Beam bunch length measurement for the ARIEL e-LINAC Vrielink, Alysson
The following document presents measurements conducted to determine and manipulate the beam bunch length in an electron linear accelerator at TRIUMF's new ARIEL facility. This method uses a radiofrequency resonant cavity that operates in a mode with transverse electromagnetic fields on axis. These fields deflect electrons by a magnitude dependent on the phase of the radiofrequency fields at the arrival time of the electron. Therefore, it is possible to convert the spread of electrons after the deflection to a profile of the beam in the time domain before it reached the deflecting cavity. This relationship between bunch length and vertical spread after the deflecting cavity requires that certain properties of the cavity be measured. In particular, the effective shunt impedance must be known, a parameter that gives the relationship between input power and effective cavity voltage. To obtain this measurement, we look at the effect of the deflecting cavity on the beam and compare this with results obtained from simulations using a 3D beam dynamics program, General Particle Tracer. Preliminary results suggest the effective shunt impedance of the deflecting cavity is 0.81MΩ, however this should be confirmed with further data collection. In addition to beam bunch length measurements, this project characterized the bunching cavity, a radio-frequency resonant cavity designed to focus the beam in the time domain. Using the bunching cavity in conjunction with the deflector cavity, we were able to measure the nominal bunching power for optimal focusing at the deflecting cavity to be 48.5W. When compared with computer simulations, the shunt impedance of the buncher was calculated to be 0.326MΩ. This shunt impedance can be used to determine the input power required for optimal temporal focusing of the beam at any point along the beamline. This is important as a focused bunch is highly desirable for downstream users. As these results are only a first estimate, it would be useful to do further data collection to confirm these values and obtain error bounds. In addition, the computer simulations could be improved by using more recent estimates of the input beam characteristics (standard deviation of the bunch in the x, y and z directions, beam energy, etc...). Having obtained the most accurate results possible for the shunt impedance of the deflecting cavity, a final recommendation would be to develop a program that can rapidly analyze images of the beam after the deflector and output the beam bunch length.
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