H. Bluhm

Underwater streamer propagation analyzed from detailed measurements of pressure release

In this paper we describe experimental observations connected with the propagation of primary and secondary streamers in water. Using a Mach-Zehnder interferometer we determined the pressure field surrounding the streamer channel at a given instant in time with high temporal and spatial resolution. This pressure field contains information on the time evolution of the pressure pulse inside the discharge channel. The pressure history in the channel has been reconstructed by comparing the experimentally obtained fringe shifts in the interferograms with those derived from one-dimensional hydrodynamic calculations in cylindrical geometry. Assuming different trial pressure pulses, it has been possible to establish the channel pressure iteratively. A reproduction of the experimental data from secondary streamers requires short (2–3ns) pressure pulses with amplitudes of 2–3GPa. These findings are inconsistent with the assumption of bubble-initiated propagation of secondary streamers. It has also been inferred fro...

Pulsed electric field treatment for bacteria reduction and its impact on hospital wastewater.

During the last years the pulsed electric field (PEF) method entered several fields of application. A promising application is the decontamination of hospital wastewater effluents, which are loaded with pathogenic and increasingly with antibiotic-resistant bacteria. For this study, Pseudomonas putida suspended in buffer solution or wastewater from university hospital was used as reference strain. To prove whether the descendent of the survival bacteria develop an adaptation to electric field, surviving PEF treated bacteria were recultivated and pulsed in serial experiments with 10 pulses (100kVcm(-1) and 600ns pulse duration). This procedure was repeated for 30 generations. The inactivation rate was calculated with 3.5+/-0.8 log of colony forming units and remained constant over 30 cycles. Investigations of the variable intergenic spacer region of the ribosomal operon demonstrated no visible changes in this highly variable part of the genome structure during the serial PEF treatment experiments. The mutagenicity of PEF treated hospital wastewater, buffer solutions and tap water was analyzed by the umu-test. Most hospital wastewater samples exhibit a considerable genotoxicity already before PEF treatment, but this was not increased by the PEF treatment, not even for higher treatments energies over 250JmL(-1). No genotoxicity was induced in buffer solutions and tap water by PEF treatment. This study supports, that PEF treatment is a sustainable non-chemical method for bacterial decontamination without any adverse effects.

Triggered Marx generators for the industrial-scale electroporation of sugar beets

Electroporation is a method to induce pores in the membranes of biological cells by applying an electric field. During the last few years, some research and development has been done for sugar beets to transfer this method from the laboratory into the industrial scale in order to replace the thermal denaturation process. High-efficiency pulsed electric fields with short pulse durations are applied to the beets as a whole. This paper deals with some aspects concerning the electrode geometry and a new trigger concept for the Marx generators used to create the required electric fields.

Processing of sugar beets with pulsed-electric fields

The treatment of biological cells with strong pulsed-electric fields can lead to irreversible formation of large pores in the cell membrane and thus destroy the cell and give access to its content. This well-known process of electroporation has been successfully applied to the inactivation of bacteria in many laboratories. However, few efforts have been made to utilize the technique on a large industrial scale for the production of nourishment from food plants. We have built the mobile test device Karlsruher Elektroporations Anlage (KEA), which consists of a 300-kV Marx generator operating at 10 Hz and delivering its pulses to a cylindrical reaction chamber with axially and azimuthally distributed electrodes. The reaction chamber has a large cross section, sufficient for the treatment of entire sugar beets in a continuous stream. KEA has been used in an experimental campaign to demonstrate the advantages of electric pulse treatment for the production of sugar from beets compared with conventional techniques. Although the process has not yet been optimized, it was found that appreciable energy savings are possible since the treated beets could be extracted at much lower temperatures with the same result. To demonstrate the technical and economic feasibility on a large scale, we plan to build a pilot plant with a throughput of several tens of tons per hour and to use it in the next seasonal campaign. Although the results are convincing, important details of the effect are not yet understood. In particular, the interaction between the cell membrane and the cell wall in the plant organism under the action of the electric field needs further investigation. Therefore, we also plan to establish a basic research program.

The Potential of Pulsed Underwater Streamer Discharges as a Disinfection Technique

In this paper, we investigate the effectiveness of pulsed underwater streamer discharges for water disinfection and its scalability to large throughputs. For this paper, we have built a coaxial streamer reactor containing a central cylindrical anode covered by a thin porous layer of almandine. The reactor was supplied by electric pulses with amplitudes of up to 100 kV and pulse durations between 200 and 400 ns at a repetition rate of up to 20 Hz. Different chemical probes have been used as the main diagnostic to determine the production rates and the accumulation of oxidants in the bulk water. It has been found that the number and the length of streamers scaled linearly with the applied voltage amplitude, whereas the pulse duration only affected the streamer length. All oxidant production rates scaled linearly with the number-length product of streamers. OH-radical concentrations of up to 100 mM appear at the streamer-water-boundary. However, because of the limited volume of streamers and the limited range of OH radicals, the most important oxidant with respect to decontamination is H2O2 which accumulates in the water and, by interacting with the intense UV radiation and the shock waves from the streamers, produces OH radicals in the bulk water. Placing water-filled cuvettes from different materials in the reactor, we found that the contribution of shock waves to the production of oxidants in the bulk water seems more important than the contribution of UV radiation. The production of oxidants can be enhanced by percolating suitable gases through the active zone of the reactor. Most effective is the use of oxygen. About 20 J/cm3 of electrical energy was required to reduce the concentration of Pseudomonas putida bacteria by six orders of magnitude. However, up to 180 J/cm3 of specific energy input was necessary to reduce the bacterial freight in waste water from a municipal sewage plant by just two orders of magnitude. This has been explained by the high conductivity of waste water which drastically reduces the length of streamers for the same electrical parameters. Therefore, it seems unlikely that underwater streamer discharges alone can become competitive with other advanced oxidation processes like ozonation, UV irradiation, or irradiation with megaelectronvolt electron beams. However, it seems conceivable that a combination of underwater streamer discharges with ozonation or other processes can lead to a more effective and economic decontamination technique.

