Wladimir An

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...

Characterization of high-current electron beam interaction with metal targets

The process of electron beam interaction with metal targets was characterized using electrical and optical diagnostics. Electron beams with current density of 5–10 A/cm2, electron energy up to 120 keV, pulse duration up to 200 μs, and cross-sectional area of 8–30 cm2 at the target surface were generated by GESA I and GESA II facilities. Streak imaging of the target surface specular reflectivity was used to determine the onset of melting and re-solidification of the target surface. Using time- and space-resolved schlieren imaging, the evolution of surface irregularities was studied. Experimental and numerical investigations of the neutral flow evaporated from the target surface showed a neutral density of ∼1019 cm−3 in the vicinity of the target and neutral velocities up to 2 × 105 cm/s. Framing and streak images of visible light emission were used to study the temporal evolution of the target surface plasma and vapors. Time- and space-resolved spectroscopy was applied to determine the surface plasma densi...

Surface Layer Dynamics During E-Beam Treatment

The interaction of a pulsed intense electron beam with a metal target leads to rapid heating and subsequent cooling of the surface layer, accompanied by a series of phase transitions among the solid, liquid, vapor, and plasma phase. As a consequence of the treatment, depending on the beam parameters, the metal target is eroded and a topographical pattern (waviness, craters, etc.,) evolves on its surface. Surface roughening, a major drawback of pulsed intense electron beam treatment, is well known but lacks comprehensive understanding. In this paper, the process of pulsed intense electron beam interaction with metal targets is studied with special attention to the dynamics of the target surface layer and the development of surface roughness. The pulsed electron beam facility GESA generates electron beams with power density 0.5-2 MW/cm2, electron energy up to 120 keV, and pulse duration up to 200 μs. Different fast in situ diagnostics are applied to study the various processes occurring at the target surface: 1) melting and resolidification are visualized by time and space resolved imaging of the surface specular reflectivity; 2) spectroscopy is used to characterize the plasma phase adjacent to the target surface; 3) the evolution of irregularities and bubbles at the surface is studied by high-resolution microscopy; and 4) a stroboscopic imaging technique is applied to catch the evolution of the surface topography. The experimental data are compared with numerical simulations of heat transfer. All results and processes involved in pulsed intense electron beam treatment are discussed with respect to the target surface layer dynamics.

Numerical simulation of the plasma generated by the interaction high- current electron beam with Al target

The results of one dimensional particle-in-cell simulations of the dynamics of plasma generated during the interaction of a high-energy (≤200 keV) and high-current (≤15 A/cm2) electron beam with an aluminum target are presented. The generated target plasma is low-ionized and characterized by non-Maxwellian electron energy distribution. The density and electron temperature of the plasma, which expands toward the anode at a typical velocity of ∼105 cm/s, does not exceed 4 × 1014 cm−3 and 1 eV, respectively, which is in satisfactory agreement with the experimental results presented in W. An et al., J. Appl. Phys. 110, 093304 (2011). The results of the simulations showed also acceleration of the ions from the target plasma toward the anode by the potential of the non-compensated space charge of the electron beam. The typical velocity of these energetic ions is ∼108 cm/s and depends on the electron current density and energy. These ions partially compensate the space charge of the electron beam, which leads to...

Investigation of Pulsed Corona Discharges in Water by Fast Imaging Diagnostics

Summary form only given. Pulsed underwater corona discharges are considered to be an effective method for killing bacteria, yeasts, and other microorganisms. Nevertheless, many of the physical phenomena causal to the initiation and propagation of the streamers and to the production of oxidants are not well understood yet. This is why we are investigating various properties of positive streamers in point-plate geometry by fast imaging techniques. Two high-resolution intensified CCD-cameras with minimum gate times of 3 ns are used to record light emission, shadowgraph, and schlieren pictures, and Mach-Zehnder interferograms. By imaging streamers using an intermediate 300 L/mm reflective grating we were able to take temporally and spatially resolved emission spectra covering the 280 nm to 800 nm wavelength range. Limiting the voltage to just above the inception level delays the formation of secondary streamers (SS) and allows to observe the development of primary streamers (PS). These bush-like structures start from a single root-point and consecutively branch to ~100 individual tips spanning perfect hemispheres with diameters reaching up to 1 mm. The propagation velocity is about 2-3 km/s. Compared with the ohmic current no clear current signal could be assigned to the PS. Light emission is weak compared with SS and limited to a few main branches. The continuum-like emission spectrum could be due to extreme broadening of hydrogen Lyman lines. Intense light emission by the SS starts at one of the tips at the periphery of the PS. A single luminous channel advances in the direction of the root-point whereas a bush-like structure of ~10 branches develops into the hemisphere towards the cathode at 30-40 km/s. Thereof, only 2-3 persist for several microseconds until the end of the discharge. Light emission lasts a few 10 ns per branch switching alternately between the branches at repetition rates of 5-100 MHz. The light bursts can be assigned to current peaks of 1-2 A. The diameter of the active branches increases for about 100 mus up to 0.5 mm. Then the channels collapse. The initial radial expansion rate is 300 m/s suggesting driving pressures in the channel of ~5 kbar. Schlieren images reveal many faint, short-lived (100 ns) ~1 mm-long twigs growing out of the branches. From interferograms, pressure pulse durations of 20-30 ns, peak pressures of a few kbar, and channel diameters 1019 cm-3 at the propagating tips of the SS. As the channel expands, densities drop to ~1015 cm-3, the luminous channel remaining limited to a fairly narrow central zone. Hydroxyl emission in the 309 nm-band shows ~300 ns.

