Optical investigation of plasma formation process by interaction of intense electron beams with metallic targets

Abstract

Summary form only given. Intense pulsed electron beams are investigated for surface modifications since several years. Different applications like surface hardening or improvement of corrosion resistance are investigated. However, physical processes of beam target interaction and plasma formation are not yet fully understood. These effects might have influence on the functional parameters of such an electron accelerator. The target plasma as an ion source might change the electron beam parameters. Especially for longer pulses in the 10th of microsecond range an intensification of this feedback is expected. To investigate the influence of the plasma formation process on the most important parameters of the accelerator like course of impedance, beam profile and beam stability an optical diagnostic system with high time and spatial resolution was developed. The system consists of a spectrometer combined with a framing and streaming camera. The target plasma has been analyzed with a 0.5m polychromator with fiber-optically couples array of 6 photo- multiple. This allows the simultaneous detection of different spectral lines emitted from the target plasma. The moment of plasma formation is under normal target conditions, without excessive adsorbents at the surface, about 20mus after the onset of the electron beam. The timely evolution of the carbon lines indicates a non thermal plasma formation mechanism. The start of plasma formation or evolution of characteristic emission lines depends on the target material. The somewhat earlier visibility in case of Ta target is related to the release of hydrogen stored in the bulk. Until the formation of the plasma ions are mainly produced by direct ionization of gas molecules. The ignition of the plasma above the target in a later stage is a result of ionization by secondary electrons.