impedance

Paper	Title	Other Keywords	Page
MOPPH006	Longitudinal Wake Field for an Electron Beam Accelerated through a Ultra-High Field Gradient	electron, acceleration, undulator, radiation	26
	E. Saldin, E. Schneidmiller, M. V. Yurkov, G. Geloni DESY, Hamburg
	Electron accelerators with higher longitudinal field gradients can produce high-energy beams with compact, cheap setups. Laser-plasma acceleration appears to constitute the more promising breakthrough in this direction, delivering field gradients up to TV/m. Here we describe the impact of longitudinal wake fields on the electron beam, based on solution of Maxwell's equations for the longitudinal field. We consider an acceleration distance much smaller than the overtaking length (the length that electrons travel as a light signal from the tail of the bunch overtakes the head of the bunch), that is the case for laser-plasma devices. We give expressions for impedance and wake function that may be evaluated numerically. We show that the rate of energy loss in the bunch due to radiative interaction is equal to that of coherently radiated energy in the far-zone. A limiting expression is found for a large distance of the electron beam from the accelerator compared with the overtaking length. We derive analytical solutions for a Gaussian transverse and longitudinal bunch shape. We apply our analytical asymptote by studying the feasibility of a Table-Top FEL based on laser-plasma driver. Numerical estimations indicate that the effects of the time-dependent energy change induced by the longitudinal wake pose a serious threat to the operation of this device. (See DESY 06-222)
MOCAU04	Impact of Longitudinal Space-charge Wake from FEL Undulators on Current-enhanced SASE Schemes	undulator, space-charge, radiation, electron	196
	E. Saldin, E. Schneidmiller, M. V. Yurkov, G. Geloni DESY, Hamburg
	In this article we present a description of longitudinal wake fields in X-ray Free-Electron Lasers (XFELs) that is of relevance in relation with Enhanced Self-Amplified Spontaneous Emission (ESASE) schemes. We consider wakes in XFELs, in the limit when the electron beam has gone inside the undulator for a distance longer than the overtaking length (the length that electrons travel as a light signal from the tail of the bunch overtakes the head of the bunch). We find that the magnitude of the resulting energy chirp constitutes a reason of concern for the practical realization of ESASE schemes.
	Slides
WEPPH019	Determination of the Wakefield Budget for the FERMI FEL Undulator System	vacuum, undulator, electron, coupling	374
	C. Bontoiu, P. Craievich, L. Rumiz ELETTRA, Basovizza, Trieste M. Castronovo Università degli Studi di Trieste, Trieste R. Vescovo, A. A.G. Lutman DEEI, Trieste
	The FERMI project aims to achieve very high-brightness photon beam pulses of minimum bandwidth. These goals can be marred by the presence of large wakefields generated along the length of the undulator small-gap vacuum chamber. Estimations of the induced energy-spread caused by the resistive wall and surface roughness wakefields along the length of the vacuum chamber of the FERMI FEL undulator are presented. The energy spread and losses induced by the resistive wall wakefield are determined for three possible transverse geometries of the vacuum chamber, namely circular, rectangular and elliptical cross-section, while the energy spread and losses induced by the surface roughness wakefields are obtained for the circular cross-section case. In this last case in-house surface profile measurements are used to provide realistic estimates.