L. Szalai

EFFECT OF DIFFERENT ELEMENTARY PROCESSES ON THE BREAKDOWN IN LOW PRESSURE HELIUM GAS

We investigated the breakdown in low-pressure helium gas both experimentally and by computer simulations. At low breakdown voltages (VBR 1000 V) the experimental and simulation results show a good agreement (differences are within 20%), while at higher voltages the simulations and experiments agree qualitatively. Our simulations indicate that several processes contribute to the particular shape of the Paschen curve in helium at low pressures. These processes are: (1) the dependence of the (ion-induced) secondary electron emission yield on the ion energy, (2) the appearance of ion impact ionization of the gas at high electric fields and (3) the secondary electron emission from the cathode due to fast neutral atoms.

Comparison of Cu-II 781 nm lasers using high-voltage hollow-cathode and hollow-anode-cathode discharges

Voltage-current characteristics and the Cu-II 780.8 nm laser performances are described for a novel segmented hollow cathode and for three- and four-slot hollow-anode cathode (HAC) tubes. Each of these operate at a higher voltage and with higher slope resistance than a conventional hollow cathode and produce improved laser performance. The best laser performance is obtained with the segmented tube. The application of a longitudinal magnetic field raises the discharge voltage and enhances the laser performance for the segmented tube and raises the voltage for the four-slot HAC tube. The magnetic field lowers the voltage and reduces the laser performance with the three-slot HAC tube. The voltage effects are attributed to the deflection of the fast electrons by the magnetic field and represent experimental evidence for the oscillation of electrons in a hollow-cathode discharge. >

Au-II 282 nm segmented hollow-cathode laser-parametric studies and modeling

Laser operation on the Au-II 282.3 nm ultraviolet transition is obtained using a high-voltage segmented hollow-cathode discharge tube. The metal vapor is produced by means of cathode sputtering. A small amount of argon is added to the helium buffer gas in order to achieve higher sputtering yield. Measurements of the laser power and small signal gain indicate that the optimal partial concentration of argon is in the range of 0.25%–0.75%. Quasi-continuous wave output power of 100 mW is obtained from a 34-cm-long active region while the highest small-signal gain is 52% m−1. To explain the basic features of the laser operation we present a model of the segmented hollow-cathode discharge. All the discharge characteristics are calculated in a self-consistent way except the temperature of slow electrons. The trajectories of fast electrons emitted from the cathode are followed by Monte Carlo simulation. Rate equations of ion, metastable and metal atom densities are solved in the negative glow, while another Monte...

High-gain ultraviolet Cu-II laser in a segmented hollow cathode discharge

We report measurements of the discharge and laser characteristics of an ultraviolet (UV) copper ion laser radiating on the 248.6- and 270.3-nm transitions. Our 50-cm active length laser operates in a high-voltage segmented hollow cathode discharge with Brewster windows and external mirrors. The lowest threshold currents were 7.3 and 5.6 A for the above transitions, respectively. The threshold current was found to increase with decreasing active length. We obtained superior gain characteristics (to 11.2 and 8.8%/m gain for the 248.6-and 270.3-nm transitions, respectively, at 0.3 A/cm linear current density) compared with previous studies on UV Cu ion lasers in conventional hollow cathode discharges.

Dependence of gain and laser power for Cu-II 780.8-nm transition on the diameter of a segmented hollow cathode discharge

The dependence of laser performance and discharge characteristics on the diameter of a segmented hollow cathode discharge for the Cu-II 780.8 nm transition is presented. This transition has a special importance since its upper level is common to potential CW VUV laser transitions (150-170 nm). Laser tubes with internal diameters of 2, 3, 4, and 5 mm were investigated. Decreasing the diameter resulted in an increased gain for a given current (up to 100 %/m in the 2-mm diameter, 5-cm-long tube at 1-A current). The highest output power was obtained from the large-diameter tubes (20 mW from a 5-cm-long, 5-mm-diameter tube at 2-A current, without optimizing the output coupler). This work is a part of a series of investigations aimed at the optimization of the segmented hollow cathode discharge which has already been found to be the most efficient type of discharge for cathode sputtered metal ion lasers. >

