Magnetic focusing of a relativistic electron beam: experiment

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Abstract
A magnetic mirror is used to control and focus a 2.0‐MeV 70‐kA electron beam. Mirror ratios of 3.7 and 5.9 and magnetic fields of 2.0, 2.5, and 5.0 kG are used. A 500‐μ air pressure is used in the drift tube to minimize the beam's electrostatic and magnetic self‐forces. The surface‐energy deposition in a sample placed at the peak of the 3.7 to 1 bottle is approximately twice as large as that obtained with beam self‐focusing without an external field. The fluence and energy as a function of axial position in the region of low‐grad Bz agree with the results of adiabatic theory and are found to be independent of magnetic field strength and nearly independent of mirror ratio for the selected ranges. Reproducibility in delivered energy, fluence, and surface‐energy deposition is significantly greater than that obtained without an external magnetic field. The induced Bz field due to beam rotation or plasma diamagnetism is approximately 1% of the applied field.