Direct Measurements of Electron Beams in Symmetric and Asymmetric X pinches

Summary form only given. It is well known that X pinches produce very small, dense plasma micropinches that emit short bursts of 1.5-8 keV radiation. Immediately after the softer X-ray burst, higher energy emission in the range 8-100 keV is seen and attributed to energetic electrons accelerated in the gaps that are visible in the X pinch structure in the images recorded using X pinch point projection radiography. Electron beam parameters in the X pinches (current, average energy, and divergence) were not previously measured. Such measurements on symmetric X pinches are compromised by strong absorption and electron scattering that occurs between the X pinch crossing point and the anode. Cold, relatively high density plasma fills this space as a result of the ablation of the wire cores. An asymmetric X pinch scheme that decreases plasma density and size along the path of propagation of the electron beam has been tested. In this configuration, the anode side of the X pinch was made 4-7 times shorter than the cathode side and the angle between wires was almost 180 deg. Electron beam parameters were determined using two low impedance coaxial Faraday cups (with the filter of the inner cup working as the collector for the outer one). The filters on these cups enabled simultaneous measurements of the beam current in two energv bands. For two-wire. 17 mum, asymmetric, tungsten X pinches the electron current reached 5-10 kA with a pulse duration of 3-5 ns and a peak electron energy of more than 80 keV. These electrons were emitted in a solid angle of about 10^-2 steradians. In symmetrically configured X pinches the measured electron beam current is much less and has much higher divergence. The asymmetric configuration may also be useful for ion beam studies by reversing the pulse polarity.