SHORT IMPULSE OF TRANSITION RADIATION EXCITED BY A RELATIVISTIC ELECTRON BUNCH ∗ N.I. Onishchenko ξ ,V.A. Balakirev, D.Yu. Sidorenko, and G.V. Sotnikov NSC “Kharkov Institute of Physics and Technology” Academic Str.1, Kharkov 61108, Ukraine ∗ Work supported in part by the STCU under project number 1569 ξ email: onish@kipt.kharkov.ua Abstract The theoretical investigations and simulation are performed for the emission of a relativistic electron bunch during the injection into a semi-infinite vacuum/dielectric waveguide. The exact solution of the problem is obtained for the first time. Contrary to the previously used saddle- point technique, we applied a number of successive conform transformations of integration area in order to carry out the inverse Fourier transformation. The power and the frequency spectrum of high intensity pulse of ultra- wideband transition radiation, excited by a finite size electron bunch in a vacuum waveguide, are calculated numerically. It is shown that in a dielectric waveguide the short pulse of Cerenkov wake field drifts behind the relativistic bunch with the group velocity. I. INTRODUCTION At present, much attention is focused on the problem of generation of short intense electromagnetic pulses whose frequency spectra width is comparable with their mean frequency, the so-called “ultra-wideband” (UWB) pulses [1]. The current interest to the problem of pulsed emission of high-power electromagnetic signals arises primarily due to their application in UWB radiolocation. Along with traditional methods of generation of short UWB pulses based on the use of UWB antennas (TEM- horns, spiral and biconical antennas, and others) the pulsed intense relativistic electron beams (IREBs) can be used. Short (and ultra-short) IREBs with durations of 1–100 ps, energies of 0.5–1 MeV, and peak currents of 1–100 kA can be either produced by transformation of a continuous IREB into a sequence of electron pulses (modulated beams) [2] or directly generated in high-current devices (explosive- emission diodes). For the most efficient generation of electromagnetic UWB pulses the non-resonant (impact) mechanisms of IREBs irradiation should be used, such as charging of a rod antenna by REB [5] or impact excitation of TEM-horn antenna by IREB [6]. High-power UWB pulses can be generated not only in direct processes of the excitation of UWB antennas by IREBs but also due to the effect of coherent transition radiation [5]. The transition radiation from an individual charged particle is one of the fundamental elementary radiation processes [6]. This effect occurs when a charged particle moves through the electrically non-uniform medium. The transition radiation effect can be greatly enhanced by using small dense bunches of charged particles [7]. The radiation from a bunch consisting of N particles is N 2 times more intense than that from one particle. Because of this effect, electron bunches provide efficient means of intense electromagnetic transition radiation generation. Here we theoretically investigate the excitation of UWB transition radiation during the injection of a short-pulsed IREB into a semi-infinite circular cross-section waveguide whose entrance (through which the beam is injected) is short-circuited by a conducting diaphragm. In such a waveguide, the large dispersion of electromagnetic waves is the pecularity of the transition radiation, so that the shape of a UWB pulse of transition radiation propagating in a waveguide will deform permanently. In the case of dielectric filling the intense Cerenkov wake field excitation can take place. It can be applied for wake- field acceleration of charged particles [8-9] or for radiation sources [10,11]. In the case of short-pulsed IREB the interference of big number of radial modes leads to the essential peaking of the wake field with formation of narrow spikes of alternative sign [12]. The spatial structure of excited field is determined by the spatially limited Cerenkov field and transition radiation field. II. NON-RESONANT WIDEBAND EMISSION IN A SEMI-INFINITE WAVEGUIDE We consider a semi-infinite (0 ≤ z < ∞ ) cylindrical metal waveguide of radius b which is filled with a homogeneous dielectric with permittivity ε. The waveguide input (z = 0) is short-circuited by a metal wall transparent to relativistic electrons. An axisymmetric monoenergetic electron bunch 0-7803-7915-2/03/$17.00 ©2003IEEE. 1250