We propose to use existing LCLS copper S-band linac double bunch infrastructure to significantly improve LCLSII hard X-ray performance. In our setup, we use the first bunch to generate a strong seeding X-ray signal, and the second bunch, initially traveling off-axis, to interact with the seed in the amplifier undulator and generate a near TW, 15 fs duration X-ray pulse in the 4 to 8 keV photon energy range. We investigate, via numerical simulations, the required transverse beam dynamics and the four crystals X-ray monochromator to be added to the existing LCLS-II beamline and discuss the final properties of the hard X-ray pulses and their potential application in high intensity, high-field physics experiments, including QED above the Schwinger critical field.

We discuss a proposal to test the operation of an X-ray cavity consisting of Bragg reflectors. The test will constitute a major step demonstrating the feasibility of either an X-ray regenerative amplifier FEL or an X-ray FEL Oscillator. These cavity-based X-ray FELs will provide the full temporal coherence lacking in the SASE FELs. An X-ray cavity of rectangular path will be constructed around the first seven LCLS-II undulator units. The Cu-linac will produce a pair of electron bunches separated by the cavityround-trip distance during each linac cycle. The X-ray pulse produced by the first bunch is deflected into the cavity and returns to the undulator where it is amplified due to the presence of the second bunch. The key challenges are: the precision of the cavity mechanical construction, the quality of the diamond crystals, and the electron beam stability. When the LCLS-II super-conducting linac becomes available, the cavity can then be used for high-repetition rate studies of the X-ray RAFEL and XFELO concepts.

When a beam travels near collimator jaws, it gets an energy loss and a transverse kick due to the back reaction of the beam field diffracted from the jaws. The effect becomes very important for an intense short bunch when a tight collimation of the background beam halo is required. In the Linac Coherent Light Source (LCLS) at SLAC a collimation system is used to protect the undulators from radiation due to particles in the beam halo. The halo is most likely formed from gun dark current or dark current in some of the accelerating sections. However, collimators are also responsible for the generation of wake fields. The wake field effect from the collimators not only brings an additional energy jitter and change in the trajectory of the beam, but also rotates the beam on the phase plane, which consequently leads to a degradation of the performance of the Free Electron Laser (FEL) at the LCLS. In this paper, we describe a model of the wake field radiation in the SLAC linac collimators. We use the results of a numerical simulation to illustrate the model. Based on the model, we derive simple formulas for the bunch energy loss and the average kick. We also present results from experimental measurements which confirm our model.

The Linac Coherent Light Source is an x-ray FreeElectron Laser (FEL) project being commissioned at SLAC. The very bright electron beam required for the FEL is subjected to various sources of jitter along the accelerator. The peak current, centroid energy, and trajectory of the electron bunch are controlled precisely at the highest repetition rate possible with feedback systems. We report commissioning experience for these systems.

H. Badakov, M. Berry, I. Blumenfeld, A. Cook, F.-J. Decker, M. J. Hoganβ, R. Ischebeck, R. Iverson, A. Kanareykin, N. Kirby, P. Muggliγ, J.B. Rosenzweigα, R. Siemann, M.C. Thompson, R. Tikhoplav, G. Travishα, D. Walz Department of Physics and Astronomy, University of California, Los Angeles Stanford Linear Accelerator Center University of Southern California Lawrence Livermore National Laboratory Euclid TechLabs, LLC Spokesperson

A Super B-Factory, an asymmetric ee collider with a luminosity of 2.5 to 7 x10 cms, can provide a sensitive probe of new physics in the flavor sector of the Standard Model. The success of the present B-Factories, PEP-II and KEKB, in producing unprecedented luminosity with very short commissioning times has taught us about the accelerator physics of asymmetric ee colliders in a new parameter regime. From this experience, it is possible to build on this success to advance the state of the accelerator art by constructing a collider with a luminosity approaching 10 cms. Such a collider would produce an integrated luminosity of 10,000 fb (10 ab) in a running year. Design studies are underway to arrive at a complete parameter set based on a collider in the PEP-II tunnel at SLAC but with an upgraded RF system, vacuum system, magnet system, and interaction region [1-7]. The present injection system based on the SLAC linac needs no improvements and is ready for the Super-B-Factory.

