Bhp Bart Broks

Experimental investigation of an atmospheric photoconductively switched high-voltage spark gap

We report on the experimental investigation of the photoconductively switched gas-filled spark gap. When the laser intensity of a femtosecond laser is high enough (around 1018Wm−2), a plasma can be created that spans the complete distance between the electrodes. The gas-filled spark gap is then closed on a femtosecond time scale, similar to photoconductive switching of a semiconductor switch. Stochastic breakdown processes, such as avalanche and streamer formation that cause the breakdown in laser triggered spark gaps, are passed over, which results in faster rise time and less jitter. Measurements of the switched pulses as a function of laser energy were performed in a 1-mm gap at an applied voltage of 4.5 kV. A clear transition from triggering to switching was measured with increased laser energy. Measurements of the output pulses with the gap filled with nitrogen at 1 atm showed results very similar to measurements in air in the same gap. In the switching regime, the amplitude of the switched pulse did...

Parameter study of a pulsed capillary discharge waveguide

Slow pulsed capillary discharges are currently under investigation for use as plasma channel waveguides in laser-wakefield acceleration and XUV generation. In this work, a parameter study is performed on this device using a combination of two models, namely a non-local thermal equilibrium (non-LTE) plasma model and a wall temperature model that is coupled to it. This model has been validated against experiments. In the present study, two parameters are varied, the initial density and the channel radius. These parameters have a strong influence on the guiding properties. The results of this parameter study can be summarized in a single, empiric formula describing the matched spot size as a function of the initial density and the channel radius. This formula is expected to give a good prediction of the matched spot size, provided that no wall ablation occurs, diffusion is limited and that the current pulse is sufficient in amplitude and duration for formation of a well-ionized, stable plasma. This has been verified for the parameter range studied here.

Modeling of a square pulsed capillary discharge waveguide for interferometry measurements

Slow pulsed capillary discharges in round capillaries are currently under investigation for use as plasma channel laser waveguides in laser-wakefield acceleration, x-ray lasers, and higher-harmonic generation. In this study, a capillary discharge with a square cross section is presented. The electron density, which determines the laser guiding properties, can be measured by means of transverse interferometry in this device. Using a numerical model of the plasma and the capillary wall, an analysis of the discharge is made. The results predict that the square channel is capable of guiding circular laser pulses. The guiding properties are quite similar to those of a round channel with nearly the same diameter as the channel width. This suggests the results obtained by measuring the square capillary discharge are applicable for round channels as well. It was found that the wall heating was inhomogeneous, which makes the wall more susceptible to ablation. The heating of the wall changes the transverse optical pathlength in the interferometry experiments.

A microwave plasma model for a PCVD setup

A numerical model is constructed using the PLASIMO toolkit to simulate a microwave configuration which is similar to that which is in use for optical glass fibre production. The simulations offer the flow patterns and the electromagnetic (EM) energy incoupling of a two-temperature argon plasma. The Yee algorithm was used for the EM module, whereas the Semi implicit method for pressure linked equations algorithm was used to calculate the pressure and velocity field. It is found that at 400 W coupling of 2.46 GHz EM radiation in 1000 Pa argon results in a plasma that is not in local thermodynamic equilibrium, in the sense that Te/Th ≥ 4 and the ionization degree is smaller than that predicted by the Saha equations. The model results are subjected to various sanity checks.

Disturbed Bilateral Relations: a guide for plasma characterization and global models

The study of the competition between equilibrium disturbing and equilibrium restoring mechanisms, reveals that the various equilibrium departures as found in different plasmas, have much in common. They can be seen as disturbances of bilateral relation; relations effectuated by forward and corresponding backward processes in the sense of detailed balancing. The deviations from the equilibrium form of the atomic state distribution function and the electron energy distribution function of atomic plasmas, which are the result of the escape of photons and electron-ion pairs, can be given in a simple equation in which the escape per balance time plays a leading role. The same idea can be used to construct a characterisation method that relates external control parameters to average values of internal plasma properties such as the electron density, electron temperature and the gas temperature. The global discharge model presented here includes gas heating and is applicable to atomic plasmas for which convection can be neglected.

Theoretical investigation of a photoconductively switched high-voltage spark gap

In this contribution, a photoconductively switched high-voltage spark gap with an emphasis on the switching behavior is modeled. It is known experimentally that not all of the voltage that is present at the input of the spark gap is switched, but rather a fraction of it drops across the spark gap. This voltage drop depends on the voltage that is present at the input of the spark gap with higher voltages resulting in a smaller drop. We have investigated two possible causes of this: the cathode fall and the resistance of the plasma arc. Using an analytical model of the cathode fall, we have established that the cathode fall can be excluded as the cause of the observed voltage drop. A one-dimensional, time-dependent non-local thermal equilibrium fluid model of the arc plasma has been made. Using this model, the plasma properties have been analyzed for various values of the switched current with emphasis on the conductivity. A good qualitative match between the observed and the simulated dissipation in the ga...

Modelling of a pinched cascaded arc light source using argon

In this work, we study a cascaded arc in argon that is used as a broadband light source for spectroscopic applications. In this arc, the arc channel is geometrically constricted. A numerical model is used to investigate the plasma parameters and light output of the arc. It is found that constricting leads to a higher electron density in the constricted area, which strongly enhances the local broadband emission of the plasma. A parameter study, in which the current is varied, is performed. The simulated arc voltages are compared with measured arc voltages, and excellent agreement is found. Furthermore, it is found that the emissivity increases strongly for increasing current, making the current a suitable control parameter to control the light output of the arc.

Creating a global plasma model using disturbed bilateral relations

Disturbed Bilateral Relations offer a way of categorizing plasma processes by their deviation from equilibrium. This method can be used to create a simple plasma model that solves key plasma parameters, namely the electron temperature, electron density and heavy particle temperature. An implementation of this method is presented, and the results validated against a detailed plasma simulation, for a wide range of parameters.

Characterization of a high-pressure hydrogen microwave plasma torch using the method of dBR

A hydrogen plasma flame produced by an axial injection torch powered at the microwave frequency of 2.45 GHz is studied using the method of disturbed Bilateral Relations (dBR). The application of this method which relates the influence of equilibrium disturbing, as produced by transport, to equilibrium restoring processes, reveals that the most dominant excitation balance is the Excitation Saturation Balance. Moreover, a global discharge model leads to an electron density of ne = 4.1020m?3 and an electron temperature of Te = 0.86 eV. The gas temperature was estimated to be Th = 0.3 eV. The values of ne and Te were found to be in good agreement with the value obtained with the modified Boltzmann-plot and the crossing method of Stark broadening. The dBR method points out that the first level in partial local Saha equilibrium will be the level with principal quantum number p = 10 which is in fair agreement with experimental results.

Characterization of highly transient EUV emitting discharges

The method of disturbed Bilateral Relations (dBR) is used to characterize highly transient plasmas that are used for the generation of Extreme Ultra Violet (EUV), i.e. radiation with a wavelength around 13.5 nm. This dBR method relates equilibrium disturbing to equilibrium restoring processes and follows the degree of equilibrium departure from the global down to the elementary plasma-level. The study gives global values of the electron density and electron temperature. Moreover, it gives a method to construct the atomic state distribution function (ASDF). This ASDF, which is responsible for the spectrum generated by the discharge, is found to be far from equilibrium. There are two reasons for this: first, systems with high charge numbers radiate strongly, second the highly transient behaviour makes that the distribution over the various ionization stages lags behind the temperature evolution.