Automatic Gain-Controlled Amplifier for Beam Profile Monitors

An amplifier for beam profile scanners of the moving-wire type has been developed in which light-controlled resistors vary the gain so as to provide constant peak amplitude. The peak output signal is independent of the intercepted beam current over the range 4 x 10-3 to 4 x 10-10 amps. The rms noise is less than 8 x 10-12 amps, and the rise time at maximum gain is 20 ¿sec. The control loop maintains approximately constant gain during a single scanner period, yet can change the gain over several decades in one second. At any given gain the amplifier response is linear. With this amplifier in operation, it is not necessary to change the gain manually, as the beam intensity varies over the range of beam currents commonly encountered with particle accelerators.