A Super B-Factory, an asymmetric ee collider with a luminosity of 2.5 to 7 x10 cms, can provide a sensitive probe of new physics in the flavor sector of the Standard Model. The success of the present B-Factories, PEP-II and KEKB, in producing unprecedented luminosity with very short commissioning times has taught us about the accelerator physics of asymmetric ee colliders in a new parameter regime. From this experience, it is possible to build on this success to advance the state of the accelerator art by constructing a collider with a luminosity approaching 10 cms. Such a collider would produce an integrated luminosity of 10,000 fb (10 ab) in a running year. Design studies are underway to arrive at a complete parameter set based on a collider in the PEP-II tunnel at SLAC but with an upgraded RF system, vacuum system, magnet system, and interaction region [1-7]. The present injection system based on the SLAC linac needs no improvements and is ready for the Super-B-Factory.

First results and beam measurements are presented for the recently installed linac bunch compressor chicane. The new bunch compressor produces ultra-short electron bunches for the Sub-Picosecond Photon Source (SPPS) and for test beams such as the E164 Plasma Wakefield experiment. This paper will give an overview of the first experiences with tuning and optimizing the compressor together with a description of the beam diagnostics and beam measurements. These measurements form the basis for further detailed study of emittance growth effects such as CSR and wakefields in a previously unmeasured regime of ultra-short bunch lengths.

During the last several decades there have been tremendous advances in the power and the techniques of particle accelerators. In parallel, there have been remarkable advances in the understanding of how charged particle beams interact with themselves and with external environments. The current status of beam dynamics is such that some of the mathematical tools for the collective instabilities, phase-space dilutions and beam cooling methods, nonlinear phenomenon, etc. have become practical design tools for currently operating accelerators. As we contemplate the next generation of large-scale accelerator projects, there are more challenges ahead, both in improving the predictive power of the current calculations as well as in developing new topics. This report is a summary of the discussions of the Beam Dynamics (T5) Working Group at Snowmass on the progress and challenges for the beam dynamics of future accelerators.

Introduction SLAC experiment E-157 is a study of plasma wakefield acceleration. The experiment was proposed and approved in 1997 with the goal of measuring acceleration by an electron beam-driven plasma wave. This is a proposal to extend these measurements to a positron beamdriven plasma wave where the physics is qualitatively different. There are strong motivations for proposing this extension • The study of positron wakefield acceleration is a unique scientific opportunity. • The E-157 apparatus is complete and performing as designed. • The SLAC schedule is such that the experiment could be performed this Fall.

