Wire scanners are very useful to measure beam sizes at micron rms level, in particular when small beam sizes in combination with high current exist. The wire scanner built at DESY Zeuthen is equipped with three 7 micron carbon fibres precisely positioned and oriented at 0, 45 and 90 degrees, so that both transverse dimensions and the beam ellipse orientation can be measured. The way to enable these measurements relies on measuring the current in the wires which results from secondary particle emission.
The signal from each wire is fed directly into a charge sensitive amplifier positioned close to the device. This component is followed by a second amplification stage to be able to drive a long cable. The sensitivity was chosen to be 500mV/pC. The signals are digitized by a standard 12-bit ADC (COMET).
The relative position of the wires are measured with high resolution rotary encoders counting 2000 steps per revolution. The scanner is driven by a DC motor which can be programmed to run over a predefined distance with a predefined time. Its position is reproducible within few microns. The range of the moving distance is close to 78 mm and its speed does not exceed 40mm/sec.
The wire scanner is installed into the TTF beam line and responds studies are ongoing.
The enclosed figures show an overall view of the wire scanner, a more detailed view with the wire positions and expectations of the scanner responds (charges) as a function of the wire position, for four Gaussian widths of the beam.
General view
Detail
