The production of ultra-bright electron bunches using ionization injection triggered by two transversely colliding laser pulses inside a beam-driven plasma wake is examined via three-dimensional (3D) particle-in-cell (PIC) simulations. The relatively low intensity lasers are polarized along the wake axis and overlap with the wake for a very short time. The result is that the residual momentum of the ionized electrons in the transverse plane of the wake is much reduced and the injection is localized along the propagation axis of the wake. This minimizes both the initial thermal emittance and the emittance growth due to transverse phase mixing. 3D PIC simulations show that ultra-short (∼8 fs) high-current (0.4 kA) electron bunches with a normalized emittance of 8.5 and 6 nm in the two planes respectively and a brightness greater than 1.7 × 10 19 A · rad −2 · m −2 can be obtained for realistic parameters. The demonstration of the Linac Coherent Light Source (LCLS) as an X-ray free electron laser (X-FEL) [1] has given impetus to research on the fifth-generation light sources [2]. The goal is to make X-FELs smaller and cheaper while decreasing their wavelength and increasing their coherence and intensity. The FEL performance is partially determined by the brightness of the electron beam that traverses the undulator. The brightness is defined as B n = 2I/ǫ 2 n where I is the beam current and ǫ n is the normalized emittance of the beam. In order to make the length of the undulator needed to drive the SASE-FEL [3] into saturation, shorter, high current (∼kA), multi GeV electron beams with ǫ n ∼ 10nm will be needed. These emittances are an order of magnitude smaller than those from state-of-the-art photoinjector RF guns [4]. In this letter, we show the generation of ultrabright electron bunches using ionization injection triggered by two transversely overlapping laser pulses inside a beam-driven wake in plasma. In our scheme, the relatively low intensity lasers are polarized along the wake axis and overlap with the wake for a very short time. Particle-in-cell (PIC) simulations using OSIRIS [5] show that this geometry reduces the residual momentum of the ionized electrons in the transverse plane and localizes them along the propagation axis of the wake leading to an electron beam with a brightness greater than 10 19

The evolution of beam phase space in ionization-induced injection into plasma wakefields is studied using theory and particle-in-cell (PIC) simulations. The injection process causes special longitudinal and transverse phase mixing leading initially to a rapid emittance growth followed by oscillation, decay, and eventual slow growth to saturation. An analytic theory for this evolution is presented that includes the effects of injection distance (time), acceleration distance, wakefield structure, and nonlinear space charge forces. Formulas for the emittance in the low and high space charge regimes are presented. The theory is verified through PIC simulations and a good agreement is obtained. This work shows how ultra-low emittance beams can be produced using ionization-induced injection.The field of plasma based acceleration has experienced significant progress in the past decade [1]. GeV energy gain in centimeter-scale laser driven wakes (LWFA) has been achieved in many recent experiments [2][3][4][5]. In beam driven wakes (PWFA), high gradient acceleration has been sustained over meter-scale distances leading to more than 40GeV energy gain [6][7][8]. For future applications of wakefield accelerators such as FELs and colliders, the quality of the self-injected beams in plasma waves, namely the transverse and longitudinal emittances, need to be improved and controlled. Among the many injection schemes [9,10], ionization-induced injection methods have attracted significant interests due to its simplest and flexibility [5,[11][12][13][14][15][16]. However, the injection process involves complex phase space dynamics, and the achievable final beam quality strongly depends on this evolution process. This area of research is of fundamental importance for achieving beam quality well beyond what is achievable with current technology.In this letter, we examine carefully the effects that affect the beam phase space evolution in ionization-induced injection using a combination of theory and simulations. We found the evolution typically has three stages, and each stage can impact the final beam quality. In typical cases where the injection time is limited to few inverse plasma periods (2πω −1 p ) and the charge is low, the three stages are as follows. First, when ionization is occurring, the emittance of the injected beam grows quickly in time from the initial thermal emittance. Second, immediately following ionization, the emittance slowly decreases to a minimum value. Finally, the emittance again gradually increases to saturated values. If the ionization time is more than ∼ πω −1 p then the emittance grows to the saturated level during the first stage including an oscillatory behavior before it slowly decreases. In the "high" charge limit the emittance evolves monotonically towards the same saturated value.The theory reveals that the evolution in emittance described above is due to special longitudinal and transverse phase mixing of electrons born at different times.The derived expressions clearly show how the emittance dep...

The simulation optimization and an experimental demonstration of improved performances of mega-electron-volt ultrafast electron diffraction (MeV UED) are reported in this paper. Using ultrashort high quality electron pulses from an S-band photocathode rf gun and a polycrystalline aluminum foil as the sample, we experimentally demonstrated an improved spatial resolution of MeV UED, in which the Debye-Scherrer rings of the (111) and (200) planes were clearly resolved. This result showed that MeV UED is capable to achieve an atomic level spatial resolution and a approximately 100 fs temporal resolution simultaneously, and will be a unique tool for ultrafast structural dynamics studies.

With progress of photoinjector technology, thermal emittance has become the primary limitation of electron beam brightness. Extensive efforts have been devoted to study thermal emittance, but experiment results differ between research groups and few can be well interpreted. Besides the ambiguity of photoemission mechanism, variations of cathode surface conditions during cathode preparation, such as work function, field enhancement factor, and surface roughness, will cause thermal emittance differences. In this paper, we report an experimental study of electric field dependence of copper cathode quantum efficiency (QE) and thermal emittance in a radio frequency (rf) gun, through which in situ cathode surface parameters and thermal emittance contributions from photon energy, Schottky effect, and surface roughness are extracted. It is found the QE of a copper cathode illuminated by a 266 nm UV laser increased substantially to 1:5 Â 10 À4 after cathode cleaning during rf conditioning, and a copper work function of 4.16 eV, which is much lower than nominal value (4.65 eV), was measured. Experimental results also show a thermal emittance growth as much as 0:92 mm mrad=mm at 50 MV=m due to the cathode surface roughness effect, which is consistent with cathode surface morphology measurements.

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