CSR Simulations in EEX Experiment

1. CSR Simulations in EEX Experiment Ray Fliller

2. Coherent Synchrotron Radiation • Coherent Synchrotron Radiation occurs when a short bunch passes through a bending magnet. Synchrotron radiation from the rear of the bunch can interact with the front of the bunch. This leads to • Beam Energy loss • Energy spread changes within the bunch • Large decrease in energy at the tail of the bunch • Smaller increase in energy at the head of the bunch • Transverse emittance growth • This is from the particles with changed energies not following the orbit given by the original energy deviation and dispersion. When the dispersion is eliminated, these particles have a betatron offset.

3. CSR Induced Emittance Growth • CSR causes emittance growth because of the redistribution of the energy inside of the bunch. This causes particles to follow paths dictated by their energy loss. After a dipole this leads to position and angular offsets. Figure Taken from “A Bunch Compressor forSmall Emittances and High Peak Currents at athe VUV Free-Electron Laser” Frank Stulle, Ph.D. thesis University of Hamburg

4. CSR in the EEX Experiment • When the EEX experiment was installed, ports were included to measure any radiation from dipoles 2,3,4, spectrometer. • On December 20 we tried to measure any Coherent Radiation at dipole 3. We measured • Coherent Transition Radiation prior to dipole to determine if the bunch was compressed. • Radiation through straight-through port of Dipole 3. • Varied the charge to investigate charge dependence. • The goal was not a precise measurement of anything. We wanted to know if we would see anything at all!

5. Experiment Setup Fused Silica Window • Pyroelectric detectors measure the THz radiation expected from the Coherent Transition and Coherent Synchrotron radiation. • Transition radiation used to ensure that bunch is compressed. • Not a measure of length per se, more of an “optimization knob”. • Straight ahead detector measures edge radiation from D2 and synchrotron radiation from D3. • Optimally a crystalline quartz window should be placed on both ports as fused silica reduces the transmission of certain wavelengths. Removable OTR Screen D3 D2 2 1 Crystalline Quartz Window Pryoelectric Detectors

6. Results – Detector 2 Signal • Detector 2 shows increased signal with a compressed beam. • Doubling the charge increases the compressed beam signal a factor of 3. • The coherent power scales as q2. • The incoherent power only scales as q. • So we have some confirmation that some sort of coherent radiation is emitted! • Be it edge radiation from D2 or synchrotron radiation from D3.

7. Simulations • Astra simulates Cathode to just before EEX line • These simulations here benchmarked against measurements we we have some confidence • CSRtrack was used to simulate each dogleg. • Simulations done with CSR/Space charge on and off • Simulations without CSR were compared to ELEGANT to at least make sure that the simulations agree! • 10k particles used • Took a week on my desktop per dogleg • If I had the parallel version (not under Windows), would take 2 days. • ASTRA was used to track through the cavity and the “diagnostic section” to the spectrometer. • Checked my ASTRA sims with Manu’s work. • Ok! • Recheck that I get the correct matrix for the cavity • Ok! • ASTRA was used even with cavity off • CSRtrack was used to simulate from the spectrometer to the dump • Some finageling needs to be done to make CSRtrack bend vertically • Like rotating the bunch 90 degrees.

8. Beam Parameters WITHOUT CSR • Into the EEX line • Epsx =4 mm-mrad • Epsz=120 mm-mrad • Dp/p=1.4% • Sigz=2mm • Out of the EEX line cavity off: • Epsx=255 mm-rad • This is the projected emittance, contains dispersion, but it what would be measured with slits or pepperpot (FNAL Beams Doc 3052-v1) • Epsz=385 mm-mrad • Again a projected emittance • Dp/p=1.4% • Sigz=1.6 mm • Out of the EEX line cavity on • Epsx=112 mm-mrad • Epsz=10 mm-mrad • Dp/p=0.14% • Sigz=0.4 mm

9. Beam Parameters WITH CSR • Out of the EEX line cavity off: • Epsx=276 mm-mrad • This is the projected emittance, contains dispersion, but it what would be measured with slits or pepperpot (FNAL Beams Doc 3052-v1) • Epsz=280 mm-mrad (27% lower!) • Again a projected emittance • Dp/p=1.3% • Sigz=1.6 mm • Out of the EEX line cavity on • Epsx=142 mm-mrad (26% higher) • Epsz=50 mm-mrad (5x higher!) • Dp/p=0.5% (3.6x higher!) • Sigz=0.6 mm • “Tim’s EEX Movie” after the spectrometer looks like a factor of 3 drop in spot size. • Tim did not have the proper focusing at the cavity, so the jury is still out.

10. Where does that leave the measurement? • With the avalible diagnostics, Tim can only measure • Bunch Length • Momentum spread • So he can only make an upper bound on the longitudinal emittance. • This upper bound will drop a factor of 6.5 when comparing the input longitudinal emittance bound to the output longitudinal emittance bound. • The emittance itself drops a factor of 7. • The apparent transverse emittance after the exchange DROPS almost a factor of 2 (with or without CSR) • This is because the dispersion contaminates the measurement. • You can correct for it if you know