Measuring Emittance of High-Brightness Photoinjector Beams: Techniques and Challenges, Exams of Physics

Download Measuring Emittance of High-Brightness Photoinjector Beams: Techniques and Challenges and more Exams Physics in PDF only on Docsity! J.B. Rosenzweig UCLA Dept. of Physics and Astronomy USPAS, July 2, 2004 Pulse length manipulation and measurement of high-brightness photoinjector beams Measuring the emittance of photoinjector beams • The low emittance of, and huge forces (internal and external) applied to these beams makes them behave very differently than emittance dominated beams • In addition, investigation of the behavior of these beams, as well as optimization of the beam’s end use, requires accurate measurement of the beam emittance • In order to produce accurate measurements, the emittance diagnostic must take into account the nature of photoinjector beams Quadrupole Scan Measurements • Neglecting space charge we can write an equation for 2 based on the Twiss parameters of the beam. • The procedure then, is to measure 2 (the mean square beam size) versus the focal length of the lens and fit the resulting curve to calculate the emittance. – Thick lens treatment often necessary in compact beamlines. 2 2 = 1 2 1L + L 2 1( ) f 2L 1 2L 2 1( ) + f 2 L2 1( ) L Thin Lens Focal Length f Screen Experimental Procedure • Emittance was measured using both the quad scan and slits for different beam plasma frequencies. – The plasma frequency was changed by changing the laser pulse length, by altering the grating pair separation in the laser system. – For each set of measurements, the laser spot size and energy, grating pair separation, beam charge, and injection phase were recorded to calculate the plasma frequencies. Quadrupole Slits YAG Screen LLNL/UCLA photoinjector beamline Quad Scan Vs Slit Data 0 5 10 15 20 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 Slit Emittance Quad Scan Emittance E m itt an ce [m m m ra d] k p L d The strength of the space-charge forces are parameterized in the scan by product drift length between quadrupole and detector and the plasma wave number. Space-charge in the Quad Scan • There are two relevant normalized numbers two parameterize space charge strength, one measuring drift length, and the other measuring emittance v. space charge. • The white plot points locate the positions of the experimental data. The normalized emittance used as input to the simulations was 5 mm mrad. • kp is a measure of the ratio of the space-charge to emittance forces at the quadrupole. Interplay of Space-charge and Emittance: Simulation • Data and simulation both show asymmetry about minimum spot size. • Asymmetry is due to differential emittance forces. If waist is emittance dominated, then envelope looks very different before and after waist. • Asymmetry makes fitting to a parabola problematic. 0 0.2 0.4 0.6 0.8 1 1.5 10 -3 2 10 -3 2.5 10 -3 3 10 -3 3.5 10 -3 x2 [ m m 2 ] 1/f [1/mm] 0 0.2 0.4 0.6 0.8 1 1.2 1 10 -3 2 10 -3 3 10 -3 4 10 -3 5 10 -3 6 10 -3 7 10 -3 x2 [ m m 2 ] 1/f [1/mm] 0 0.5 1 1.5 2 2.5 2 3 4 5 6 7 Emittance Included Space-Charge Only x2 [ m m 2 ] 1/f [1/m] Fitting on either side of minimum gives different result Summary of Emittance Measurement Techniques • Quad scans are ill-suited for highly space-charge dominated beams because the beam evolves under the influence of both space-charge and emittance effects. • Rules of thumb – Quad scan data may not be valid if – Asymmetry in quad scan data is an indicator of trouble. kpLd > 1 kp > 1 CTR Interferometer • Changing one arm of the interferometer allows to measure the autocorrelation of the CTR signal: S( ) ~ I t( )I t +( )dt light source window filter membrane photodiode Golay cell CTR Autocorrelation Analysis Fourier transform measured autocorrelation function: ˜ S ( ) ~ ˜ I ( ) 2 kLOSSES ( ) Missing long wavelengths due to: –Golay Cell window acceptance –Beamsplitter Efficiency