Y.-E. Sun

Tunable subpicosecond electron bunch train generation using a transverse-to-longitudinal phase space exchange technique

We report on the experimental generation of a train of subpicosecond electron bunches. The bunch train generation is accomplished using a beam line capable of exchanging the coordinates between the horizontal and longitudinal degrees of freedom. An initial beam consisting of a set of horizontally separated beamlets is converted into a train of bunches temporally separated with tunable bunch duration and separation. The experiment reported in this Letter unambiguously demonstrates the conversion process and its versatility.

Photoinjector-generation of a flat electron beam with transverse emittance ratio of 100

The generation of a flat electron beam directly from a photoinjector is an attractive alternative to the electron damping ring as envisioned for linear colliders. It also has potential applications to light sources such as the generation of ultrashort x-ray pulses or Smith-Purcell free electron lasers. In this paper, we report on the experimental generation of a flat beam with a measured transverse emittance ratio of 100{+-}20 for a bunch charge of {approx}0.5 nC; the smaller measured normalized root-mean-square emittance is {approx}0.4 {micro}m and is limited by the resolution of our experimental setup. The experimental data, obtained at the Fermilab/NICADD Photoinjector Laboratory, are compared with numerical simulations and the expected scaling laws.

First observation of the exchange of transverse and longitudinal emittances

An experimental program to demonstrate a novel phase-space manipulation in which the horizontal and longitudinal emittances of a particle beam are exchanged has been completed at the Fermilab A0 Photoinjector. A new beam line, consisting of a TM(110) deflecting mode cavity flanked by two horizontally dispersive doglegs has been installed. We report on the first direct observation of transverse and longitudinal emittance exchange.

Generation of Relativistic Electron Bunches with Arbitrary Current Distribution via Transverse-to-Longitudinal Phase Space Exchange

We propose a general method for tailoring the current distribution of relativistic electron bunches. The technique relies on a recently proposed method to exchange the longitudinal phase space emittance with one of the transverse emittances. The method consists of transversely shaping the bunch and then converting its transverse profile into a current profile via a transverse-to-longitudinal phase-space-exchange beamline. We show that it is possible to tailor the current profile to follow, in principle, any desired distributions. We demonstrate, via computer simulations, the application of the method to generate trains of microbunches with tunable spacing and linearly-ramped current profiles. We also briefly explore potential applications of the technique.

Ugrades of beam diagnostics in support of emittance-exchange experiments at the Fermilab A0 photoinjector

The possibility of using electron beam phase space manipulations to support a free-electron laser accelerator design optimization has motivated our research. An ongoing program demonstrating the exchange of transverse horizontal and longitudinal emittances at the Fermilab A0 photoinjector has benefited recently from the upgrade of several of the key diagnostics stations. Accurate measurements of these properties upstream and downstream of the exchanger beamline are needed. Improvements in the screen resolution term and reduced impact of the optical systems depth-of-focus by using YAG:Ce single crystals normal to the beam direction will be described. The requirement to measure small energy spreads (<10 keV) in the spectrometer and the exchange process which resulted in bunch lengths less than 500 fs led to other diagnostics performance adjustments and upgrades as well. A longitudinal to transverse exchange example is also reported.

Angular momentum measurement of the FNPL electron beam

In the flat beam experiment at Fermilab/NICADD Photoinjector Laboratory(FNPL), it is essential to have a nonvanishing longitudinal magnetic field on the photocathode. The canonical angular momentum of the electron beam generated by this magnetic field is an important parameter in understanding the round to flat beam transformation. In this paper, we report our measurements of the canonical angular momentum, which is directly related to the skew diagonal elements of the beam matrix before beam is made flat. The measurements of the other elements of the beam matrix are also reported.

Demonstration of Transverse‐to‐Longitudinal Emittance Exchange at the Fermilab Photoinjector

Phase space manipulation techniques within two degrees of freedom are foreseen to enhance the performances of next generation accelerators such as high-energy physics colliders and accelerator based light sources. At the Fermilab A0 photoinjector, a proof-of-principle experiment to demonstrate the exchange of the transverse and longitudinal emittances is ongoing. The emittance exchange beamline consists of a 3.9 GHz normal conducting deflecting mode cavity flanked by two doglegs. Electron bunches with charges of 250 pC and energy of 14.3 MeV are routinely sent through the exchanger. In this paper, we report our latest results on the demonstration of emittance exchange obtained with significantly improved beam diagnostics. We also compare our experimental results with a simple numerical model.

Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line

We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunchs horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beam line to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations.

Beam dynamics simulations of the transverse‐to‐longitudinal emittance exchange proof‐of‐principle experiment at the Argonne Wakefield Accelerator

Transverse‐to‐longitudinal emittance exchange has promising applications in various advanced acceleration and light source concepts. A proof‐of‐principle experiment to demonstrate this phase space manipulation method is currently being planned at the Argonne Wakefield Accelerator. The experiment focuses on exchanging a low longitudinal emittance with a high transverse horizontal emittance and also incorporates room for possible parametric studies e.g. using an incoming flat beam with tunable horizontal emittance. In this paper, we present realistic start‐to‐end beam dynamics simulation of the scheme, explore the limitations of this phase space exchange.

Production of relativistic electron bunch with tunable current distribution

We proposed a novel method for tailoring the current distribution of relativistic electron bunches. The technique relies on a recently proposed transverse‐to‐longitudinal phase space exchange. The bunch is transversely shaped and the phase space exchange mechanism converts this transverse profile into a current profile. The technique provides a tool for generating arbitrary current profiles in a tunable fashion. We demonstrate, via computer simulations, the method and its application to tailor specific current profiles such as, e.g., linearly ramped profiles and train of femtosecond micro‐bunches that have application in plasma and dielectric wakefield accelerators.