Properties of the nonequilibrium plasma from a pulsed sliding discharge in a hydrogen gas layer desorbed from a metal hydride film

The properties of a proton source developed for use in a pulsed high power magnetically insulated ion diode have been determined. The source is created from a sliding gas discharge on the surface of a thin double layer of TiH and Pd deposited on an insulating substrate. By driving a short (<20 ns) high current pulse through the metal films hydrogen is released from the Ti store and a multichannel electrical breakdown is created in the desorbed gas layer. The uniformity of this breakdown depends on the capacitance per area of the multilayer setup. It has been found that the breakdown always occurs after the same areal gas density has been released. The density as well as the temperature of the plasma depend on a continuous influx of hydrogen from the reservoir and on the expansion. The electron temperature decreased from up to 12 early in the pulse to less than 3 eV late in the pulse. Over a distance of 1 mm the plasma density falls from 1017 at the surface to 1015 cm−3. The plasma expansion is stopped by ...

Electroporation-Assisted Dewatering as an Alternative Method for Drying Plants

During the last few years, electroporation has been introduced to the food-processing industry as an effective method to open cell membranes for the extraction of substances. Currently, it has been investigated whether electroporation is also useful as a part of an energy-efficient drying process for plant material for the production of biofuel. To omit the use of additional water, the material has been placed in contact with the electrodes by means of its own juice after a first pressing step. This paper describes a laboratory-scale parameter study on the electroporation-assisted drying of whole maize plants, grass, and lucerne. The influence of the applied electric-field strength and the number of applied pulses on the drying curves is presented. For the electroporated material, an increased yield of juice during a pressing step after the electroporation and a faster drying have been observed.

Influence of electrons reflected from a target on the operation of triode-type electron sources

An external magnetic field between the electron source and a target causes the reflected electrons to move along the magnetic force lines towards the source region. There they get reflected back to the target by the electric field of the source. These electrons distort the electric field in the source region, and change the source’s operational parameters. Penetration of reflected electrons back into the source region reduces essentially the space charge limited emission current density of a cathode and the minimum current density to form a potential minimum and a virtual cathode. How much the emission current density decreases depends on the target material’s atomic number, Z, which determines the reflection coefficient and the energy spectrum of the reflected electrons. The calculations are in good agreement with experimental data. The analysis shows that reflected electrons must be considered for a correct calculation of the beam parameters at the target for the distribution of the energy density depos...

High‐power proton beam‐matter interaction diagnostics by analysis of the hydrodynamic response of solid targets

The hydrodynamic response of thin planar targets to the ablative pressure pulse induced by a high‐power proton beam has been investigated experimentally at the Karlsruhe Light Ion Facility using time‐resolved laser‐Doppler velocimetry. An analytical acoustic model was established which allows a semiquantitative interpretation of the phenomena observed. Details in the measured ablation pressure history could be explained by the particularities of the proton beam. The evolution in time of the depth of the energy deposition zone deduced from our experiments is in good agreement with the proton energy and stopping range increase during the voltage rise of the generator pulse.

Formation of a homogeneous hydrogen plasma layer for the production of terawatt ion beams

A suitable anode plasma source layer for a high power ion-diode must provide an ion current density of up to 10 kA/cm/sup 2/ for a pulse duration of 50 ns, a minimum of 3*10/sup 15/ ions/cm/sup 2/ is needed. This layer should uniformly cover an area of more than 100 cm/sup 2/ and smoothly conform to the desired anode shape. A proton source layer meeting these requirements is described. It is based on a thin metal-hydride film. A 500-AA-thick Ti layer is used as the hydrogen store. It is covered with a 100-300-AA thick Pd layer to prevent oxidation of the Ti surface. A fast heating pulse desorbs a large fraction of the hydrogen within a few ns from the metal hydride. After a sufficient amount of gas has been released, an electrical discharge is initiated in the gas layer to create the plasma. The dynamics of hydrogen desorption and of plasma layer formation are described. Experimental investigations are conducted to determine the characteristics of the discharge and to measure the plasma parameters. Since the plasma is far from thermodynamic equilibrium, spectroscopic methods with high spatial and temporal resolution are applied. >