We investigated the breakdown in low-pressure helium gas both experimentally and by computer simulations. At low breakdown voltages (V BR 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.

We carried out molecular dynamics experiments to determine the reduced heat diffusion coefficient D ‫ء‬ th and the reduced thermal conductivity l ‫ء‬ of the three-dimensional classical electron one-component plasma, for the 1 # G # 20 range of the plasma coupling parameter G. In our simulations the temperature of the system was spatially modulated, and D ‫ء‬ th and l ‫ء‬ were calculated from the relaxation time of the temperature profile. D ‫ء‬ th was found to decrease with increasing G, while l ‫ء‬ decreased with increasing G for G # 4 (from l ‫ء‬ Х 2.3 at G 1.1 to l ‫ء‬ Х 0.6 at G 4) and was approximately constant (l ‫ء‬ Х 0.4) in the 8 # G # 20 range. [S0031-9007(98)06853-7]

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 magnetiqfield 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. .

We have investigated the effect of constriction on the characteristics of low-pressure glow discharges in argon. In a series of experiments four discharge tubes with plane-parallel electrodes of different diameters (D = 31.4, 20, 10 and 5 mm) and same electrode separation (L 0 = 45 mm) were studied. The discharges were surrounded by floating-potential metal tubes. We measured voltage-current characteristics of the discharges and recorded the spatial intensity distribution of selected spectral lines (Ar-I 750.3 nm, 811.5 nm and Ar-II 476.5 nm) in the electrode gap at current densities 0.2 mA cm −2 j 1 mA cm −2. We also observed copper lines in the spectrum originating from the sputtering of the copper cathode, even at these relatively low discharge current densities. The 'effective surface' of the cathode-that actively participates in discharge operation-is always smaller than the cathode surface. This results from the radial electric field formed in the cathode dark space due to the accumulation of charges on the metal tube surrounding the discharge. Using a simple model we could calculate the effective cathode diameter (which determines the current density for given current). Taking into account the current density obtained this way, the voltage of the tubes was found to obey the V = f (j/p 2) scaling relation. The increasing loss of charge on the metal wall with decreasing tube diameter also resulted in structural changes in the discharge. While at large diameter the cathode dark space and the negative glow filled the interelectrode gap, at low D a positive column-like part was also formed.

The dependence of laser performance and discharge characteristics on the diameter of a segmented hollow cathode discharge for the Cu-I1 780.8 nm transition is presented. This transition has a special importance since its upper level is common to potential CW W V 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-cmlong, 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.

No abstract

The urine concentrating function of the kidney is essential to maintain the water homeostasis of the human body. It is mainly regulated by the arginine-vasopressin (AVP), which targets the type 2 vasopressin receptor (V2R) in the kidney. The inability of V2R to respond to AVP stimulation leads to decreased urine concentration and congenital nephrogenic diabetes insipidus (NDI). NDI is characterized by polyuria, polydipsia, and hyposthenuria. In this study, we identified a point mutation (S127F) in the AVPR2 gene of an NDI patient, and we characterized the impaired function of the V2R mutant in HEK293 cells. Based on our data, the S127F-V2R mutant is almost exclusively located intracellularly in the endoplasmic reticulum (ER), and very few receptors were detected at the cell surface, where the receptor can bind to AVP. The overexpressed S127F-V2R mutant receptor has negligible cAMP generation capability compared to the wild-type receptor in response to AVP stimulation. Since certain misfolded mutant proteins, that are retained in the ER, can be rescued by pharmacological chaperones, we examined the potential rescue effects of two pharmacochaperones on the S127F-V2R. We found that pretreatment with both tolvaptan (an established V2R inverse agonist) and MCF14 compound (a cell-permeable high-affinity agonist for the V2R) were capable of partially restoring the cAMP generating function of the receptor in response to vasopressin stimulation. According to our data, both cell permeant agonists and antagonists can function as pharmacochaperones, and serve as the starting compounds to develop medicines for patients carrying the S127F mutation.

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 Ϫ1. 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 Carlo code is applied for the fast heavy particles in the cathode sheath. The spatial distribution of the gas temperature and the thermalization of sputtered metal atoms are calculated as well. The laser characteristics predicted by the model are in reasonable agreement with the experimental results.