Alexander Zholents

Experimental observation of energy modulation in electron beams passing through terahertz dielectric wakefield structures.

We report the observation of a strong wakefield induced energy modulation in an energy-chirped electron bunch passing through a dielectric-lined waveguide. This modulation can be effectively converted into a spatial modulation forming microbunches with a periodicity of 0.5-1 ps and, hence, capable of driving coherent terahertz radiation. The experimental results agree well with theoretical predictions.

Experimental demonstration of energy-chirp compensation by a tunable dielectric-based structure.

A tunable energy-chirp compensator was used to remove a correlated energy chirp from the 60-MeV beam at the Brookhaven National Laboratory Accelerator Test Facility. The compensator operates through the interaction of the wakefield of the electron bunch with itself and consists of a planar structure comprised of two alumina bars with copper-plated backs separated by an adjustable beam aperture. By changing the gap size, the correlated energy chirp of the electron bunch was completely removed. Calculations show that this device, properly scaled to account for the electron bunch charge and length, can be used to remove residual correlated energy spread at the end of the linacs used for free-electron lasers. The experimental results are shown to be in good agreement with numerical simulations. Application of this technique can significantly simplify linac design and improve free-electron lasers performance.

Generation of Picosecond X-Ray Pulses in the ALS Using RF Orbit Deflection

A scheme is proposed for producing ps length pulses of x-ray radiation from the Advanced Light Source (ALS) using two RF deflecting cavities. The cavities create vertical displacements of electrons correlated with their longitudinal position in the bunch. The two cavities separated by 180 degrees of vertical phase advance. This allows the vertical kick from one cavity to be compensated by the vertical kick of the other. The location of the cavities corresponds to the end of one straight section and the beginning of the following straight section. Halfway between the cavities a bending magnet source is located. The radiation from the bend can be compressed to ∼ 1 ps in duration.

Next-Generation X-Ray Free-Electron Lasers

Research frontiers for future free-electron lasers are discussed. Attention is given to ideas for improving the temporal coherence and obtaining subfemtosecond X-ray pulses. Improving brightness of the electron bunches is considered to be a major step forward for an electron beam accelerator simultaneously supporting multiple free-electron laser lines.

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.

Suppressing Parasitic Effects in a Long Dielectric Wakefield Accelerator

Dielectric wakefield acceleration is a promising concept for increasing the accelerating gradient above the limits of conventional accelerators. Although superior gradients are reported in short dielectric wakefield accelerator tubes, problems arise when it comes to efficiency and multi-meter long interaction lengths. Here we discuss possible issues and provide some solutions backed by simulations.

X-ray Capabilities on the Picosecond Timescale at the Advanced Photon Source

The Advanced Photon Source (APS) operates with a timing structure advantageous for ultrafast dynamics experiments and, as a result, X-ray time-resolved studies on the 100-picosecond timescale have flourished. The standard operating mode, 24-bunch mode, runs 65% of the time and a hybrid singlet mode runs 15% of the time, yielding a total of 80% of beamtime easily accessible for studies of ultrafast dynamics using laser-pump/X-ray probe techniques. This article highlights recent enhanced capabilities for time-resolved studies at the APS.

A compact soft x-ray free-electron laser facility based on a dielectric wakefield accelerator

We propose an ultra-compact soft X-ray superradiant free-electron laser at 30 nm driven by a 150-MeV beam. Its total length is approximately 30 m, which is 10 times shorter than that of an ordinary free-electron laser. The key concept is to laser modulate the electron emission at a photoinjector and then compress the electron macro-bunch by a linac and chicane magnet to achieve a soft X-ray bunching frequency in front of a freeelectron laser undulator. We calculated a sub-GW radiation power at 32.2 nm from a ~3 m long undulator for a 150 MeV beam with a 0.1% bunching factor, 0.15% energy spread, 4 mm-mrad emittance, and 10kA current.

Towards a Practical Multi-Meter Long Dielectric Wakefield Accelerator: Problems and Solutions

A multi-meter long collinear dielectric wakefield accelerator is considered, and it is shown that a single bunch breakup instability is a major limiting factor for obtaining highly efficient energy transfer from the drive bunch to the main bunch. Different methods for instability suppression are studied. Numerical simulations using a 6D particle tracking computer code are performed and tolerances to various errors are defined.

Transformer ratio enhancement with triangular beam

Transformer ratio, which is defined as the ratio of the maximum energy gain of the witness bunch to the maximum energy loss of the drive bunch, is an important concept in collinear wakefield acceleration (structure or plasma based). For gaussian drive beam transformer ratio equals to 2. For a given drive beam energy, higher transformer ratio acceleration means higher energy gain by the witness beam. A number of methods were proposed to increase transformer ratio by shaping the drive beam. At the Accelerator Test Facility we studied experimentally triangle drive beam and observed transformer ratio enhancement.