Z. Yusof

Observation of wakefield generation in left-handed band of metamaterial-loaded waveguide

We report on a design of a TM-mode based metamaterial-loaded waveguide. Network analyzer measurements demonstrated a left-handed propagation region for the TM11 mode at around 10 GHz. A beamline experiment was performed with the metamaterial-loaded waveguide. In this experiment, a 6 MeV electron beam passes through the waveguide and generates a wakefield via the Cherenkov radiation mechanism. We detected a signal in the left-handed frequency band at 10 GHz. This is an indirect demonstration of reverse Cherenkov radiation as predicted in the work of Veselago [Sov. Phys. Usp. 10, 509 (1968)] and discussed in the works of Lu et al. [Opt. Express 11, 723 (2003)], Averkov and Yakovenko [Phys. Rev. B 72, 205110 (2005)], and Tyukhtin et al. [IEEE, Proceedings of the PAC, 2007 (unpublished), pp. 4156–4158]. Cherenkov radiation in artificially constructed materials [metamaterials (MTMs)] can provide unusual engineered features that can be advantageous for particle detector design.

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.

Quasiparticle liquid in the highly overdoped Bi(2)Sr(2)CaCu(2)O(8+delta).

Results from the study of a highly overdoped (OD) Bi(2)Sr(2)CaCu(2)O(8+delta) with a T(c) = 51 K using angle-resolved photoemission spectroscopy are presented. We observe a sharp peak in the spectra near ( pi,0) that persists well above T(c), a nodal self-energy which approaches that seen for the Mo(110) surface state, and a more k-independent line shape at the Fermi surface than the lower-doped cuprates. This allows for a realistic comparison of the lifetime values to the experimental resistivity measurements. These observations point to the validity of the quasiparticle picture for the OD even in the normal state.

A novel method for the absolute fluorescence yield measurement by AIRFLY

One of the goals of the AIRFLY (AIR FLuorescence Yield) experiment is to measure the absolute fluorescence yield induced by electrons in air to better than 10% precision. We introduce a new technique for measurement of the absolute fluorescence yield of the 337 nm line that has the advantage of reducing the systematic uncertainty due to the detector calibration. The principle is to compare the measured fluorescence yield to a well known process—the Cherenkov emission. Preliminary measurements taken in the BFT (Beam Test Facility) in Frascati, Italy with 350 MeV electrons are presented. Beam tests in the Argonne Wakefield Accelerator at the Argonne National Laboratory, USA with 14 MeV electrons have also shown that this technique can be applied at lower energies.

Tunneling spectroscopy of Tl2Ba2CuO6

Abstract New results from tunneling spectroscopies on near optimally doped single crystals of Tl 2 Ba 2 CuO 6 (Tl-2201) junctions are presented. The superconductor–insulator–normal metal (SIN) tunnel junctions are obtained using the point-contact technique with a Au tip. The tunneling conductances reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with a consistent asymmetry, and weakly decreasing backgrounds. A rigorous analysis of the SIN tunneling data is performed using two different models for the d x 2 – y 2 (d-wave) density of states (DOS). Based on these and earlier results, the tunneling DOS of Tl-2201 has exhibited the most reproducible data that are consistent with a d-wave gap symmetry. We show that the dip feature at 2 Δ that is clearly seen in SIN tunneling data of Bi 2 Sr 2 CaCu 2 O 8+ δ is also present in Tl-2201, but at a weaker level. The gap values for crystals with a bulk T c =86 K are in the range of 19–25 meV.New results from tunneling spectroscopies on near optimally doped single crystals of Tl{sub 2}Ba{sub 2}CuO{sub 6} (Tl-2201) junctions are presented. The superconductor-insulator-normal metal (SIN) tunnel junctions are obtained using the point-contact technique with a Au tip. The tunneling conductances reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with a consistent asymmetry, and weakly decreasing backgrounds. A rigorous analysis of the SIN tunneling data is performed using two different models for the d{sub x{sup 2}-y{sup 2}} (d-wave) density of states (DOS). Based on these and earlier results, the tunneling DOS of Tl-2201 has exhibited the most reproducible data that are consistent with a d-wave gap symmetry. We show that the dip feature at 2{Delta} that is clearly seen in SIN tunneling data of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} is also present in Tl-2201, but at a weaker level. The gap values for crystals with a bulk T{sub c}=86 K are in the range of 19-25 meV.

