GTF Experimental Program for FY 2003

John Schmerge

October 8, 2002

The following is a list of experiments that will be conducted at the GTF during FY 2003 as part of the LCLS R&D program. The list may not be complete and is not necessarily in chronological order.

1. Measure and balance the rf gun cell field ratio. Measure the rf dipole and quadrupole field in gun and install a new Mg cathode for increased charge production. We will also install and calibrate a half cell field probe to allow for dynamic measurements of the gun cell field ratio. Measure and reduce the quadrupole field component in the solenoid and measure the spectrometer magnet fringe field and pole face rotations. Optimize the e-beam beamline screens, cameras and quadrupoles for best emittance measurement resolution using the quadrupole scan technique on both the YAG screens and spectrometer screen. The combination of all these optimizations is expected to decrease both the longitudinal and transverse projected emittance. It should also enable the measurement of slice emittance exiting the gun utilizing a solenoid scan technique. In addition the transverse and longitudinal pulse shape of the UV laser pulse striking the cathode will be measured before the gun is re-installed to determine if any deleterious effects occur due to the optics between the vacuum window and cathode.

2. Measure the integrated QE, QE as a function of position, Fowler Nordheim field enhancement factor at the cathode, total emitted dark current and maximum applied electric field for the friction welded Mg cathode. A Cs²Te cathode with contamination preventing coating is also being prepared for installation at the GTF. If installed during FY2003 similar measurements would be performed on this cathode. These measurements will help determine the usefulness of alternative cathodes for the LCLS gun.

3. Repeat longitudinal and slice emittance measurements with newly optimized beam components. With the entire beamline component optimizations completed in part 1, the slice emittance and the correlated energy spread previously measured at the GTF can be optimized with user adjustable field levels and charges up to 1 nC.

4. Install an optimized IBM laser beam apodizer to properly transversely shape the laser pulse. The IBM apodizer should allow for approximately 90% transmission as opposed to the less than 50% transmission with the clipping aperture transverse shaping technique currently utilized. The combination of increased transmission efficiency and higher QE from the Mg cathode, will allow for temporal pulse shaping measurements using both the time domain Michelson interferometer and frequency domain pulse shaping experiments with up to 1 nC of charge. The effect of temporal pulse shaping according to simulation is a decrease of projected transverse emittance of up to 50%.

5. Install and commission an LCLS prototype electro-optic based e-beam pulse length measurement device. This device will be tested and compared with pulse length measurements conducted as part of the longitudinal emittance measurements. The device could also be tested for applicability of utilizing the output in a feedback circuit to control the e-beam pulse length.

6. Commission a new 5045 klystron powering the GTF 3 m SLAC linac section. Once commissioned to full 64 MW power, the drift distance between the gun and linac will be increased from 90 cm to 150 cm. According to simulation this should decrease the projected transverse emittance from 1 mm-mrad down closer to the slice emittance level. In addition longitudinal and transverse slice emittances will be measured.

7. Test the ability of the klystron and fast attenuator rf system to perform necessary temporal pulse shaping to reduce the heat load on the rf gun. Utilizing a 1 kW linear amplifier, a fast low power attenuator will be ramped to rapidly change the klystron output power. Tests will be conducted to see if the klystron output power can be changed from 15 MW output for 800 ns to 10 MW output for 200 ns and then terminating the output pulse with the klystron connected to a SLAC linac section as a load. This test can be conducted at the GTF or SLAC linac. After a successful completion of this test, the rf gun will be connected to the klystron and the test repeated. Measurements will be made of the klystron forward power and phase as well as the gun cell probes field and phase. The measured cell field probes will then be compared with the expected values using the rf gun model and the forward power amplitude and phase inputs. This is a test to see if this pulse shaping technique can be used to reduce the heat load on the LCLS rf gun.