Alejandro Clausse

A Lattice-Boltzmann solver for 3D fluid simulation on GPU

Abstract A three-dimensional Lattice-Boltzmann fluid model with nineteen discrete velocities was implemented using NVIDIA Graphic Processing Unit (GPU) programing language “Compute Unified Device Architecture” (CUDA). Previous LBM GPU implementations required two steps to maximize memory bandwidth due to memory access restrictions of earlier versions of CUDA toolkit and hardware capabilities. In this work, a new approach based on single-step algorithm with a reversed collision–propagation scheme is developed to maximize GPU memory bandwidth, taking advantage of the newer versions of CUDA programming model and newer NVIDIA Graphic Cards. The code was tested on the numerical calculation of lid driven cubic cavity flow at Reynolds number 100 and 1000 showing great precision and stability. Simulations running on low cost GPU cards can calculate 400 cell updates per second with more than 65% hardware bandwidth.

Demonstration of neutron production in a table-top pinch plasma focus device operating at only tens of joules

Neutron emission from a deuterium plasma pinch generated in a very small plasma focus (6 mm anode diameter) operating at only tens of joules is presented. A maximum current of 50 kA is achieved 140 ns after the beginning of the discharge, when the device is charged at 50 J (160 nF capacitor bank, 38 nH, 20–30 kV, 32–72 J). Although the stored energy is very low, the estimated energy density in the plasma and the energy per particle in the plasma are of the same order as in higher energy devices. The dependence of the neutron yield on the filling pressure of deuterium was obtained for discharges with 50 and 67 J stored in the capacitor bank. Neutrons were measured by means of a system based on a 3He proportional counter in current mode. The average neutron yield for 50 J discharges at 6 mbar was (1.2 ± 0.5) × 104 neutrons per shot, and (3.6 ± 1.6) × 104 for 67 J discharges at 9 mbar. The maximum energy of the neutrons was (2.7 ± 1.8) MeV. Possible applications related to substance detection and others are discussed.

Conceptual engineering of plasma-focus thermonuclear pulsors

The basic engineering criteria for conceptual design of plasma focus devices is derived from a thermonuclear model, and applied successfully to the operation of small neutron pulsors. The theory is able to explain the variation of the neutron yield with the gas pressure in deuterium-filled chambers, the current evolution, and the electrode geometrical parameters. The performance of a prototype designed to optimize the flux/fluence ratio is presented, contrasting the experimental outcomes with the model. A set of effective design parameters is deduced, which ensure a band confidence of 20%.

Experimental study of the hard x-ray emissions in a plasma focus of hundreds of Joules

An experimental study on hard x-ray production in a small plasma focus device operating in a few hundreds of Joule range is presented. A threshold in the voltage drop on the pinch was observed for x-ray emission. A comparison with Dreicer theory for electrons runaway in plasmas yields significant agreement. The study was performed at a constant pressure (1.8 mbar) of deuterium with three different anode lengths.

A plasma focus driven by a capacitor bank of tens of joules

As a first step in the design of a repetitive pulsed neutron generator, a very small plasma-focus device has been designed and constructed. The system operates at low energy (160 nF capacitor bank, 65 nH, 20–40 kV, and ∼32–128 J). The design of the electrode was assisted by a computer model of Mather plasma focus. A single-frame image converter camera (5 ns exposure) was used to obtain plasma images in the visible range. The umbrellalike current sheath running over the end of the coaxial electrodes and the pinch after the radial collapse can be clearly observed in the photographs. The observations are similar to the results obtained with devices operating at energies several orders of magnitude higher. The calculations indicate that yields of 104–105 neutrons per shot are expected with discharges in deuterium.

A lumped parameter model of plasma focus

In this paper, a fast running computer model of a plasma focus device is presented. The model is based on the snowplow model constructed with effective parameters validated against experimental results. A pinch model is included to calculate the temporal evolution of the focal variables. The resulting neutron yield predictions are compared against available data at different pressures, electrode length, and capacitor voltages from experimental measurements, finding good agreements. The model ultimately calculates the neutron production given the geometric parameters and the filling deuterium pressure.

Plasma focus as a powerful hard X-ray source for ultrafast imaging of moving metallic objects

Using high energy, pulsed X-rays, generated by a 4.7 kJ Plasma Focus operated with deuterium - argon admixtures, images of fast rotating metallic pieces were obtained. This illustrates the potential application of Plasma Focus devices for non invasive scanning of moving metallic objects. The samples for radiographic imaging were located outside the plasma focus chamber. High-sensitivity, fast-response commercial radiographic film was used as X-ray detector. The use of small doses of argon helps to enhance both the neutron production and the hard X-ray emission. By analyzing the mean attenuation of the studied radiation in different metals, an X-ray effective energy of about 100 keV was obtained.

A combined region growing and deformable model method for extraction of closed surfaces in 3D CT and MRI scans

Image segmentation of 3D medical images is a challenging problem with several still not totally solved practical issues, such as noise interference, variable object structures and image artifacts. This paper describes a hybrid 3D image segmentation method which combines region growing and deformable models to obtain accurate and topologically preserving surface structures of anatomical objects of interest. The proposed strategy starts by determining a rough but robust approximation of the objects using a region-growing algorithm. Then, the closed surface mesh that encloses the region is constructed and used as the initial geometry of a deformable model for the final refinement. This integrated strategy provides an alternative solution to one of the flaws of traditional deformable models, achieving good refinements of internal surfaces in few steps. Experimental segmentation results of complex anatomical structures on both simulated and real data from MRI scans are presented, and the method is assessed by comparing with standard reference segmentations of head MRI. The evaluation was mainly based on the average overlap measure, which was tested on the segmentation of white matter, corresponding to a simulated brain data set, showing excellent performance exceeding 90% accuracy. In addition, the algorithm was applied to the detection of anatomical head structures on two real MRI and one CT data set. The final reconstructions resulting from the deformable models produce high quality meshes suitable for 3D visualization and further numerical analysis. The obtained results show that the approach achieves high quality segmentations with low computational complexity.

Plasma-focus-based tabletop hard x-ray source for 50ns resolution introspective imaging of metallic objects through metallic walls

A tabletop 4.7kJ, 30kV plasma focus device was used as a pulsed hard x-ray source for fast radiography (50ns exposure time) of metallic pieces even through several millimeter thick metallic walls. An experimental estimation of the effective average energy of the x-ray beam (found to be around 100keV) and a numerical estimation of the induced voltage on the focus during the compressional stage of a plasma focus are briefly discussed.

Characterization of the axial plasma shock in a table top plasma focus after the pinch and its possible application to testing materials for fusion reactors

The characterization of plasma bursts produced after the pinch phase in a plasma focus of hundreds of joules, using pulsed optical refractive techniques, is presented. A pulsed Nd-YAG laser at 532 nm and 8 ns FWHM pulse duration was used to obtain Schlieren images at different times of the plasma dynamics. The energy, interaction time with a target, and power flux of the plasma burst were assessed, providing useful information for the application of plasma focus devices for studying the effects of fusion-relevant pulses on material targets. In particular, it was found that damage factors on targets of the order of 104 (W/cm2)s1/2 can be obtained with a small plasma focus operating at hundred joules.