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## Beam Physics Department

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**Beam Physics Department**Yunhai Cai DOE Program Review, SLAC June 13, 2007**Professors:**Alex Chao Ron Ruth Post-doctor: Yuantao Ding Students: Daniel Ratner Jerry Wang Administrative: Tom Knight Margie Bangali Head: Yunhai Cai Staff: Gennady Stupakov Sam Heifets Karl Bane Zhirong Huang Yiton Yan Yuri Nosochkov Min-Huey Wang Associates: Bob Warnock Martin Lee John Irwin Members in Beam Physics Department**Accounts Charged for the Activities**• ~50% accelerator research: beam dynamics and instabilities, FEL physics • ~50% program support: PEP-II, ILC, LCLS, SEBAR • Year: 2006&2007, published 67 papers among them 16 on peer-reviewed journals**Core Competencies in Beam Physics Department**• Lattice design and single-particle beam dynamics in storage rings • Designed optics for PEP-II, SABER, SPEAR3 upgrades, ILC • Accelerator modeling and improvement, PEP-II • Wakefield and impedance, collective effects and instabilities of intensely charged beam • Impedance at very high frequency • Coherent synchrotron radiation and its dynamical effects • Physics related to ultra-short bunches • Beam-beam effects in the colliders • Simulation and parallel computing • Lie-algebra-based linear and nonlinear analysis codes: LEGO and Zlib • PIC simulation of beam-beam interaction and luminosity: BBI • Nonlinear Vlasov solver for microwave instability • Theory of free-electron laser • Regenerative Amplifier FEL • Teach at Stanford University and US Particle Accelerator School • Single-particle dynamics, FEL physics, impedance and instabilities**Improvements of Online Model**VAX: SCP database: R-Matrix Or Twiss steering package beta measurement Linux: PEP-II optics Optics codes MAD DIMAD LEGO AT Model codes MIA buffer data • Parameters in optics model are used as inputs of beam-beam simulation • to figure out what to do as the next step. • Several computer programs are developed to correct optics, such as • beta beating, dispersion, and coupling.**Comparison Between Measurement and Calculation Based of**Beam-Based Online Model nx ny x-oscillation dispersions**Predictive Power of Precision ModelNonlinear Beam Dynamics**• Measured chromatic optics and dynamic aperture in HER • Excellent agreement between measurements and LEGO model in the chromatic optics • Improvement of understanding of nonlinear dynamics including sextupoles**Developed Accurate Online and Offline Models**Comparison of quadrupole strength between LER model and configuration, May 30, 2007**Improvement of Machine Optics and Luminosity for PEP-II**Achieved peak luminosity: 1.2x1034cm-2s-1, August 15, 2006**An Alternative Approach to Double the LuminosityWithout**Increase Beam Currents • Beam-beam codes were benchmarked against our KEKB colleagues and • many measurement. Simulations show a significant increase of luminosity • as coupling decreases in both machine. • Some skew quadrupoles are necessary to reduce the design vertical • emittance. Extensive tuning and MD may be necessary as well.**LER lattice with low vertical emittance**The new IR local coupling correction is implemented. It uses 12 additional permanent skew quads (PSK) to reduce the design vertical emittance from 0.50 nm to 0.034 nm. SK5,6L SK5,6 The main source of emittance in the design lattice is the local coupling and dispersion from SK5,6 skew quads on left/right sides of IR. old 5 PSK 7 PSK In the new solution, the IR coupling is better localized by setting the strengths of SK5,6 to zero, and including the 12 PSK quads closer to IP. Their positions and strengths are optimized for minimum vertical emittance. new IP**Mode Emittances in the PEP-II Ringsafter the PSK lattice was**implemented into the LER May 30, 2007 models. At this moment, luminosity is 10% below the best achieved value at currents of 2450/1775 mA.