Experimental Demonstration of Double Emittance Exchange toward Arbitrary Longitudinal Beam Phase-Space Manipulations.

Many of the most significant advances in accelerator science have been due to improvements in our ability to manipulate beam phase space. Despite steady progress in beam phase-space manipulation over the last several decades, future accelerator applications continue to outpace the ability to manipulate the phase space. This situation is especially pronounced for longitudinal beam phase-space manipulation, and is now getting increased attention.

Generation of High-Power, Reversed-Cherenkov Wakefield Radiation in a Metamaterial Structure.

We present the first demonstration of high-power, reversed-Cherenkov wakefield radiation by electron bunches passing through a metamaterial structure. The structure supports a fundamental transverse magnetic mode with a negative group velocity leading to reversed-Cherenkov radiation, which was clearly verified in the experiments. Single 45 nC electron bunches of 65 MeV traversing the structure generated up to 25 MW in 2 ns pulses at 11.

In Situ Observation of Dark Current Emission in a High Gradient rf Photocathode Gun.

Undesirable electron field emission (also known as dark current) in high gradient rf photocathode guns deteriorates the quality of the photoemission current and limits the operational gradient. To improve the understanding of dark current emission, a high-resolution (∼100  μm) dark current imaging experiment has been performed in an L-band photocathode gun operating at ∼100  MV/m of surface gradient. Scattered strong emission areas with high current have been observed on the cathode.

Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure.

We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic-band-gap (PBG) accelerating structure at 11.700 GHz. It has been long recognized that PBG structures have the potential for reducing long-range wakefields in accelerators.

Observation of Field-Emission Dependence on Stored Energy.

Field emission from a solid metal surface has been continuously studied for a century over macroscopic to atomic scales. It is general knowledge that, other than the surface properties, the emitted current is governed solely by the applied electric field. A pin cathode has been used to study the dependence of field emission on stored energy in an L-band rf gun.

Schottky-enabled photoemission in a rf accelerator photoinjector: possible generation of ultralow transverse thermal-emittance electron beam.

We present a clear signature of the Schottky effect in a rf photoinjector using photons with energy lower than the Mg cathode work function. This signature is manifested by the shift in the rf phase angle for the onset of the detection of photoelectrons via single-photon absorption and allows for a reasonable estimate of the field enhancement factor. This is a viable method to generate an electron beam with very low thermal emittance and thus, a high brightness beam.