SLAC, Menlo Park, California, USA
Monitoring the pulse by pulse output intensity and profile of an FEL is a critical measurement both for users and for optimising the accelerator drive beam. The diagnostic challenge is to find a technique that is not susceptible to damage at high output power, is noninvasive and can be performed at high repetition rates. Fluorescent screens are invasive, susceptible to damage and limited in repetition rate by the camera readout. Gas cell monitors are noninvasive but only yield intensity information and suffer from residual ionisation at high repetition rates. The technique described here uses the scattering of a beam of low-energy electrons as they are scanned across the photon beam to measure the transverse intensity profile of the photon beam. Two different geometries are compared. One is where a finely focused electron beam is scanned transversely across the photon beam to measure the transverse profile. The second is where the electrons are bent onto the axis of the photon beam and are scattered by the counter propagating beam of photons. Here the electron beam is kept larger in diameter than the photon beam so that the photon pulse intensity can be measured by the scattering.
SLAC, Menlo Park, California, USA
A new fast wire scanner based on a linear DC servo motor acting through dual bellows has been developed at SLAC. After successful beam testing at LCLS we are now replacing all the old style stepping motor driven scanners with the new type. The fast scanner design allows full emittance scans to be completed in seconds rather than minutes as before, facilitating speedier tuning of the accelerator. The low vibration design allows for wire speeds up to 1 m/s, making it also suitable for use in the new LCLS-II machine where high wire speeds are essential to prevent wire breakage from the high power electron beam with a 1 MHz repetition rate. The wire scanner design is presented along with beam measurements demonstrating its performance.