SPECTRAL-INTERFEROMETRY MEASUREMENTS OF COPPER ATOM CONCENTRATION IN A SEGMENTED HOLLOW CATHODE DISCHARGE

Abstract We report spectral-interferometry measurements of the concentration and the spatial distribution of copper vapor in a high voltage, segmented hollow cathode discharge, which is an efficient source for laser light generation. The concentration of sputtered copper atoms was found to be in the order of 1014 cm-3 at currents of 0.6-1.0 A in a 5 cm long, 4 mm bore diameter discharge tube, operating in argon buffer gas. At high pressures and relatively low current densities the sputtered metal concentration forms two maxima near the cathode surface. With increasing current and decreasing pressure these two maxima move towards the center of the discharge. This result may account for the fact that at certain discharge parameters the segmented hollow cathode lasers operate in higher order transverse modes near the threshold. The measurements of copper vapor concentration show increased sputtering compared to conventional hollow cathodes, which is considered to be one of the reasons for segmented hollow cathode metal ion lasers being more efficient.

Optimum operating conditions of a hollow-cathode Au-II laser

Laser operation on the Au-II 282.3 nm ultraviolet transition was obtained from a high-voltage, segmented hollow-cathode discharge tube. The excitation mechanism of the laser is charge transfer collisions of He+ ions and ground state Au atoms resulting from the sputtering of the gold- coated cathode. The high gain provided by the special hollow cathode construction made it possible to operate the laser with external mirrors on the ultraviolet 282.3 nm line. For the efficient pumping of the laser, high density of both He+ ions and Au atoms must be present in the discharge. Increased cathode sputtering, thus higher Au atom density can be achieved using a small amount of a heavier noble gas (e.g. Ar) in the helium buffer gas. On the other hand, the presence of Ar--due to its low ionization potential and high ionization cross section--reduces the energy of electrons and thus the ionization rate of helium. Measurements of the laser power and small signal gain indicate that the optimum partial pressure of argon is in the order of 0.25 - 0.75%.

Optimization of hollow cathode discharges for pumping metal ion lasers

Brief overview of recent optimization studies of hollow cathode discharges, mainly the segmented hollow cathode arrangement for the excitation of metal ion lasers is given here. Results of parametric studies (on the 282.3 nm Au II and 780.8 nm Cu II transitions) including optimization of buffer gas composition, cathode-anode area ratio and inner diameter of the discharge tube are presented here together with some modeling results.

Continuous Uv Metal Ion Lasers Excited In Segmented Hollow Cathode Discharges

The copper vapor laser (CVL) is the most high-power and high-efficiency laser in the visible spectral portion (511 nm and 578 nm), and therefore it is a subject of considerable interest. The main problem in the development and application of CVLs comes from the high-temperature operation regime (1500C) of the laser tube. This high temperature is necessary to maintain such a copper atom density that laser oscillation becomes possible. One approach to this problem is the copper bromide vapor laser (CuBrVL) that is a new variant (proposed by us) of the CVL. The CuBrVL works at about 500 C temperature of the active medium. The present paper is a summary of our basic investigations on the development and practical application of the CuBrVL. Our studies were performed in three main directions described below.

Novel Low-Temperature Pulsed Metal-Vapour Lasers

Hollow-cathode discharge sputtering allows the immediate production of metal vapour together with efficient charge-transfer pumping of ultraviolet (~ 220–290 nm) metal ion laser transitions, most notably those for copper, silver and gold. This paper discusses a novel high-voltage hollow-cathode discharge with increased efficiency and gain to facilitate the development of compact pulsed lasers for applications to biomedical instruments.