High Magnetic Field Microwave Conductivity of 2D Electrons in an Array of Antidots

We measure the high magnetic field ($B$) microwave conductivity, Re$\sigma_{xx}$, of a high mobility 2D electron system containing an antidot array. Re$\sigma_{xx}$ vs frequency ($f$) increases strongly in the regime of the fractional quantum Hall effect series, with Landau filling $1/3<\nu<2/3$. At microwave $f$, Re$\sigma_{xx}$ vs $B$ exhibits a broad peak centered around $\nu=1/2$. On the peak, the 10 GHz Re$\sigma_{xx}$ can exceed its dc-limit value by a factor of 5. This enhanced microwave conductivity is unobservable for temperature $T \gtrsim 0.5$ K, and grows more pronounced as $T$ is decreased. The effect may be due to excitations supported by the antidot edges, but different from the well-known edge magnetoplasmons.Comment: 4 pages, 3 figures, revtex

Beam Position Monitoring with Cavity Higher Order Modes in the Superconducting Linac FLASH

FLASH (Free Electron Laser in Hamburg) is a user facility for a high intensity VUV-light source [1]. The radiation wavelength is tunable in the range from about 40 to 13 nm by changing the electron beam energy from 450 to 700 MeV. The accelerator is also a test facility for the European XFEL (X-ray Free Electron Laser) to be built in Hamburg [2] and the project study ILC (International Linear Collider) [3]. The superconducting TESLA technology is tested at this facility, together with other accelerator components

Using Higher Order Modes in Superconducting Accelerating Cavities for Beam Monitoring

Dipole modes have been shown to be successful diagnostics for the beam position in superconducting accelerating cavities at the Free Electron Laser in Hamburg (FLASH) facility at DESY. By help of downmixing electronics the signals from the two higher order mode (HOM) couplers mounted on each cavity are monitored. The calibration, based on singular value decomposition, is more complicated than in standard position monitors. Position like signals based on this calibration are currently being in the process of being included in the control system. A second setup based on digitizing the spectrum from the HOM couplers has been used for monitoring monopole modes. The beam phase with respect to the RF has been thus monitored. The position calibration measurements and phase monitoring made at the FLASH are presented