Plasma Production Mechanism Research Articles

Stark broadening of the cesium 52D5/2 → 102F line shape

Pump modulated laser absorption spectroscopy was used to measure the cesium 52D5/2→102F5/2,7/2 line shape. Alkali density in an optically pumped low pressure cell was increased from 0.5 to 85×1012cm−3, resulting in a factor of 200 increase in Lorentzian width due to Stark broadening (2.76GHz/1012cm−3). The observed red shift is consistent with literature, and the shift-to-width ratio δ/γ=−0.356±0.015 informs an electron temperature of 1820±100K. A linear trend between alkali density and electron density is seen, suggesting that electron impact is the primary plasma production mechanism. A maximum ionized concentration of 28% was observed.

Studies on plasma production in a large volume system using multiple compact ECR plasma sources

This paper presents a scheme for large volume plasma production using multiple highly portable compact ECR plasma sources (CEPS) (Ganguli et al 2016 Plasma Source Sci. Technol. 25 025026). The large volume plasma system (LVPS) described in the paper is a scalable, cylindrical vessel of diameter ≈1 m, consisting of source and spacer sections with multiple CEPS mounted symmetrically on the periphery of the source sections. Scaling is achieved by altering the number of source sections/the number of sources in a source section or changing the number of spacer sections for adjusting the spacing between the source sections. A series of plasma characterization experiments using argon gas were conducted on the LVPS under different configurations of CEPS, source and spacer sections, for an operating pressure in the range 0.5–20 mTorr, and a microwave power level in the range 400–500 W per source. Using Langmuir probes (LP), it was possible to show that the plasma density (~1 − 2 × 1011 cm−3) remains fairly uniform inside the system and decreases marginally close to the chamber wall, and this uniformity increases with an increase in the number of sources. It was seen that a warm electron population (60–80 eV) is always present and is about 0.1% of the bulk plasma density. The mechanism of plasma production is discussed in light of the results obtained for a single CEPS (Ganguli et al 2016 Plasma Source Sci. Technol. 25 025026).

Low-power radio-frequency excitation as a plasma source in a Penning–Malmberg trap: a systematic study

A novel method was experimentally demonstrated to produce a low-density electron plasma in a Penning–Malmberg trap, exploiting the static electric and magnetic confinement fields together with a periodic excitation with amplitudes as low as 0.5–5 V and frequencies in the MHz range. This unusual technique proved to be applicable as a replacement for conventional electron sources in Penning devices and presents interesting aspects both in terms of basic science and technological applications. Nevertheless, the experimental observations demonstrate the high sensitivity of plasma features of interest (charge, mean density and density distribution) to the experimental conditions, namely trap configuration and excitation parameters, and as a consequence clear trends have not been identified so far. We present an experimental campaign of measurements where several parameters were systematically changed leading to a better assessment of the plasma production mechanism and to the identification of common trends.

The new TORPEX in-vessel toroidal conductor for the generation of a poloidal magnetic field

TORoidal Plasma EXperiment (TORPEX) is a Simple Magnetized Torus featuring open helical magnetic field lines obtained from the superposition of a small vertical component on the main toroidal field. This work introduces the experimental setup developed to include a poloidal magnetic field. The toroidal and poloidal fields generate a rotational transform, making the magnetic geometry of TORPEX closer to that of a tokamak. This upgrade opens the possibility to deal with closed and open flux surfaces, as well as with the transition region across the last closed flux surface. The main technical solutions are discussed together with the physical considerations at the basis of the system design. Selected examples of the magnetic configurations accessible with the set of magnetic field coils available on TORPEX are discussed, ranging from single-null X-points to magnetic snowflakes. The simplest magnetic configuration of quasi-circular concentric flux surfaces is tested experimentally. Measurements of the two-dimensional electron plasma density profiles and the particle confinement time are presented, together with the first steps towards the understanding of plasma production mechanisms.