Losses –Radiator size kLOSSES ( ) = e 2 2 -0.2 0 0.2 0.4 0.6 0.8 0 5 1 0 1 5 2 0 2 5 3 0 N or m al iz ed A m pl itu de Position [psec] S( ) ~ e 2 4 t 2 2 t t 2 + 2 e 2 4 t 2 + 2( ) + 2 t t 2 + 2 2 e 2 4 t 2 +2 2( ) High frequency cutoff: 100 µm Neptune compression experiments • 300 pC beam • Initial pulse length of 4.5 ps • Initial normalize emittance ~6 mm- mrad • Pulse compressed to 0.6 ps 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 10 12 14 16 18 20 22 Autcorrelation Data N or m al iz ed S ig na l Delay [psec] t = 0.63 ps Simulation tools • Different codes model different processes (acceleration fields versus velocity fields.) • Codes employed: – TREDI: Solves Lienard-Wiechert potentials. – PARMELA: Provides input distributions for TREDI. Point-to- point space charge for comparison. – ELEGANT: CSR only calculation. • Simulations indicate that for this experiment, acceleration fields do not contribute much emittance growth; space charge fields are dominant. Simulation results • Simulation is difficult. Number of macro-particles is low (2500-10,000) because of time- intensive space-charge calculations. PARMELA and TREDI are 3D point-by-point codes • Sharp emittance increase is missing in simulations. Trace space bifurcation not as severe • Coherent synchrotron radiation is insignificant effect (note ELEGANT point, similarity of TREDI/PARMELA) 0 5 10 15 20 25 55 60 65 70 75 80 85 90 Emittance data PARMELA simulation TREDI simulation ELEGANT simulation E m itt an ce [ m m m ra d] PWT Phase [deg.] Illustrative PARMELA results 19.5 20 20.5 21 21.5 22 22.5 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 z z [mm] 0 100 200 300 400 500 600 700 19.55 20.3 21.05 21.8 co un t Long. phase space Energy distribution (bifurcation here!) Slice model simulation Trace space bifurcation 0 5 10 15 20 0 0.2 0.4 0.6 0.8 1 1.2 Input size dependence Beam size (mm) E m it ta n c e ( m m -m r a d ) (x,x’) distribution function Repulsive force is maximized in peak strength and length of integration when beam is small. Velocity Bunching: a Cure for “The Bends”? • Proposed by Serafini, Ferrario; tool for SASE FEL injector, avoids magnetic compression • Inject emittance-compensated beam at 5-7 MeV into slow-wave linac • Effectively compresses at low energy — good for energy spread control • Perform one-quarter of synchrotron oscillation to compress beam in slow wave – Gentle, low gradient option – Also “thin-lens” option for ORION Longitudinal phase space schematic for velocity bunching from Hamiltonian picture ˜ H ,p( ) = p 2c2 + m0c 2( ) 2 v p + qE0 kz cos kz[ ] Velocity bunching optimization and implementation at ORION • Serafini-Ferrario proposal: long slow-wave structure from tuning (new source) or k (structure)? • Expensive in structure and real estate • Alternative for ORION: use only short bunching section to split functions of bunching and acceleration (ballistic bunching) • Example buncher: PWT (Neptune model, 14 MW) 0 0.05 0.1 0.15 0.2 0 1 2 3 4 5 6 7 z (m m ) z (m)S-band PWT Buncher S-band RF gun X-band travelling wave linacs Simulation of proposed ORION system First linac “catches”beam, second removes energy spread (accel. + phase). 10 pC case. PLEIADES phase space rotation • Hamiltonian picture • Do NOT need slow- wave • Note “cross-over” for full compression • Nonlinear transformation due to form of rf forces. • Very accurate model 77 77.5 78 78.5 79 79.5 80 80.5 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 (degrees) (b) 7.94 7.95 7.96 7.97 7.98 7.99 8 8.01 8.02 -4 -2 0 2 4 (degrees) (a) Longitudinal phase space at injection Longitudinal phase space after compression Velocity bunched beam used in ICS experiments (a) (b) • Electron beam compressed by factor of 12-15 • X-ray yield decreased by 4. • X-ray brightness increased by factor of 3. Single shot, false-color X-ray beam images measured by the CCD for the (a) uncompressed and (b) compressed electron beams.