Current density monitor for intense relativistic electron beams

We describe a new type of electric probe which is capable of measuring the time‐resolved current density profile of a stable, reproducible, high‐energy (>4‐MeV) high‐current (>1‐kA) electron beam. The sensing element of this probe is an open‐ended but capped‐off 50‐Ω coaxial line constructed of graphite. The graphite sensor is 4.3 mm in diameter, 6 cm long, and is range thin to the primary beam electrons. The probe produces a signal proportional to the intercepted beam current. When the sensor is scanned radially through the beam during repeated pulses, a curve of signal versus depth of insertion is produced from which the radial current density profile can be determined. Measurements are presented of the profile of the electron beam from the Experimental Test Accelerator (4.5 MeV, 10 kA) at Lawrence Livermore National Laboratory. Good agreement is shown between measurements made with this probe and the beam radius as predicted by transport codes. The advantage of the electric probe lies in its ruggedness...