Biomedical subjects

W J Goedheer

Publications and source records attributed to W J Goedheer.

8 recordsLinked to original sources

PubMed2006

Particle-in-cell Monte Carlo simulations of an extreme ultraviolet radiation driven plasma.

A self-consistent kinetic particle-in-cell model has been developed to describe a radiation driven plasma. Collisions between charged species and the neutral background are represented statistically by Monte Carlo collisions. The weakly ionized plasma is formed when extreme ultraviolet radiation coming from a pulsed discharge photoionizes a low pressure argon gas. The presence of a plasma close to optical components is potentially dangerous in case the ions that are accelerated in the plasma sheath gain enough energy to sputter the optics. The simulations predict the plasma parameters and notably the energy at which ions impact on the plasma boundaries. Finally, sputter rates are estimated on the basis of two sputtering models.

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PubMed2005

Dust transport in a magnetized radio-frequency discharge under microgravity conditions.

Dust is found in plasmas used in industrial applications, such as microelectronics and solar cell manufacturing, in fusion plasmas, where it is usually the result of plasma-wall interactions, and in plasmas in space, such as planetary atmospheres, cometary tails, planetary rings, interstellar molecular clouds, and star and planet formation regions. In plasma applications, magnetic fields are occasionally used, mainly to confine the plasma. In space, however, magnetic fields are very often present and they may strongly influence the behavior of dusty plasma, for instance in the formation of stars and planets. We extended a fully self-consistent two-dimensional fluid model for radio-frequency discharges by adding a homogeneous axial magnetic field and the effect it has on the transport of plasma species in a low-temperature dusty discharge. We show that the magnetic field has an important effect on the (ambipolar) diffusion of ions and electrons in the bulk of the discharge. This causes an important change in the force balance of the dust particles and in the time scales of the formation of a dust-free void. Finally, we compare the parameters of the modeled discharge with the parameters of a planet formation region around a young stellar object (YSO). We conclude that a magnetic field in both low-temperature rf discharges under micro-gravity conditions and dusty plasmas around YSO's has an important effect on the transport of dust and must be important for the formation of planets and stars.

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PubMed2003

Vortices in dust clouds under microgravity: A simple explanation.

Clouds of dust particles in radio frequency discharges often show a periodic vortexlike motion, especially near the edges of the electrodes or near the tip of an electrostatic probe. These vortices often last as long as the discharge is powered. In a previous paper we have followed a small number of individual dust particles in a discharge under microgravity conditions, moving under the influence of forces computed by means of a self-consistent two-dimensional hydrodynamic model, and interacting via a screened Coulomb potential. The resulting motion showed the vortexlike rotation. In this paper we discuss this phenomenon in more detail, using a simplified model with harmonic forces, but extending the simulations to three dimensions. Stable vortices are observed, which show a more chaotic behavior than in the two-dimensional situation. Particles frequently jump up and down between two counterrotating vortices. The generation of the vortices can be ascribed to a nonzero rotation of the net global force vector field, which is the sum of the ion drag force, the electric force, and the thermophoretic force in case of the experiments. Comparison of experimental data with simulations using a model potential may open a way to unravel the forces inside a cloud of dust particles.

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PubMed2003

Modeling the effect of dust on the plasma parameters in a dusty argon discharge under microgravity.

A dusty radio-frequency argon discharge is simulated with the use of a two-dimensional fluid model. In the model, discharge quantities, such as the fluxes, densities, and electric field are calculated self-consistently. The charge and density of the dust are calculated with an iterative method. During the transport of the dust, its charge is kept constant in time. The dust influences the electric potential distribution through its charge and the density of the plasma through recombination of positive ions and electrons on its surface. Results are presented for situations in which the dust significantly changes the discharge characteristics, both by a strong reduction of the electron density and by altering the electric potential by its charge. Simulations for dust particles having a radius of 7.5 microm show that a double space charge layer is created around the sharp boundary of the dust crystal. A central dust-free region (void) is created by the ion drag force. Inside this void a strong increase of the production of argon metastables is found. This phenomenon is in agreement with experimental observations, where an enhanced light emission is seen inside the void.

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PubMed2003

Modeling of self-excited dust vortices in complex plasmas under microgravity.

A two-dimensional hydrodynamic model for a dusty argon plasma in which the plasma and dust parameters are solved self-consistently has been supplemented with a separate dust particle tracing module to study the behavior of dust vortices. These coherent vortices appear in plasma crystal experiments performed under microgravity conditions. The nonconservative total force exerted by the discharge on the dust particles is responsible for the generation of the vortices. The contribution of the thermophoretic force driven by the gas temperature gradient plays an insignificant role in the generation of the vortices, even when the gas heating via the dust particles is taken into account. The forces related to the electric field, including the ion drag force, are dominant.

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PubMed2001

Local and fast relaxation phenomena after laser-induced photodetachment in a strongly electronegative rf discharge.

A one-dimensional self-consistent particle in cell/Monte Carlo method is used to study the local (at the laser impact region) and fast relaxation phenomena after laser-induced photodetachment in a strongly electronegative SiH(4)/H(2) rf discharge. The relaxation process of the local densities of the charged plasma species has been studied in association with the time evolution of the local electric field. The phenomena predicted theoretically about the relaxation processes, such as the potential well, the electrostatic oscillation, the long lasting potential structure, the distortion of the early potential perturbation on the measurement of negative ion temperature, and the depression in the positive ion density profile at the edges of the laser impact region, have been confirmed by our simulation results. Compared to the relaxation in weakly electronegative discharges, the local and even the global electric field in strongly electronegative discharges, has been weakened strongly after photodetachment. The relaxation of the local electric field lasts 100 rf cycles with the recovery of the local electron density and the local electron energy. The electrostatic oscillation exhibited as the deviation in quasineutrality, is very strong and continues over several rf cycles in our case. The large dip in the center of the positive ion density profile, observed in the experiment, is also reproduced by our model.

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PubMed2001

Modeling of voids in colloidal plasmas.

A two-dimensional fluid model for a dusty argon plasma in which the plasma and dust parameters are solved self-consistently, is used to study the behavior of voids, i.e., dust-free regions inside dust clouds. These voids appear in plasma crystal experiments performed under microgravity conditions. The ion drag force turns out to be the most promising driving force behind these voids. The contribution of the thermophoretic force, driven by the temperature gradient induced by gas heating from ion-neutral collisions, can be neglected in the quasineutral center of the plasma.

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PubMed2001

Kinetic modeling of relaxation phenomena after photodetachment in a rf electronegative SiH4 discharge.

The global relaxation process after pulsed laser induced photodetachment in a rf electronegative SiH4 discharge is studied by a self-consistent kinetic one-dimensional particle-in-cell-Monte Carlo model. Our results reveal a comprehensive physical picture of the relaxation process, including the main plasma variables, after a perturbation up to the full recovery of the steady state. A strong influence of the photodetachment on the discharge is found, which results from an increase of the electron density, leading to a weaker bulk field, and hence to a drop in the high energy tail of the electron energy distribution function (EEDF), a reduction of the reaction rates of electron impact attachment and ionization, and a subsequent decrease of the positive and negative ion densities. All the plasma quantities related to electrons recover synchronously. The recovery time of the ion densities is about 1-2 orders of magnitude longer than that of the electrons due to different recovery mechanisms. The modeled behavior of all the charged particles agrees very well with experimental results from the literature. In addition, our work clarifies some unclear processes assumed in the literature, such as the relaxation of the EEDF, the evolution of the electric field, and the recovery of negative ions.

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