Numerical investigations of radially converging electron beam generated in cylindrical GESA IV facility

The cylindrical triode-type electron accelerator GESA IV was developed for treatment of metallic rods, specifically cladding tubes for nuclear reactors. The target (anode) diameter is therefore fixed at about 10 mm by the application, which leads to problems of homogeneity and stability of the radially converging beam. Due to the large difference between cathode diameter (about 150 mm) and anode diameter, a virtual cathode may form between grid and anode, electrons may miss the target and start to circulate around the anode, and the self-induced magnetic field may lead to large distortion of the electron trajectories. In this study, we investigate the influence of various crucial effects on the beam performance by PIC code simulations using the software package MAGIC. In particular, we consider monopolar and bipolar flow (i.e., the influence of ions generated at the target and moving towards the cathode), the effects of scattering at the grid and of backscattering at the target, the angular velocity spread of the electrons at emission, and the influence of the grid potential. The numerical results are compared with experiments performed at the GESA IV facility, where the influence of the target material and of the self-induced magnetic field on the beam performance are investigated.

Cathode plasma as electron source in long pulse accelerator gesa

Summary form only given. The use of plasma as an emission source has many advantages such as a rather simple implementation and very high emissivity and reproducibility. Therefore, plasma emission sources have established themselves as an essential part of modern electron accelerators. One drawback is that each application requires a specific control of the emission source, which represents a challenge. Of special concern are accelerators with pulse duration similar to or longer than the characteristic timescale of the plasma dynamics. In such cases, plasma motion into the accelerating gap crucially influences the beam quality and operation stability. At Karlsruhe Institute of Technology, a new pulsed electron beam facility was designed and constructed, which is specially equipped to investigate the cathode plasma performance. Proven diagnostic tools such as fast framing imaging, X-ray diagnostics, and Langmuir probes were installed to investigate the plasma dynamics and its influence on the beam characteristics. The new results on the axial and azimuthal plasma motion and its correlation with the beam performance provide new material for discussion of well-known but hitherto unresolved phenomena such as the impedance collapse and the precession of the electron beam.

Optical investigations of cathode plasma dynamics of long pulse electron accelerator gesa

Summary form only given. A specific problem of long pulse (several tens of μs) electron accelerators is the uncontrolled decrease of the machine impedance, caused by filling of the cathode-grid space with emission plasma. Because the intensity of cathode plasma generation and the decrease of the impedance are related by a positive feedback, soon an unstable operation of the accelerator is reached, resulting in beam breakdown. Our study concerns the search for an optimum operation regime of the accelerator. For this task, comprehensive optical diagnostics of the plasma dynamics was performed, combined with X-ray analysis of the beam profile. The characteristics of the plasma generation process was observed, as function of the electron flow backscattered at the target, of the magnetic field configuration, and of the potential distribution between the electrodes. As possible solutions, a change of the electrode geometry and limitation of the total current were tested.Optimization of the accelerator geometry by means of MAGIC3D PIC code simulations resulted in a new machine called GESA-SOPHIE.

Investigation of radially converging electron beams generated by GESA IV

Summary form only given. The pulsed electron beam facility GESA IV generates radially converging beams by means of a cylindrically shaped explosive emission cathode. Along the cylinder axis a rod-shaped target serves as anode of the triode system. In this study, the generation, homogeneity, dynamics, and stability of electron beams in GESA IV are investigated numerically using MAGIC-3D. Thereby, the reflection of electrons at the target is taken into account as well as the magnetic field induced by the current along the target. Various geometrical details of the GESA IV facility are modified to find the optimum parameters. An apparent anode is introduced with diameter exceeding the dimensions of the target. Also, the effect of lamellae placed radially around the target is studied. Although the target current could be enhanced considerably by the presence of the lamellae, the divergence of the beam along the symmetry axis could not be avoided. The numerical findings are discussed and compared with experimental measurements conducted at the GESA IV facility.

Measurement and simulations of electron beam precession in GESA 1 and GESA 2 facilities

Transportation of intense electron beams in vacuum by an applied magnetic field leads to precession of the beam around the axis of the transport channel with a frequency of about 1 MHz. The frequency, amplitude and onset time of the beam precession were measured by e.g. x-ray diagnostic showing a dependency on magnitude and distribution of the applied magnetic field and on target material. In this paper we present results of further investigations to clarify the reasons for appearance of beam precession. A systematic experimental study of beam precession parameters as function of magnetic field configuration is presented. The precession parameters were estimated by the similar x-ray diagnostic described in [1]. The analyses of the experimental results were supported by simulations using the PIC-code ESRAY. The results imply that a prerequisite for beam precession is a spatial pulsation of the beam profile, resulting from deviation of the real magnetic field from Brillouin-conditions and is supposed to be triggered or even maintained by the dynamic of the cathode plasma (movement towards the anode), leading to continuously alteration of the beam perveance.