Metallic plasma production by laser ablation

Laser production of plasma at the surface of a metallic target was modeled. The mechanism of plasma production comprises several common phenomena at the target surface, at the plasma-target interface and in the plasma expansion region. Atom vaporization and electron emission due to target heating by high intensity laser radiation and the plasma were considered. The heat flux from the plasma is convected by ions and returned electrons. At the plasma-target interface, an electrical sheath appears in which all charged particles are accelerated. The accelerated electron emission beam conveys energy into the plasma and together with partial absorption of the laser energy heats the plasma electrons. A mathematical model was formulated, which describes the phenomena in the solid target, space charge sheath, electron beam relaxation region, plasma expansion region and the Knudsen layer, where the vapor flux from the target was formed. The model allows determination of the target temperature, plasma density and plasma temperature, potential drop in the sheath and electrical field at the target surface, electron and ion current density, the rate of target vaporization, vapor degree of ionization and plasma velocity for the model was applied to an example of an Al target, with laser pulse durations of 1 ns, 10, 103 and 10 ms and laser power density of q L=10−3−1 GW/cm2. The plasma parameters and rate of target ablation were compared with the experimental data.

Analysis of Electron and Microwave Behavior in Microwave Discharge Neutralizer

In the MUCES-C mission conducted by JAXA (Japan Aero Exploration Agency), a microwave neutralizer is mounted with a microwave ion engine on the HAYABUSA space probe. The neutralizer consists of an L-shaped antenna to inject microwaves and samarium cobalt magnets to provide ECR (electron cyclotron resonance). Plasma production of a higher density than the cutoff density is expected in the discharge chamber, but the neutralizer is so small that high-precision measurements using a probe are difficult. To clarify the plasma production mechanism in the microwave neutralizer, numerical analysis was conducted using a code coupling PIC (particle-in-cell) method, and a FDTD (finite-difference-time-domain) method. This paper describes effects caused by varying magnetic field configuration and antenna position in the neutralizer. The calculation results show that bringing the antenna closer to the ECR region is effective for plasma production.

Plasma production experiments using a folded waveguide antenna on LHD

A folded waveguide (FWG) antenna was used in the ion cyclotron range of frequency (ICRF) on the large helical device (LHD) in the National Institute for Fusion Science. The FWG antenna is a waveguide antenna folded several times in order to make its size smaller. The FWG antenna in the LHD is designed so that slow waves are excited preferentially. It was used in the fourth experimental campaign in 2000-2001 for the purpose of plasma production and plasmas with an average electron density up to 3.0×1018 m-3 were obtained. This will be a high enough density for initial plasmas of neutral beam injection or ICRF to obtain plasmas with higher densities and temperatures. This is the first demonstration of the utility of an FWG antenna in magnetic confinement devices. Further investigations were made in order to understand the mechanism of plasma production. The maximum achievable plasma density increased with injection power and gas-puffing rate, and became saturated. The density became higher as the magnetic field strength was increased. Such experimental observations were explained by the wave accessibility conditions of a shear Alfvén wave.

Ion density variation effects in plasma source ion implantation

Plasma-source ion implantation (PSII) is a well-developed technique for introducing ions into a material specimen for enhancement of the surface properties. The technique is moving from the research to the industrial level, where specimens of large surface area need to be treated simultaneously. However, such large areas demand an enormous amount of ion current from the plasma during application of the negative bias. As in most cases the rate and mechanism of plasma production is independent of the bias conditions of the specimen, there can be a drop in the plasma density during the application of the bias for large areas of the specimen. This paper analytically investigates the consequences of this drop in density on sheath expansion and ion flux using quasi-static expansion of Child's law. The results indicate a drastic drop in ion flux to the specimen and the production of large ion sheaths if the plasma production mechanism is not strong enough to cope with the ion loss.

Investigation of microwave plasmas produced in a mirror machine using ordinary-mode polarization

This paper presents results of plasma production in a mirror machine with the microwaves being coupled from a side port, in the ordinary-mode polarization. In this method of coupling, contamination due to the launching antenna is avoided; also, there is very little window degradation since plasma bombardment on the side port is negligible. The coupling of the microwaves to the plasma in this method relies on the existence of certain surface waves that exist at low values of the magnetic field (below that for electron cyclotron resonance) and have a dominant Ez component. The existence of such surface waves is predicted by a recent theory (Ganguli A et al 1998 Phys. Plasmas 5 1178). The plasma characterization at microwave power levels of about 200 W, indicates moderately overdense plasmas (density 1011 cm-3), with fairly uniform radial and axial profiles. From the data, the mechanism of plasma production and confinement in the mirror has been analysed. To gain insight into the nature of the waves in the plasma-loaded waveguide, the wavelengths of the waves within the plasma and the radial profiles of the wave electric field were measured. The results are compared with the theory of Ganguli et al for wave identification.

