• O. Grimm, H. Delsim-Hashemi, L. Fröhlich
  DESY, Hamburg
• E. Chiadroni
  Universita di Roma II Tor Vergata, Roma

Various activities at the TTF linear accelerator at DESY, Hamburg, that drives the VUV-FEL are geared towards measuring the longitudinal charge distribution of electron bunches with coherent far-infrared radiation. Examples are beam lines transporting synchrotron or transition radiation to interferometers mounted inside or outside the tunnel, and studies of single-shot grating spectrometers. All such approaches require a good understanding of the radiation generation and transport mechanism and of the detector characteristics to extract useful information on the charge distribution. Simulations and measurements of the expected transverse intensity distribution and polarization of synchrotron radiation emitted at the first bunch compressor of TTF have been performed. The transverse intensity scanning provided for the first time at DESY a visual image of the footprint of terahertz radiation. Detector response measurements have been performed at the FELIX facility, Netherlands, for wavelengths between 100-160 microns, and first studies with blackbody radiation and band pass filters in the terahertz regime have been done at PTB, Berlin. The paper will summarize these results.

• L. Fröhlich, O. Grimm
  DESY, Hamburg

The longitudinal charge distribution of short electron bunches can be characterized by a measurement of their coherent far-infrared radiation spectrum. This paper will present the results obtained at the DESY VUV-FEL linear accelerator by observation of synchrotron radiation with a Martin-Puplett interferometer. The reconstructed bunch shapes are strongly asymmetric with a full width at half maximum of about 1 ps.

• O. Grimm, J. Feldhaus, J. Rossbach, E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Funding: University of Hamburg

A special electromagnetic wiggler generating infrared radiation in the range 1-200 microns is planned to be installed at the DESY VUV-FEL in Hamburg by autumn 2006. The device is located after the FEL undulators, using the spent electron beam. The purpose is two-fold: first, it will serve longitudinal electron beam diagnostics, similar to other methods currently investigated using the coherent emission of radiation at wavelengths similar to the bunch length, and second it will be used as a powerful (100 MW peak) source for short (few ps) infrared radiation pulses. The natural, perfect synchronization with the VUV pulses will allow for pump-probe experiments with high timing precision. This paper will give an overview of the project, including the infrared beam transport line.

• H. Delsim-Hashemi, O. Grimm, J. Rossbach, H. Schlarb, B. Schmidt, P. Schmuser
  DESY, Hamburg
• A.F.G. van der Meer
  FOM Rijnhuizen, Nieuwegein

Funding: DESY

FEL facilities are pushing to achieve higher peak currents mainly by means of compressing bunches longitudinally. This process defines a machine parameter that has to be fine-tuned empirically. Among the operational types of diagnostic tools for longitudinal phase-space are those based on IR spectroscopy. The most commonly used IR spectrometers at the FEL facilities are operating in the scanning mode and are not fast enough to be applicable for monitoring bunch compression. On the other hand, any non-scanning spectrometer may suffer from the low intensity that is available from coherent IR radiation in short time intervals in different wavelengths. The proposed "Single Shot Spectrometer" is based on using gratings as dispersive elements. Pioneering tests with a transmission grating have shown the feasibility of the concept. In a second step, a version with "Reflective Blazed Grating" will be tested and should allow getting the maximum available signal for the whole spectrum and improved resolution. Parallel to the study of optical parts, an array of pyroelectric detectors with integrated multi-channel readout is under development.