Microbunching and ODR Issues Alex Lumpkin Presented at AARD Workshop May 12, 2009 Lake Geneva, Wisconsin
Emittance Preservation Issues • Preservation of beam transverse emittance following bunch compression is one of the challenges for advanced accelerators using high-brightness beams. • Microbunching instabilities are now recognized as a concern in recent workshops (µBI 08, CHBB 08) – Coherent synchrotron radiation (CSR) and longitudinal space charge (LSC) effects are now under study. – The diagnosis of the microbunching instability has recently and surprisingly moved to the visible light regime using COTR- and COSR-related techniques at LCLS, APS, and FLASH. • LCLS staff observed strong enhancements at 135 -4000 Me. V • AAI/ANL staff saw localized spatial structures at 150 Me. V. • Laser heater suppresses instabilities (Huang). (Dec. 08) • Investigations of these using NML are proposed. A. H. Lumpkin AARD WS May 12, 2009 2
Microbunching Aspects • Microbunching instability observed via COTR at LCLS and APS linacs after first chicane compressor. • Longitudinal space charge (LSC), coherent synchrotron radiation (CSR), and linac wakefields (WF) are invoked with LSC more at shorter wavelengths. • Laser-induced microbunching (LIM) possible to probe micropulse bunch length and slice emittance (a. k. a. optical replica (OR)). • Can one use 3 rd harmonic of undulator at γ=80 to make optical replica? A. H. Lumpkin AARD WS May 12, 2009 3
Enhanced OTR Images Seen at APS after Chicane Compression • Examples of images and profiles for L 2 phase at 12 deg. (L-uncompr. ) and L 2 phase at 14. 9 deg. (R-compr. ) Y Y X Intensity X X X A. H. Lumpkin AARD WS May 12, 2009 A. Lumpkin et al. , BIW 08 4
Off-line Analysis on OTR Image Peak Intensity Confirms Phase Effect • Localized peak intensity from single-column samples of 150 -Me. V data. Peak intensity and image averages processed for 375 -Me. V data. A. Lumpkin et al. , BIW 08 A. H. Lumpkin AARD WS May 12, 2009 5
Spectrometer results (10 -30 -08) • Grating offset adjusted to probe the red/NIR end of the COTR spectra using ICCD readout with Ga. As PC. X(ch) 880 nm 650 nm ITT web site 900 nm Wavelength(ch) 880 nm Intensity 880 nm 1250 nm offset, 10 um Slit, ND 1. 0, ICCD A. H. Lumpkin AARD WS May 12, 2009 6
COTR Case at 250 Me. V • Estimate OTR/COTR spectral effect in LCLS case. COTR (3 ke. V) OTR CCD Resp. -UV- Submitted to PRST-AB A. H. Lumpkin AARD WS May 12, 2009 7
Upgraded Photoinjector for NML • Propose use of the Upgraded injector to look for LSC effects at higher charge (1 -6 n. C), lower gamma (60 -100) than present experiments. • Unique opportunity to compare Chicane compression effects and x-z emittance exchange effects. OTR, COTR A. H. Lumpkin AARD WS May 12, 2009 8
Optical Replica Mode or LIM • Use laser to selectively energy modulate a slice of the longitudinal profile or the whole profile within the short undulator. The Chicane’s R 56 then converts to density modulation in z direction or microbunching. LASER pulse e-beam Chicane Al foil Undulator Also possible to use COSR from dipole bend A. H. Lumpkin COTR Slice x, y beam size AARD WS May 12, 2009 9
Gamma Impacts K and Coupling Coefficients • Coupling coefficients JJn for n = 1, 3, 5 versus K show rapid decrease below K~1. 5 for n = 3, 5. Lower γ pushes K and λu lower for fixed λ = 780 nm. Case 0: γ=30, K= NA Case 1: γ=60, K= 0. 51 Case 2: γ=80, K= 1. 13 2 1 Musumeci, Pellegrini, Rosenzweig PRE-72, 016501 (2005) A. H. Lumpkin AARD WS May 12, 2009 10
ODR is a Potential Nonintercepting Diagnostic for Ge. V Lepton Beams and Te. V Hadron Beams • • At left, schematic of ODR generated from two vertical planes (based on Fig. 1 of Fiorito and Rule, NIM B 173, 67 (2001). We started with a single plane. At right, calculation of the ODR light generated by a 7 -Ge. V electron beam for b =1. 25 mm in the optical near field based on a new model (Rule and Lumpkin). a/2=b~γλ/2π A. H. Lumpkin AARD WS May 12, 2009 11
An Analytical Model has been Developed by D. Rule for ODR Near. Field Distributions Based on the Method of Virtual Quanta • We convolved the electron beam’s Gaussian distribution of sizes σx and σy with the field expected from a single electron at point P in the metal plane (J. D. Jackson) where ω = radiation frequency, v = electron velocity ≈ c = speed of light, q = electron charge, N is the particle number, K 1(αb) is a modified Bessel function with α= 2π/γλ and b is the impact parameter. A. H. Lumpkin AARD WS May 12, 2009 9