Cold-cathode, pulsed-power plasma discharge switch

CROSSATRONTMmodulator switches are cold-cathode, grid-controlled, plasma-discharge devices that are used for thyratron and hard-tube replacement in high-voltage, pulsed-power applications. CROSSATRON modulator switches have been used to produce square pulses of up to 100 kV and 1000 A, and CROSSATRON laser-discharge switches have switched peak discharge currents of up to 10 kA at 40 kV. The major advantage that CROSSATRON switches offer over other plasma switches is a rapid deionization time that permits high pulse-repetition frequencies (103 to 106 pulses per second depending on the application), and a long life associated with the cold-cathode plasma production mechanism. Compared to hard tubes, CROSSATRON switches have a relatively low forward voltage drop (500 V), the ability to close and open up to 1 kA of peak current, and lower grid-drive power requirements. In this article, we describe the physical mechanisms for how the switch works based on simple models and experimental data. The design of CROSSATRON switches is explained, and characteristic performance in closing and opening applications is described and explained.

One-Dimensional Modeling for Magneto-Microwave Plasma Using the Monte Carlo Method

This study discusses modeling of a magneto-microwave plasma used in semiconductor manufacturing equipment such as etching reactors and chemical vapor deposition (CVD) reactors. A one-dimensional simulation program for magneto-microwave plasma was developed. This combines a Monte Carlo particle plasma model and an electromagnetic wave damping model. With the use of this simulation, a plasma production mechanism with electron cyclotron resonance and electromagnetic wave damping in a partially ionized gas can be analyzed. Typical results of the effect of gas pressure on the plasma distribution in plasma processing equipment are presented.

Multiply charged metal ion beams

The metal vapor vacuum arc is a prolific generator of highly ionized metal plasma in which the ions are in general multiply stripped. We have utilized this plasma production mechanism to make a high current metal ion source. The metal vapor vacuum arc ion source is thus a laboratory tool for providing high quality, high current beams of a wide range of metal ion species. Charge state spectra have been measured using a time-of-flight diagnostic. In this paper we review the ion source development program, describe the various embodiments of the concept that have been made, and summarize those experimental results relating to the ion charge states that can be produced.

Development of a formed-ferrite flash plasma light source for gas laser applications

A new type of plasma light source using amorphous filaments formed on ferrite substrates has been developed for application to gas laser excitation and preionization. A formed filament acts as a leading guide to produce a long-distance, high-current discharge plasma which emits hard ultraviolet photons. The plasma production mechanism and technological advantages over conventional ways of producing plasmas of the formed-ferrite flash are described.

Electron and ion kinetics and anode plasma formation in two applied B r field ion diodes

Two magnetically insulated ion diodes that utilize a radial applied-B field are described. Both diodes generate an annular beam that is extracted along the diode axis. The first diode operated at 1.2 MV and 600 kA for 25 ns and generated a 300-kA ion beam. The second operated at 300 kV, 100 kA and generated 15 kA of ion current. The first diode was used to study diode performance as a function of inner and outer anode-cathode gaps, the applied-B field, and transmission line current ratios. The second diode was used to study anode plasma formation. The diodes were operated below Bcrit, resulting in electron leakage to the anode, especially near the outer cathode. A definition of Bcrit applicable to extraction diodes is given and methods of improving ion production efficiency in these diodes are suggested. The strong correlation of ion production with visible light emission suggests, however, that the electron loss played an important role in anode turn-on. The breakdown of neutral gas desorbed by electron impact is thought to be the anode plasma production mechanism. The grazing incidence leakage electrons affect the breakdown by significantly enhancing space-charge-induced electric fields in the dielectric-filled anode grooves.