Wall Current Monitors

A wall current monitor is a device to measure the instantaneous value of the beam current. As the beam flies through the vacuum pipe, it is accompanied by a current flowing along the inside of the pipe's wall, opposite in direction to the beam current. This is the image or wall current (Red arrows).

Imagine that we now cut the pipe, and connect some load across the gap. In the figure here, that load has been represented as a coaxial cable. The image current can do nothing but flow through the load, producing a voltage which is the output signal.

Of course, cutting the pipe would break the vacuum. So we put a box around the gap, which will also shield it, so that interfering signals cannot get in, and the beam-induced signals cannot get out. This box constitutes a short circuit across the gap, but that can be alleviated by filling the box with ferrite rings (brown), so that high frequency signals, at least, do not see the short circuit. Another often used solution is to solder a ceramic ring in the gap. The shielding box is still needed, but now its contents may be at atmospheric pressure.

The properties of the ferrite set the lower cut-off frequency of the output signal, usually around 100kHz. The upper cut-off frequency depends on the load across the gap and the residual capacitance between the pipe ends, and is usually situated in the GHz range. The load often consists of many resistors in parallel, distributed all around the gap. In the PS WCMs, the load is eight evenly spaced 50 Ohm coaxial cables.