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.