid tumors.

Ferroinductor coupled discharge

An inductively coupled discharge was obtained at low gas pressure. Unlike an ordinary inductively coupled discharge with an inductorlike rf antenna, we used a magnetic core with a primary winding (“ferroinductor”). In this way, we increased the electric field and eliminated the magnetic field of the winding. Gas breakdown was obtained at a pressure as low as 10−4Torr, the discharge plasma ionization rate reached almost 100%, and the maximum plasma density was about 1013cm−3. The high inductance of the ferroinductor allowed us to work even with single pulses. The efficiency of such a discharge as a plasma source could reach 90%, which makes this kind of discharge attractive for many applications.

Optimized performance of a powerful hollow-cathode rf oscillator

We have studied the power, efficiency, and frequency ranges achievable for a hollow-cathode based rf oscillator. We have found that the rf component of the discharge current can be much larger (up to ∼4.5 times) than its dc component. In the achievable frequency range of 15–120 MHz, this very simple cold cathode device produces up to 40 kW of rf power with efficiency of up to 25%. The internal resistance of this device always remained within a convenient range of 20–40 Ω. The rf pulse duration could be prolonged up to tens of μs. The physical processes governing the oscillator parameters are discussed.

Probe measurements in a nonstationary plasma

Single and double probes are simple and common tools for plasma measurements. In the case of nonstationary plasmas, the values of the plasma density obtained with these tools may differ significantly from the correct values measured, e.g., by microwave methods. The reason for such discrepancy could be the Bohm criterion failure during the plasma transition to the steady state. Indeed, the Bohm criterion, which is commonly used as a boundary condition at the plasma-sheath edge, directly determines the ion saturation current to the probe surface. The transition-time duration is studied and explained quantitatively for various plasmas produced by a version of a ferroinductor-coupled plasma source, which has its magnetic core fully immersed in the plasma. Corresponding conversion factors for probe measurements have been evaluated. Also, the influence of a certain amount of “hot” non-Maxwellian electrons on probe characteristics has been investigated.

Input impedance of a powerful single-core ferromagnetic inductively coupled plasma source

An experimental study of the recently developed version of the ferromagnetic inductively coupled plasma source has shown that under certain circumstances its input impedance becomes almost independent of the delivered rf driving power and (therefore) of the produced plasma density. This plasma source consists of a large ferromagnetic core, which is fully immersed in plasma. This core is surrounded by a primary winding and plasma appears due to gas discharge driven by an rf voltage applied to this primary winding. We have found values of parameters which determine the input impedance in such an “independent” regime and derived a quantitative theory which is in good agreement with the measured impedance values.

Plasma uniformity enhancement in a single-core ferromagnetic inductor coupled plasma source

We describe the phenomenon of plasma uniformity enhancement in the recently developed simple version of the ferromagnetic inductor coupled plasma source in which a single thin ferromagnetic core having a large diameter (ferroinductor) was fully immersed in plasma. The plasma appeared due to the gas discharge driven by a comparatively low ac voltage (?350?V) applied to the primary winding of this core. Under certain circumstances the plasma nonuniformity did not exceed a few per cent in the radial direction as well as in the axial direction while the plasma density reached almost 1013?cm?3. A qualitative explanation of such uniformity enhancement is suggested.

X-band microwave generation caused by plasma-sheath instability

It is well known that oscillations at the electron plasma frequency may appear due to instability of the plasma sheath near a positively biased electrode immersed in plasma. This instability is caused by transit-time effects when electrons, collected by this electrode, pass through the sheath. Such oscillations appear as low-power short spikes due to additional ionization of a neutral gas in the electrode vicinity. Herein we present first results obtained when the additional ionization was eliminated. We succeeded in prolonging the oscillations during the whole time a positive bias was applied to the electrode. These oscillations could be obtained at much higher frequency than previously reported (tens of GHz compared to few hundreds of MHz) and power of tens of mW. These results in combination with presented theoretical estimations may be useful, e.g., for plasma diagnostics.

Effect of the reference electrode size on the ionization instability in the plasma sheath of a small positively biased electrode

It is well known that additional ionization in the vicinity of a positively biased electrode immersed into a weakly ionized plasma is responsible for a hysteresis in the electrode current–voltage characteristics and the current self-oscillations rise. Here we show both experimentally and theoretically that under certain conditions these phenomena cannot be correctly interpreted once considered separately from the reference electrode current–voltage characteristics. It is shown that small electrodes can be separated into three groups according to the relation between the electrode and the reference electrode areas. Each group is characterized by its own dependence of the collected current on the bias voltage.