SLAC, Menlo Park, California
LBNL, Berkeley, California
Funding: This work was supported by U.S. DOE contracts DE-AC03-76SF00515 and under the auspices of the Office of Science, U.S. DOE under Contract No. DE-AC02-05CH11231.
Recently Stupakov* has suggested a scheme entitled echo-enabled harmonic generation (EEHG) for producing short wavelength FEL radiation that allows far higher harmonic numbers to be accessed as compared with the normal limit arising from incoherent energy spread. We have studied the feasibility of a single EEHG stage to generate coherent radiation in the "water window" (2- 4 nm wavelength) directly from a UV seed laser at ~200-nm wavelength. By adjusting the temporal overlap region of the two lasers producing energy modulation in the EEHG scheme, we find it may be possible to vary the duration of the output coherent soft x-ray pulse. We present time-dependent simulation results which explore these ideas and also examine the sensitivity of the scheme to various input electron beam parameters.
*G. Stupakov, Preprint SLAC-PUB-13445
SLAC, Menlo Park, California
Funding: This work was supported by US DOE contracts DE-AC03-76SF00515
The echo-enabled harmonic generation free electron laser (EEHG FEL) holds great promise in generation of coherent soft x-ray directly from a UV seed laser within one stage. The density modulation in the harmonic generation process is affected by the smearing effect caused by the fluctuations of energy and current along the beam, as well as the field error of the dispersive elements. In this paper we study the tolerance of the EEHG FEL on beam quality and field quality.
SLAC, Menlo Park, California
Stanford University, Stanford, California
UCLA, Los Angeles, California
Funding: This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-76SF00515.
SSRL and SLAC groups are developing a long-range plan to transfer its evolving scientific programs from the SPEAR3 light source to a much higher performing photon source that would be housed in the 2.2-km PEP-II tunnel. While various concepts for the PEP-X light source are under consideration, including ultimate storage ring and ERL configurations, the present baseline design is a very low-emittance storage ring. A hybrid lattice has DBA or QBA cells in two of the six arcs that provide a total ~30 straight sections for ID beam lines extending into two new experimental halls. The remaining arcs contain TME cells. Using ~100 m of damping wigglers the horizontal emittance at 4.5 GeV would be ~0.1 nm-rad with >1 A stored beam. PEP-X will produce photon beams having brightnesses near 1022 at 10 keV. Studies indicate that a ~100-m undulator could have FEL gain and brightness enhancement at soft x-ray wavelengths with the stored beam. Crab cavities or other beam manipulation systems could be used to reduce bunch length or otherwise enhance photon emission properties. The present status of the PEP-X lattice and beam line designs are presented and other implementation options are discussed.
SLAC, Menlo Park, California
BINP SB RAS, Novosibirsk
Funding: Work supported by US DOE contracts DE-AC03-76SF00515
We study the impedance due to coherent synchrotron radiation (CSR) generated by a short bunch of charged particles passing through a bend magnet of finite length in a vacuum chamber of a given cross section. Our method represents a further development of the previous papers*. In this method we decompose the electromagnetic field of the beam into the eigenmodes of the toroidal chamber. We derive a system of equations for the expansion coefficients in the series, and develop a numerical algorithm for practical calculations. We illustrate our general method by calculating the CSR impedance of a beam moving in a vacuum chamber of rectangular cross section.
*G. V. Stupakov and I. A. Kotelnikov, PRST-AB 6, 034401 (2003); T. Agoh, K.Yokoya, PRST-AB, 7, 054403 (2004)
SLAC, Menlo Park, California
Funding: Work supported by US DOE contracts DE-AC03-76SF00515.
A remarkable progress over the last decade in development of computer codes significantly advanced our capabilities in calculation of wakefields and impedances for accelerators. There are however a number of practical problems that, when approached numerically, require a huge mesh, and hence memory, or an extraordinary CPU power, or both. One class of such problems is related to wakes of ultra short bunches, typical for many next generation electron/positron accelerators and photon sources. Another class is represented by long shallow collimators and tapers, often with non round cross sections. The numerical difficulties with these problems can be traced to a small parameter in the system, such as, e.g., a ratio of the bunch length to the length of a taper. It is remarkably, however, that the same small parameter often allows developing approximate analytical methods that provide a simplified solution to the impedance problem. In this paper, we review recent results in the analytical theory of wakefields, which include calculation of the wakes of very short bunches, long transitions and some special cases of the resistive wall impedance.
SLAC, Menlo Park, California
Funding: Work supported by the U.S. Department of Energy under contract DE-AC02-76SF00515.
An L-band (1.3 GHz) sheet beam klystron that will nominally produce 10 MW, 1.6 ms pulses is being developed at SLAC for the ILC program. In recent particle-in-cell transport simulations of the 115 kV DC beam through the klystron buncher section without rf drive, a hose-type instability has been observed that is the result of beam noise excitation of transverse modes trapped between the rf cells. In this paper we describe analytical calculations and numerical simulations that were done to study the nature of this instability and explore the required mode damping and changes in the beam focusing to suppress it.
SLAC, Menlo Park, California
A proposed high-brightness synchrotron light source (PEP-X) is under design at SLAC. The 4.5-GeV PEP-X storage ring has four theoretical minimum emittance (TME) cells to achieve the very low emittance and two double-bend achromat (DBA) cells to provide spaces for IDs. Damping wigglers will be installed in zero-dispersion straights to reduce the emittance below 0.1nm. In this paper, we present a preliminary estimation of the threshold of the transverse mode coupling instability(TMCI). Three approaches have been used in the estimation and they agree well with each other.