**14 mrad ILC extraction line**The ILC single IR optics is re-designed to allow a fast interchange between two different detectors and different L* (push-pull detector option). Redesigned extraction optics with energy and polarimeter diagnostics, and the 2nd beam focus. Detector dependent optics Fixed optics Fast x-y kickers sweep the beam on 3 cm circle at the dump to reduce power density and prevent water boiling in the dump vessel. Disrupted large energy spread leads to low energy losses. X collimators magnets Y**f0=650 MHz**ABCI Scale = 22.05 V/pC Impedance & Instabilities in the ILC Damping Rings • A working group from several departments: PBD, ACD, ASD • Develop an impedance model and estimate threshold of instabilities, in particular, microwave instability, which may has impact of choice of momentum compaction factor for the damping rings.**RESISTIVE WALL IMPEDANCE OF A SURFACE WITH TRIANGULAR**GROOVES A surface with triangular grooves can significantly reduce the secondary emission yield below the multipacting threshold with weak dependence on the size of surface and magnetic field. Electric field lines in the groove. Impedance amplification factor as a function of the angle.**For one overall P and individual Q adjustments, the**optimized solution has • q= Q/Q0 (Q0 is the matched loaded Q) mostly in the range [1, 2] • Optimize gradient of the 26 cavities gi (while keeping boundary conditions) where • 27d optimization {qi, tb}, can be turned into 3d optimization {p, tb, qmin}: g= G/G0 ghead gtail q Optimization of ILC Linac RF Distribution System -As an example: Overall P and Individual Q Adjustments For p= 0.92, tb= 0.89, gradient at head and tail of train • One seed • Red dots give (glim)I • Loss only 3% • compare to 20% in • the worst case**SABER final focus**SABER final focus is redesigned for using existing SLAC spare quadrupoles from SLC and FFTB. Round beam at IP with E = 28.5 GeV, bx = 1.5 cm, by = 15 cm, hx,y = 0. IP With bypass: sx = 5.5 mm, sy = 6.1 mm, sz = 19.3 mm. W/o bypass: sx = 6.9 mm, sy = 6.7 mm, sz = 18.8 mm.**Orbit Response of Horizontal Kick: q in Strongly Coupled**Lattices General solution of closed orbit: Rab Ab b Aa-1 Mab a a**Emittance exchange**• “Natural” beam configuration of RF gun not optimal for FEL: transverse emittances too large, intrinsic E-spread too small • Better beam configuration obtainable through 6D phase space manipulation (flat beam gun + x-z emittance exchange) • Very small transverse emittances “achieved” in simulations for 20 pC charge, x = 0.16 m y = 0.0054 m z = 11 m k dipole deflecting cavity Emma, Huang, Kim, Piot, PRSTAB 9, 100702 (2006)**HIGH FREQUENCY IMPEDANCE CALCULATIONS**A theory of high frequency impedance is developed for various non-axisymmetric geometries such as irises/short collimators in a beam pipe, step-in transitions, step-out transitions, and more complicated transitions of practical Importance [G. Stupakov, K. Bane, Zagrodnov,PRSTAB 10, 054401 (2007) ]. For a flat iris with aperture 2g in a flat beam pipe of aperture 2b, the transverse impedances as functions of g/b.**MICROWAY INSTABILITY STUDIES FOR THE ILC DR**A new computer code is developed that solves a linearized Vlasov equation in the time domain. The code is implemented in Mathematica; it can be easily modified and augmented. Growth rate for the CSR induced microwave instability as a function of current. Phase space of the microwave instability.**Bunch Lengthening Variation along Bunch Train**Due to RF Gap Transient Transient & PWD gap transients only • Measured by Novokhatski • Figures on right show calculated variation of bunch length at different currents (shown in Amp) S. Heifets, S. Novokhatski, D. Teytelman, PRSTAB 10, 011001, (2007).**Ultra-Low Emittance Ring with FEL**• A lattice with 0.1 nm-rad emittance at 7.0 Gev and 0.05 nm-rad at 4.5 Gev. Steady-state SASE: • wavelength 5-100nm, pulse length ~10 ps, rep rate ~ 100 kHz, peak power ~ 1MW, undulator length ~ 100m • HHG seeding:~1MW, ~10 fs at • 10 nm, 10 m modulator+chicane • HGHG FEL output at ~ 3.3 nm 5 nm SASE**Conclusion**• BPD has made significant contribution to the success for the PEP-II operation in the past year as many long-term research activities baring their fruits, most noticeable: beam-beam simulation and precision optics modeling. • We continued to important contributes to accelerator projects: ILC, SABER, LCLS, and future possibilities: SuperB and 5th generation light source. • We published many accelerator research papers on peer-reviewed journals and continued to teach at Stanford University and USPAS.