ANL, Argonne
Euclid TechLabs, LLC, Solon, Ohio
Funding: Work supported by the U.S. Department of Energy under contract No. DE-AC02-06CH11357.
Dielectric loaded wakefield structures are being developed to be used as high gradient accelerator components. The high current electron beam at the Argonne Wakefield Accelerator Facility was used to excite wakefields in cylindrical dielectric loaded wakefield structures in the frequency range of 8 to 14 GHz, with pulse duration of a few nanoseconds. Short electron bunches (13 ps FWHM) of up to 86 nC drove these wakefields, and accelerating fields as high as 100 MV/m were reached. These standing-wave structures have a field probe near the outer edge of the dielectric to sample the RF fields generated by the electron bunches. Monitoring of the field probe signal serves to verify the absence of electric breakdown. Similar structures were used to extract RF power from the electron beam; however, in this case they were travelling-wave structures, driven by electron bunch trains of up to 16 bunches. RF pulses of up to 40 MW were measured at the output coupler of these structures.
UCLA, Los Angeles, California
BNL, Upton, Long Island, New York
SLAC, Menlo Park, California
USC, Los Angeles, California
Funding: Work Supported by US Department of Energy
In the plasma wakefield accelerator (PWFA), high quality accelerated electron bunches can be produced by injecting a witness bunch behind a single drive bunch or a train of N bunches. To operate at large gradient the plasma density must be in the 1017/cc range, corresponding to a typical bunch separation of the order of the plasma wavelength or ≈100μm. We have demonstrated that such a sub-picosecond temporal bunch structure can be produced using a mask to selectively spoil the emittance of temporal slices of the bunch*. The bunches spacing, as well as their length can be tailored by designing the mask and choosing the beam parameters at the mask location. The number of bunches is varied by using an adjustable width energy limiting slit. The bunches spacing is measured with coherent transition radiation interferometry. Experimental results will be presented and compared to simulations of the bunch train formation process with the particle tracking code ELEGANT.
*P. Muggli et al., to appear in Phys. Rev. Lett. (2008).
IPM, Tehran
University of Oslo, Oslo
CERN, Geneva
The aim of the CLIC Test Facility CTF3, built at CERN by an international collaboration, is to address the main feasibility issues of the CLIC electron-positron linear collider technology by 2010. One key-issue studied in CTF3 is the generation of the very high current drive beam, used in CLIC as the rf power source. It is particularly important to simulate and control the drive beam longitudinal dynamics in the drive beam generation complex, since it directly affects the efficiency and stability of the rf power production process. In this paper we describe the ongoing effort in modelling the longitudinal evolution of the CTF3 drive beam and compare the simulations with experimental results.
UMAN, Manchester
Higher order modes (HOMs) are simulated with finite element and finite difference computer codes for the ILC superconducting cavities currently under investigation for the ILC. In particular, HOMs in KEK's Ichiro type of cavity and Cornel University's Re-entrant design are focussed on in this work. The aim, at these Universities and laboratories, is to achieve an accelerating gradient in excess of 50 MV/m in 9-cell superconducting cavities whilst maintaining a high quality and stable electron beam. At these high gradients, electrical breakdown is an important cause for concern and the wakefields excited by the energetic electron beams are also potentially damaging to the beam's emittance. Here we restrict the analysis to performing detailed simulations, on emittance dilution due to beams initially injected with realistic offsets from the electrical centre of the cavities and due to statistical misalignments of the cavities. We take advantage of the latest beam dynamics codes in order to perform these simulations.
SLAC, Menlo Park, California
Funding: Work supported by DOE contract DE-AC02-76SF00515.
SLAC's Advanced Computations Department (ACD) has developed the first high-performance parallel Finite Element 3D Particle-In-Cell code, Pic3P, for simulations of rf guns and other space-charge dominated beam-cavity interactions. As opposed to standard beam transport codes, which are based on the electrostatic approximation, Pic3P solves the complete set of Maxwell-Lorentz equations and thus includes space charge, retardation and wakefield effects from first principles. Pic3P uses advanced Finite Element methods with unstructured meshes, higher-order basis functions and quadratic surface approximation. A novel scheme for causal adaptive refinement reduces computational resource requirements by orders of magnitude. Pic3P is optimized for large-scale parallel processing and allows simulations of realistic 3D particle distributions with unprecedented accuracy, aiding the design and operation of the next-generation of accelerator facilities. Applications to the Linac Coherent Light Source (LCLS) rf gun are presented.
SLAC, Menlo Park, California
UMAN, Manchester
The CLIC linear collider aims at accelerating multiple bunches of electrons and positrons and colliding them at a center of mass energy of 3 TeV. These bunches are accelerated through X-band linacs operating at an accelerating frequency of 12 GHz. Each beam readily excites wake-fields in the accelerating cavities of each linac. The transverse components of the wake-fields, if left unchecked, can dilute the beam emittance. The present CLIC design relies on heavy damping of these wake-fields in order to ameliorate the effects of the wake-fields on the beam emittance. Here we present initial results on simulations of the long-range wakefields in these structures and on beam dynamics simulations. In particular, detailed simulations are performed, on emittance dilution due to beams initially injected with realistic offsets from the electrical centre of the cavities and due to statistical misalignments of the cavities.
