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Abstract-Unique features associated with the hollow cathode recess in a pseudospark discharge have been identified from streak camera observations. A plasma is observed to form in a highly localized region immediately behind the cathode aperture prior to gas breakdown in the main discharge volume. The point-like plasma expands from an initial diameter of below 0.3 to 3 mm, the size of the cathode aperture, in about 20 ns, at which time the main discharge is formed starting from the axis. Numerical modeling based on swarm parameters has been carried out to investigate the plasma formation in the hollow cathode region. Results show that a combination of varying ionization rates due to the field geometry and differing mobilities of electrons and ions leads to the formation of a highly localized space-charge field on axis behind the hollow cathode. This space-charge field in turn brings about the rapid formation of a highly localized, high density of charge carriers behind the cathode aperture. It is suggested that this point-like formation of a plasma source is the phenomenon observed in the streak camera observation of the hollow cathode region.

In recent experiments with the superconducting injector system, beam intensity and beam quality objectives for the superconducting electron accelerator have been achieved and exceeded. At 8.5 MeV and 25 μA, spreads in phase and energy of 1.2° and 9.2 keV fullwidth at half-maximum, respectively, have been measured, and at 250 μA the spreads in the phase and energy increased by only 10%.

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Das wesentlichste Ergebnis der Simulation, das äußerst schnelle Auftreten eines gut fokussierten, hochintensiven Elektronenstrahls auf der Achse in der Nähe des Loches entspricht experimentellen Beobachtungen. Abstract A Physical Model of Prebreakdown in the Ho/low Cathode PsewJospark Discharge Based on Numerical SimulationsThe pseudospark is a gas discharge in a cylindrical geometry with a hole in the cathode and a hollow cathode region behind it. Based an the analysis of numerical simulations a physical model of the prebreakdown phase of the pseudospark is presented.The occurence of a weil focused high intensity electron beam is a result of an enhanced electric field in the hollow cathode caused by a positive space charge. The observed overexponential growth of densities in time an axis close to the hole is caused by a positive feedback mechanism. On one hand, this is based an the fad that the electric field, the ionization coefficient and the drift velocities, and as a consequence also the electron, the positive ion, and hence also the space charge density all have their maximum an axis at the hole. On the other hand, it follows from the continuity equation that the relative change in eiedran density at a given point is the sum of an ionization and of a space charge term. The latter one is the larger the more electrons have drifted by, and it is the cause for the rapid overexponential growth of densities in the ignition phase.Further, in the present work the essentials of, and tests performed an the numerical code are described. These give confidence that large numerical errors or instabilities can be excluded in the simulations, and that indeed the discharge physics is modelled properly, and that the results describe the physics qualitatively correct.The main feature of these simulations, the extremely rapid appearance of a weil focused, high intensity eiedran beam an axis, originating at the hole, is also seen in experiments. Contents