LSC/CSR Instability Introduction (origin of the instability) CSR/LSC Cure (laser heater)
Download ReportTranscript LSC/CSR Instability Introduction (origin of the instability) CSR/LSC Cure (laser heater)
Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center LSC/CSR Instability Z. Huang, M. Borland (ANL), P. Emma, J. Wu C. Limborg, G. Stupakov, J. Welch Introduction (origin of the instability) CSR/LSC Cure (laser heater) LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 1 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Introduction FEL instability in the undulator requires very “cold” electron beams (small emittance and energy spread) Such a cold beam can be subject to other “undesirable” instabilities in the accelerator… Bunch compression gives rise to a microbunching instability that may be harmful to LCLS A laser heater at the end of LCLS injector can be used to add “incoherent” energy spread to control LSC/CSR instability while preserving the FEL lasing LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 2 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center How cold is the photoinjector beam? Parmela Simulation DE/E TTF measurement 3 keV ·measured mean simulation (sec) LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 3 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Microbunching instability • Initial density modulation induces energy modulation through long. impedance Z(k), converted to more density modulation by a chicane growth of local energy spread/emittance! R56 bi Energy Z(k) DE bf >> bi or G= bf/ bi >> 1 l l Current modulation 1% t Gain=10 10% t LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 4 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center LCLS accelerator systems End of injector DL1 SC wiggler at 4.5 GeV DL2 Laser heater at 135 MeV Linac 1 Linac 2 Linac 3 BC1 BC2 • At the end of injector, e-beam carries some residual density modulations which can be amplified in the downstream accel. • Sources of impedance: CSR in dipoles, longitudinal space charge (LSC) and linac wakefields in linacs • Landau damping options: a SC wiggler before BC2 at 4.5 GeV or a laser heater before DL1 at 135 MeV LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 5 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Heating within FEL tolerance • FEL parameter r ~ 5×10-4, not sensitive to energy spread until sd ~ 1×10-4 M. Xie’s fitting formula • 3 keV initial energy spread after compression = 120 keV, corresponding to sd ~ 1×10-5 at 14 GeV can increase sd by a factor of 10 without FEL degradation LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 6 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center CSR instability energy profile long. space sd 310-6 temporal profile microbunching 230 fsec sd 310-5 LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 7 SC-wiggler damps bunching Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center SC wiggler • SC wiggler increases sd 10 times at 4.5 GeV (BC2), suppresses the CSR gain Initial modulation wavelength (mm) • Ineffective for LSC instability occurred earlier in the beamline LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 8 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Longitudinal space charge Current modulation Energy modulation • Space charge oscillation at low energies (in the photoinjector), little accumulation in energy modulation LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 9 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center LSC instability • Acceleration in linacs freezes density modulation and accumulates energy modulation, amplified by the chicane Saldin Schneidmiller Yurkov LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 10 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center LSC instability in LCLS • 3 keV energy spread is too small to suppress the LSC instability in BC1, which could induce too much energy modulation in L2 before the wiggler 1×10-4 l0≈15 mm from 3 keV LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability Elegant tracking of final energy spread with 1% initial density modulation 11 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Laser Heater 10 cm 50 cm 2 cm q 5.7º 10 period undulator 10 cm ~120 cm • Laser-electron interaction in an undulator induces rapid energy modulation (at 800 nm), to be used as effective energy spread before BC1 (3 keV 40 keV rms) • Inside a weak chicane for easy laser access, time-coordinate smearing (Emittance growth is completely negligible) LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 12 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source +60 keV P0 = 37 MW w0 3 mm large laser spot sx,y 200 mm Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center P0 = 1.2 MW w0 = 350 mm matched spot sx,y 200 mm spread by laser transverse gradient -60 keV In Chicane After Chicane less uniform heating LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability more uniform heating 13 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Microbunching Gain after Laser Heater 40 keV large laser spot matched laser spot LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 14 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source THE GOOD (w0 = 350 mm, P0 = 1.2 MW) Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center THE BAD ( w0 = 3 mm, P0 = 37 MW) Final phase space for initial 15 mm seed AND THE UGLY (no heater) LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 15 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Sliced final energy spread (a) No heater (b) w0 = 3 mm, P0 = 37 MW (c) w0 = 350 mm, P0 = 1.2 MW LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 16 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Choices of transverse laser profile • For an initial “white” noise spectrum, heating with a matched laser spot is generally more effective • Laser spot size may be used to “shape” the sliced energy distribution to suppress a particular range of modulation spectrum A 60-fs section of the final phase space with initial150-mm seed (w0 = 350 mm, P0 = 1.2 MW) LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability (w0 = 3 mm, P0 = 37 MW) 17 Zhirong Huang, SLAC [email protected] Linac Coherent Light Source Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center Summary Microbunching instability driven by LSC, CSR and machine impedance can be a “nightmare” for LCLS The photoinjector beam is too “cold” in energy spread, “heating” within the FEL tolerance (~10X) can damp the instability SC wiggler is too late for the LSC instability occurs in the lower energy end of the linac (L1, BC1 and L2) A laser heater can be effective to suppress the microbunching and is under technical design (R. Carr et al.) It also adds flexible control of sliced energy spread to study FEL physics LCLS Linac Review, 12 Dec 2003 LSC/CSR Instability 18 Zhirong Huang, SLAC [email protected]