LBNL, Berkeley, California
Laser driven plasma wakefields have recently accelerated electron beams with quasi-monoenergetic energy distributions and with gradients of ~100 GV/m. Stabilization and optimization of beam quality are now essential. Recent LBNL experiments have demonstrated control of self trapping, resulting in reproducible bunches at 0.5 GeV. Further optimization has been demonstrated using plasma density gradients to control trapping, producing beams with very low absolute momentum spread at low energies. Simulations indicate that use of these beams as an injector greatly improves accelerator performance and experiments are now underway to demonstrate such staging, which will be a crucial technology for laser driven linacs. This talk will cover recent progress in LWFAs to obtain more reproducible, higher quality beams and also cover staging prospects for high energy laser linacs.
SLAC, Menlo Park, California
UW-Madison/SRC, Madison, Wisconsin
Funding: The work of PE and JW was supported by the US Department of Energy under contract DE-AC02-76SF00515. The work of RAB and KJK was supported by National Science Foundation Award No. DMR-0537588.
For electron bunches driving a hard X-ray free electron laser, the electron bunch high qualities should be preserved as well as possible in the acceleration and compression. For typical configuration, the electron bunch is accelerated in rf cavity and compressed in magnetic chicane. Besides the rf curvature and high-order optics terms in a chicane, the collective effects during the bunch acceleration, transportation, and compression can further distort the phase space and even lead to instability. Among these collective effects, the coherent edge radiation dominates and governs the overall bunch property; while the longitudinal space charge is the main cause for microbunching instability. Random jitter couples to the wakefields and affect the final bunch properties. We study these effects and discuss their implication to general linac design and optimization.
CERN, Geneva
Recently the CLIC study has changed the operating frequency and accelerating gradient of the main linac from 30 GHz and 150 MV/m to 12 GHz and 100 MV/m, respectively. This major change of parameters has been driven by the results from a novel main linac optimization procedure. The procedure allows simultaneous optimization of operating frequency, accelerating gradient, and many other parameters of CLIC main linac. It takes into account both beam dynamics (BD) and high power rf constraints. BD constraints are related to emittance growth due to short- and long-range transverse wakefields. Rf constraints are related to rf breakdown and pulsed surface heating of the accelerating structure. The optimization figure of merit includes the power efficiency, measured as a ratio of luminosity to the input power as well as a quantity proportional to investment cost.
CERN, Geneva
KEK, Ibaraki
SLAC, Menlo Park, California
Demonstration of a gradient of 100 MV/m at a breakdown rate of 10-7 is one of the key feasibility issues of the CLIC project. A high power rf test program both at X-band (SLAC and KEK) and 30 GHz (CERN) is under way to develop accelerating structures reaching this performance. The test program includes the comparison of structures with different rf parameters, with/without wakefield damping waveguides, and different fabrication technologies namely quadrant bars and stacked disks. The design and objectives of the various X-band and 30 GHz structures are presented and their fabrication methods and status is reviewed.
Fermilab, Batavia
Northern Illinois University, DeKalb, Illinois
Euclid TechLabs, LLC, Solon, Ohio
ANL, Argonne
Funding: Work supported by US. Department of Energy, under Contract No. DE-FG02-06ER41435 with Northern Illinois University.
We present a diagnostics suite and analyze techniques for setting up the longitudinal beam dynamics in ILC electron injectors and bunch compressors. Techniques to measure first order moment and recover the first order longitudinal transfer map of the injector intricate bunching scheme are presented. Coherent transition radiation diagnotics needed to measure and monitor the bunch length downstream of the ~5 GeV bunch compressor are investigated using a vector diffraction model. We finally introduce a new diagnostics capable of measuring time-transverse correlation along a single bunch. Such a diagnostics should be valuable for controlling emittance dilution via transverse wakefield and for properly setting the crab cavities needed for maximizing luminosity for non-zero crossing angle at the interaction point.
SLAC, Menlo Park, California
Funding: Work supported by DOE contract DE-AC02-76SF00515.
The TESLA-shape cavity has been chosen as the baseline design for the 1.3 GHz SCRF linacs of the International Linear Collider. However, there is ongoing research to develop new cavity shapes that will support higher gradients and hence lower the machine cost. The critical magnetic flux (Bc) of the niobium, which is approximately 180 mT, ultimately limits the gradient achievable in a superconducting cavity. Thus far, the new designs have focused on minimizing the peak surface magnetic field (Bs) for a given on-axis gradient, while relaxing the requirement on the peak surface electric field (Es). For example, the Low Loss (LL) design reduces Bs by more than 10% relative to the baseline design, which should allow a gradient of up to 50 MV/m with a 20% reduction in cryogenics loss. However, Es is about 15% higher in this case, which enhances field emission that in practice is one of the main impediments to achieving the Bc-limited gradient. In this paper, we will present an optimized cavity shape that reduces both Bs and Es, and thus should have a better chance of reaching higher gradients. The design of HOM couplers for wakefield damping in this cavity will also be presented.
SLAC, Menlo Park, California
Funding: Work supported by the U.S. Department of Energy under contract DE-AC02-76SF00515.
I introduce a novel normal-conducting accelerator structure combining standing wave and traveling wave characteristics, with relatively open cells. I describe the concept and geometry, optimize parameters, and discuss the advantages and limitations this new structure presents.