The Argonne Wakefield Accelerator Facility capabilities and experiments.

A description of the Argonne Wakefield Accelerator is presented, pointing out the unique capabilities of the facility. A photocathode RF gun produces electron bunches with tens of nanocoulombs of charge, which are used to excite wakefields. A second photocathode RF gun generates electron bunches that are used to probe these wakefields. An overview of the experimental program carried out at the facility is also presented.

Metamaterial-loaded waveguides for accelerator applications

Metamaterials (MTM) are artificial periodic structures made of small elements and designed to obtain specific electromagnetic properties. As long as the periodicity and the size of the elements are much smaller than the wavelength of interest, an artificial structure can be described by a permittivity and permeability, just like natural materials. Metamaterials can be customized to have the permittivity and permeability desired for a particular application. Waveguides loaded with metamaterials are of interest because the metamaterials can change the dispersion relation of the waveguide significantly. Slow backward waves, for example, can be produced in an LHM-loaded waveguide without corrugations. In this paper we present theoretical studies and computer modeling of waveguides loaded with 2D anisotropic metamaterials, including the dispersion relation for a MTM-loaded waveguide. The dispersion relation of a MTM-loaded waveguide has several interesting frequency bands which are described. It is shown theoretically that dipole mode suppression may be possible. Therefore, metamaterials can be used to suppress wakefields in accelerating structures.

Kelvin probe studies of cesium telluride photocathode for AWA photoinjector

Abstract Cesium telluride is an important photocathode as an electron source for particle accelerators. It has a relatively high quantum efficiency ( > 1%), is sufficiently robust in a photoinjector, and has a long lifetime. This photocathode is grown in-house for a new Argonne Wakefield Accelerator (AWA) beamline to produce high charge per bunch ( ≈ 50 nC ) in a long bunch train. Here, we present a study of the work function of cesium telluride photocathode using the Kelvin probe technique. The study includes an investigation of the correlation between the quantum efficiency and the work function, the effect of photocathode aging, the effect of UV exposure on the work function, and the evolution of the work function during and after photocathode rejuvenation via heating.

High Gradient Wakefields in Dielectric Loaded Structures

Dielectric loaded wakefield structures have potential to be used as high gradient accelerator components. Using the high current drive beam at the Argonne Wakefield Accelerator Facility, we employed cylindrical dielectric loaded wakefield structures to generate accelerating fields of up to 86 MV/m, at 10 GHz. Short electron bunches of up to 86 nC are used to drive these fields, either as single bunches or as bunch trains. The structures consist of cylindrical ceramic tubes (cordierite) with a dielectric constant of 4.76, inserted into cylindrical copper waveguides. These standing‐wave structures have a field probe near the outer diameter of the dielectric, in order to sample the RF fields generated by the electron bunches. Monitoring the field probe signal serves to verify the absence of electric breakdown in the structures. MAFIA simulations are used to calculate the amplitude of the fields generated by the traversing electrons bunches.

A new high intensity electron beam for wakefield acceleration studies

A new RF photocathode electron gun and beamline have been built for the study of electron beam driven wakefield acceleration. The one and a half cell L-band gun operates with an electric field on the cathode surface of 80 MV/m, and generates electron bunches with tens of nanocoulombs of charge and rms bunch lengths of a few picoseconds. The beam diagnostics include a Cherenkov radiator and streak-camera for bunch length measurements, YAG screens for beam profile, integrating charge transformers (ICTs) for bunch charge, an energy spectrometer, and a pepper-pot plate for measurement of the transverse emittance. Measurements of the beam properties at various bunch charges are presented.