VIBRATING WIRE FOR BEAM PROFILE SCANNING

The method for measurement of transverse profile (emittance) of the bunch by detecting of radiation arising at the scattering of the bunch on scanning wire is wide-spread. In this work the information about bunch scattering is proposed to obtain by measuring the oscillation frequency of the tightened scanning wire. In such way the system of radiation (or secondary particles) extraction and measurement can be removed. The entire unit consist of a compact fork with tightened wire and a scanning system. Normal oscillations frequency of a wire depends on wire tension, its geometric parameters and, in second approximation, on its elastic characteristics. Normal oscillations are generated due to interaction of an alternating current through the wire with magnetic field of a permanent magnet. In this case the magnetic field of the accelerator (field of dipole magnets or quadrupole magnets) is suggested to use for excitation of oscillations. Dependence of oscillations frequency on beam scattering is determined by several factors, including changes of wire tension caused by transverse force of the beam, influence of beam self field. Preliminary calculations show that influence caused by wire heating will dominate. We have studied strain gauges on the basis of vibrating wire from various materials (tungsten, beryl bronze, niobium zirconium alloys). A scheme of normal oscillations generation by alternating current in autogeneration circuit with automatic frequency adjustment was selected. Special method of wire fixation and elimination of transverse degrees of freedom allow to achieve relative stability better than 10 -5 during several days. For a tungsten wire with a fixed end dependence of frequency on temperature was 10-5K-1 . Experimental results and estimates of wire heating of existing scanners show, that the wire heats up to a few hundred grades, which is enough for measurements. Usage of wires of μm thickness diminishes the problem of the wire thermalisation speed during the scanning of the bunch.