Ultrashort electron bunch generation by an energy chirping cell attached rf gun

Abstract

We present a new design for a rf electron gun to be used in ultrashort (∼1 ps) electron bunch generation. Using both simulation and measurement we evaluated the principle of this new type rf gun and were able to confirm an ultrashort bunch generation. During simulation, a bunch length of less than 100 fs(rms) with a 100 pC=bunch charge was confirmed at the optimum operating condition. The principle is to produce a linearly distributed longitudinal phase space by using an attached output cell specially designed for energy chirping. Such phase space distribution can be rotated by the velocity difference in the bunch. We already fabricated an energy chirping cell attached rf gun and successfully observed 0.2 THz coherent synchrotron radiation, which corresponds to less than 500 fs bunch. Such an electron gun can be used as a compact THz light source and a new electron injector with an ultrashort bunch.

Characterization of THz radiation generated by ultra-short bunch from energy chirping cell attached RF electron gun

“…In velocity bunching, it is important how linearly the energy is chirped. In our previous study [16], we confirmed with simulation codes PARMELA [18] and GPT [19] that ECC RF gun can compress a 100 pC bunch into less than 100 fs. However, in our system, the RF power from the klystron is limited to 10 MW so that the maximum electric field on the cathode was expected to be 75 MV/m.…”

Compression of a photoinjector electron bunch in the negative-mass undulator

“…The simulation results are in good accordance with estimations for the length of the compressing region and for the spread in energy encountered in the process of the compression [formulas (23) and (24)]. In the case of 0.3 nC electron bunch, I ¼ 1 kA for l 0 ¼ 0.1 mm and I ¼ 100 A for l 0 ¼ 1 mm.…”

“…However, the negative-mass compression of electron bunches may be of value in itself. In principle, the well-developed methods of сompression of photoinjector bunches based on the energy chirping allow producing bunches with as short lengths as tens fs (∼10 −2 mm) [22][23][24][25]. However, they are successful only when the Coulomb forces inside the bunch are not so important.…”

“…As a special example, let us study the behavior of estimations (23) and (24) in the situation when the initial bunch length l 0 increases, whereas the total charge of the bunch Q ≈ Il 0 =c stays constant. In the case of a long electron bunch (r ¼ R=γ 0 l 0 ≪ 1), the factor of the weakening of the Coulomb field decreases with an increase in the bunch length as follows:…”

“…Note that the simplest model considered in this section predicts the zero compression factor l=l 0 for the "bunch" consisting of two particles, which is achieved at the distance equal to the quarter period of the motion along the phase-plane trajectory determined by Eq. (23). In order to develop this analysis to the case of a multiparticle bunch, one should consider separately the motion of every pair of particles, which are placed initially inside the bunch symmetrically (with respect to the bunch center) and have a distance between themselves δz 0 < l 0 [ Fig.…”