V Colomer

The electron density stabilisation process in pulsed surface wave plasmas

The authors present an experimental study of the evolution of electron density during the formation process of a plasma column produced by a surface wave. The experiments have been performed in argon gas, using pulsed microwave power at 2.45 GHz, in the 0.3-30 Torr pressure range. The time elapsed between the arrival of the ionisation front at a given axial position and the moment the electron density reaches 90% of its stationary value (stabilisation time) has been determined. This stabilisation time increases exponentially with axial position towards the end of the plasma column, as the power of the surface wave decreases.

Study of surface-wave-produced plasma column lengths

An experimental description of the length of cylindrical plasma columns produced and sustained by a weakly damped surface wave is presented. A general experimental law for the column length is obtained that depends on the operating parameters in a wide range of discharge conditions. This general empirical law can be deduced theoretically from similarity laws already known to apply to such plasmas.

Effective recombination coefficients in argon surface-wave-produced plasma

The authors present experimental data for the effective recombination coefficient in a pulsed surface-wave-produced plasma. To determine these data they studied the relaxation of electron density to equilibrium. A 2.45 GHz Surfatron was used in order to couple the HF energy, and three different Pyrex cylindrical tubes of internal radius a=1.5, 2.5 and 4.5 mm, with a corresponding external radius b=a+1.5, were employed. The gas used in the discharge was argon, in the 0.5-12 Torr pressure range. They obtained the total losses by ambipolar diffusion and recombination, divided by pressure, as a function of the quotient between the effective surface wave electric field and gas pressure. At the lower end of the pa range diffusion losses dominate but recombination becomes more important for higher values of pa.

Experimental study of the ionization front in pulsed‐surface‐wave‐produced plasmas

The advance of the ionization front along the plasma column is studied during the formation process of plasmas produced by a surface wave. The experimental measurements have been performed in argon gas, for the 0.1–30‐Torr pressure range, using pulsed microwave power at 2.45 GHz. The ionization front velocity decreases along the plasma column, from its maximum value at the launcher to the zero value at the stationary plasma column end, similarly to the surface‐wave power diminution. This velocity can be expressed to depend only on the product collision frequency times tube radius and the surface‐wave incident power along the plasma column.

The numerical modelling of Joule heating effects in thoriated tungsten cathodes of high-current plasma arcs

The influence of the Joule heating on the performance of thoriated tungsten cathodes working at current densities of 103-104 A cm-2 is investigated. The variation of the degree of coverage of the cathode surface by thorium atoms is considered, together with the heat conduction and the diffusion of thorium in the cathode bulk. The nonlinear system of equations governing these processes is solved numerically and the time evolution of the spot temperature is obtained. The effects of different working conditions on the cathode lifetime are discussed. It is found that a critical current density exists for which the service time of the cathode has a minimum. It is shown that a small increment in the cathode cooling temperature significantly increases the cathode lifetime.

A study of spot evolution in hot refractory cathodes of high-pressure arcs

In this paper the behavior of the cathode spot in atmospheric pressure arc is studied from a dynamic point of view and later applied to a specific case of a tungsten refractory cathode. For this, a model of the cathode region was developed. In this model the tungsten atoms proceeding from the cathode evaporation, which were all supposed to be ionized in the presheath, returned to the cathode surface as ions, provoking a heating of this surface. Therefore, the model allowed us to evaluate the importance of those atoms. Furthermore, different mechanisms of the electron emission from the cathode surface were considered as function of the spot temperature and the electric-field strength. This model of the cathode region allowed getting important parameters in the study of the dynamics of the cathode spot such as total current density, which is necessary for the calculation of Joule heating effect, and the total-energy flux density, which was incorporated as a boundary condition for the solution of the heat co...

Empirical similarity laws for argon plasmas produced by a surface wave at 2.45 GHz

The authors calculate the value of the product Eeffa for Ar surface-wave-produced plasmas at 2.45 GHz. This value has been found to be variable with pressure in the range of cases studied (low pressures up to 20 Torr). Such a variation is obtained as a consequence of empirical similarity laws existing for the effective electric field in the discharge and the reduced length of plasma columns as a function of the product pa.

Modeling and experimental investigation of spot dynamics on graphite cathodes in dc plasma arcs at high pressure

From a model recently developed for refractory cathodes [Munoz-Serrano et al., J. Appl. Phys.98, 093303 (2005)], the behavior of a graphite cathode spot in a dc plasma torch at atmospheric pressure was investigated. Furthermore, an experimental study of these cathodes was made guided by the results obtained from the model. The model includes the modeling of the cathode region, the solution of the heat conduction problem in the cathode, and the simulation of the cathode ablation process. As a result of the model, the values of the parameters which characterize the cathode region were obtained, and the evolution of the spot under different working conditions determined by the value of the initial voltage drop in the cathode region, U0, was investigated. The results obtained show that the maximum spot radius diminishes when U0 increases. Furthermore, two qualitatively different conditions for the spot dynamics were found. For U0⩾31V, the spot radius continually grows over time until reaching a maximum value ...

Temporal evolution of the electric field intensity in pulsed surface-wave-produced plasmas

The authors present a study about the temporal evolution of the electric field intensity during the plasma formation process. The experimental measurements have been performed in argon gas at low pressure and using pulsed microwave power at 2.45 GHz to produce the discharge. From the electric field radial distribution of the surface wave and with experimental measurements of the electron density and electric field outside the discharge tube, the temporal evolution of the electric field within the plasma is obtained. This field reaches a maximum during the creation process, in the first moments of the plasma formation. This fact justifies the maximum of the light intensity emitted by the plasma observed experimentally.

Reexamination of recent experimental results in surface-wave-produced argon plasmas at 2.45 GHz: comparison with the diffusion-recombination model results

In this paper we comment on recently reported experimental data about some characteristic magnitudes of plasma columns produced and maintained by surface microwaves. We then compare them with theoretical values obtained from the diffusion‐recombination model of Mateev, Zhelyazkov, and Atanassov [J. Appl. Phys. 54, 3049 (1988)] and Zhelyazkov, Benova, and Atanassov [J. Appl. Phys. 59, 1466 (1986)] for the same magnitudes, in a wide range of operating conditions. Such a comparison allows us to make conclusions about the results of the model and its hypothesis.