We report various accelerator physics studies and improvements from the 1997/98 run at the Stanford Linear Collider (SLC). In particular, we discuss damping-ring lattice diagnostics, changes to the linac set up, fast control for linac rf phase stability, new emittance tuning strategies, wakefield reduction, modifications of the final-focus optics, longitudinal bunch shaping, and a novel spot-size control at the interaction point (IP). 1 DAMPING-RING LATTICE DIAGNOSTICS In 1997 the South Damping Ring (SDR) optics was characterized by an analysis of the measured orbit response matrix with the program LOCO [1]. LOCO varies the individual gradients of the quadrupoles in a computer model (such as MAD [2]) to find the gradients that best reproduce the measured orbit response data. Figure 1 compares the design optics for the SDR with the optics derived from a first statistical fit to the measured response matrices. The agreement was poor, and the χ per degree of freedom was about 100. This plus the extreme variations in the fit model optics indicated some large systematic error. Subsequent inspection of the ring showed that the longitudinal locations of many beam-position monitors (BPMs) were not correct in the model. Once the model was updated, the LOCO calculation gave the more reasonable optics shown in Fig. 2 (left). The convergence of model-based ring orbit correction also improved. Figure 1: (Left) the design SDR optics; (right) optics according to the first analysis of the response matrix. ∗Work supported by the Department of Energy, contracts DE-AC0376SF00515 and DE-FG02-92ER40715. †Present address: CERN, SL, 1211 Geneva 23, Switzerland ‡University of Massachusetts, Amherst, Massachusetts 01003. §CERN, PS, 1211 Geneva 23, Switzerland ¶Corresponding author Figure 2: (Left) the SDR optics from fit to the response matrix after correcting the BPM position errors in the model; (right) the closed orbit at 16 QF magnets according to the BPMs and according to the variation in the fit gradients. The optics in Fig. 2 (left) is still significantly different from the design optics. The model optics was fit to the measurements by only varying the gradients in the model quadrupoles, and assuming that the beam was centered in the sextupoles. Alternatively, we could also assume that the actual quadrupoles themselves have no gradient errors, and attribute the computed gradient variations entirely to orbit offsets in the adjacent sextupoles. In Fig. 2 (right), the orbit offsets so obtained are compared with the closed orbit measured at the BPMs adjacent to the 16 quadrupoles in the QF family. The good correlation indicates both that most of the calculated gradient error is caused by the orbit in the sextupoles and that the fit optics shown in Fig. 2 (left) is a reasonable representation of the true ring optics. 2 EMITTANCE TRANSPORT In 1997, a variety of new techniques were adopted in order to preserve the small emittances from the damping rings. For example, the beam loss in the ring-to-linac transport line (RTL) was reduced by a new optics with larger momentum compaction factor [3], and a more robust lattice [4] improved the chromatic and wakefield-induced emittance dilution in the SLAC linac, while also ensuring compatibility with PEP-II (B factory) operation. In previous years, one major source of linac instability had been the poor control over the rf phases, most notably over the phase reference of the 30 linac subboosters (each driving a group of 8 klystrons). In 1997, a fast subbooster phasing algorithm was implemented [5] by which the phases of all subboosters are measured within about 2 minutes. The energy variation induced by a ±20◦ subbooster change is inferred from the orbit shift at high dispersion points in the beam switch yard (BSY) at the end of the linac. Fitting the observed dependence to a cosine function determines the subbooster phase with a resolution of about one degree S-band [5]. As an illustration of the new phasing method, Fig. 3 shows a measurement of the diurnal rf phase variation in different parts of the linac.

In high energy linear colliders, the low emittance beam from a damping ring has to be preserved all the way to the linac, in the linac and to the interaction point. In particular, the R,hg-‘bLinac (RTL) section of the SLAC Linear Collider (SLC) should provide an exact betatron and dispersion match from the damping ring to the linac. A beam with a non-zero dispersion shows up immediately as an increased emittance, while with a betatron mismatch the beam filaments in the linac. Experimental tests and tuning procedures have shown that the linearized beta matching algorithms are insufficient if the actual transport line has mrne unknown errors not included in the model. Also, adjusting quadrupole strengths steers the beam if it is offset in the quadrupole magnets. These and other effects have lead to a lengthy tuning process, which in the end improves the matching, but is not optimal. Different ideas will be discussed which should improve this matching procedure and make it a more reliable, faster and simpler process. 1 Theoretical Considerat ions A mismatch inpetatron functions of the beam (a, ,9) and the lattice (a, p) and a non-zero dispersion (9 or,v’ # 0) at the beginning of the linac enlarges the epective emittance (Q/J). A dispersion q causes different beam positions for different energies AZ = qAE/E. This effect can be es timated by the following example. For an energy spread +dEc: 1 %, a dispersion of q = 10 mm will lead to an emittance growth of roughly 10% (at a beam size of 00 = fl= 316pm): ‘c//P = u2 = I$ + ~~6~ = (0.1 + 0.01) mm’, (1) if there is a similar disturbance in the angular component with q’. Otherwise the full expression has to be recognized: Cl/j = q/l + Iv2 + (P# + aqg2] < 62 > /(c/q, (2) which corresponds to a bigger (and additionally mismatched [l]) ellipse in phase space. A betatron mismatch has no immediate effect on the emittance, but will increase the emittance by the filamentation of the phase space ellipse induced by an energy *Work ~ppotied by the Department of Energy contract DE AC03-76SFOO515. spread. This magnification due to the betatron mismatch is given by: e.g.: ff = & = 0 1.0 0.25 2.0 0.67 4.0 1.60 n > 10 w n/P Fig. 1 shows the beam in real space for /3, v-mismatch and also higher order contributions. Besides the theoretical considerations, the observed practical problems during the actual minimization process will be described.