Investigations of Optical Diffraction Radiation on 7 -Ge. V Beams at APS are Relevant to Other Beams • ODR offers the potential for nonintercepting, relative beam-size monitoring with near-field imaging. This is an alternate paradigm to far-field work at KEK. Lumpkin et al. , Phys. Rev. ST-AB, Feb. 2007 A. H. Lumpkin AARD WS May 12, 2009 13
Initial OTR Images at 4. 5 Ge. V • Newly installed Al-coated Si wafer used with 5 -µA Tune beam (250 µs at 60 Hz). Polarization effects seen on σx, y. V-pol σx: 127 µm Total Intensity, ND 1. 0 σx: 150 µm, σy: 161 µm A. H. Lumpkin H-pol. σy: 134 µm AARD WS May 12, 2009 14
Basic ODR images at 4. 5 Ge. V and 10 µA CW Polarization Component effects are clear. b = 1 mm Hpol. : Double lobe Total: σx=996 µm A. H. Lumpkin AARD WS May 12, 2009 Vpol. ; σx=609 µm 15
OTR, Flying Wire, and ODR Comparison • Effects of vertical polarizer and 550 x 10 nm Bandpass Filter on ODR profile size are shown. A. H. Lumpkin AARD WS May 12, 2009 16
ODR at FLASH: 900 -Me. V Electrons Intensity Y position (ch) Intensity • Near-field ODR Image obtained in collaboration with E. Chiadroni, et al. in January 2008 studies. b = 400, 600 µm. X position (ch) 10 image sum, 6 b/macro, 800 nm BP, 0. 5 s Exposure, 5 Hz, 1 -mm slit, DC subtracted. A. H. Lumpkin Projection to Horizontal plane (sum of vertical pixels 205 -230) Projection to vertical plane (sum of horizontal pixels 220 -280) MATLAB by R. T-K AARD WS May 12, 2009 17
ODR Model Shows Beam-size Effects • NML examples for beam-size monitor for σx=200 µm (L) and 400 µm ± 20% (R) with σy=200 µm, d = 5 σy, and γ=1000. Courtesy of C. -Y. Yao, ANL A. H. Lumpkin AARD WS May 12, 2009 18
SUMMARY • NML offers new parameter space to look for µBI and COTR (detect visible, NIR, MIR, etc. ? ) • Possible EEX aspects versus chicane compression could be tested uniquely at NML. • Need simulations of LSC-induced microbunching at NML. • Development of nonintercepting beam-size measurement for NML high current beams is proposed for gamma > 1000. • Diagnostics test area in one beamline requested. Use impact parameters of about 5 -6 sigma and 0. 8 -µm imaging. • Once developed, recommend ODR installations in other beamlines to support operations and other experiments. A. H. Lumpkin AARD WS May 12, 2009 19
Schematic of LH and OR Mode for A 0 Up. Grade Laser Spectr. /Dump BC RF PC 40 -Me. V Gun Injector Transport Laser Heater Mode Und. RF PC Gun Injector DL 1 BC COTR Dump Optical Replica Mode Und. Laser A. H. Lumpkin AARD WS May 12, 2009 20
Studies Performed in FY 08 • Planning of the NML linac station at 550 -750 Me. V point. • Baseline concept is to image at about 800 nm with a 16 bit camera and use the high charge of the macropulse to generate enough photons at this wavelength. • Second concept is to image or detect in the MIR in the 3 - to 10 -µm regime, where there are more photons emitted. Possible detectors are pyroelectric arrays or cryo-cooled detectors (relevant to hadron issues). • Collaboration with INFN on 900 -Me. V electron-beam experiment at FLASH/ DESY. Studies in Jan. -Feb. 2008 with 16 -bit camera. • Collaboration at JLAB on CEBAF recirculating linac beam at location before nuclear physics target. A. H. Lumpkin AARD WS May 12, 2009 21
OTR at FLASH: 900 -Me. V Electrons • Near-field OTR Image obtained at slit edge in collaboration with E. Chiadroni, et al. in January 2008 studies. 3 image sum, 6 n. C per 6 -bunch macropulse at 5 Hz. X projection σx = 205 µm Y projection Beam size: σx = 205 µm σy = 100 µm A. H. Lumpkin MATLAB by R. T-K AARD WS May 12, 2009 22
ODR Model Shows Wavelength Effect • Examples for beam-size monitor for σx=400 ± 20% µm with σy=400 µm, d = 12 σy , and γ=1000. λ=0. 8 µm (left) and 10 µm (right). Courtesy of C. -Y. Yao , ANL A. H. Lumpkin AARD WS May 12, 2009 23
CEBAF 5 -Ge. V Recirculating Linac • 100 μAmps CW beam extracted at 1, 2, 3, 4 or 5 Ge. V Linac Photoinjector Linac Hall-A Courtesy of Alex Bogaz, JLAB A. H. Lumpkin AARD WS May 12, 2009 24
ODR at FLASH: 900 -Me. V Electrons • Near-field ODR Image obtained in collaboration with E. Chiadroni, et al. in January 2008 studies. b = 300, 700 µm. 740 µm (FWHM) X 10 image sum, 6 b/macro, 800 nm BP, 0. 5 s Exposure, 5 Hz, 1 -mm slit A. H. Lumpkin Y AARD WS May 12, 2009 25
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