BUNCH DIAGNOSTICS WITHCOHERENTINFRARED UNDULATOR RADIATION AT FLASH Arik Willner ∗ , Oliver Grimm, Hossein Delsim-Hashemi, J¨ org Rossbach, University of Hamburg Bernhard Schmidt, DESY Hamburg The Free-Electron-Laser in Hamburg (FLASH) at DESY has been complemented by an electromagnetic infrared un- dulator as a new tool for analysing the longitudinal pro- file of the short electron bunches characteristic for FLASH using coherent diagnostic techniques. This undulator has a maximum K-Value of 44 corresponding to a maximum wavelength of 200μm at an electron energy of 500 MeV. For the characterization of the emitted radiation and for the analysis of correlations between machine or undulator pa- rameters and undulator spectra, an experimental set-up has been developed and installed in the FLASH Experimental Hall containing a dispersive spectrometer as a main instru- ment. The spectrometer is designed for THz spectra us- ing reflective blazed gratings as dispersive elements and a pyroelectric detector. The final goal is the reconstruction of the longitudinal bunch shape by tuning the undulator through the wavelength range of the electromagnetic de- vice and measuring the intensity of the infrared radiation. INTRODUCTION The Free-Electron-Laser in Hamburg (FLASH) is a high-gain FEL user facility at DESY operating in the pho- ton wavelength range from vacuum ultra-violet to soft x- rays. It is driven by a superconducting linac and the gen- eration of the FEL radiation is based on the SASE process. In autumn 2007 FLASH reached its design goals being an electron energy of 1 GeV and lasing down to 6.5 nm photon wavelength [1]. For a high-gain FEL like FLASH the energy of the elec- tron beam defines the photon wavelength assuming a fixed gap of the undulator. In addition, the longitudinal charge distribution within the bunch and the transverse electron beam size are crucial factors within the lasing process of FELs. This is based on the dependence of the FEL-gain on the peak current via the gain length L G which is defined as L G = Cγ  σ 2 t I 0  1/3 where C is a constant determined by the undulator, γ the relativistic factor, σ t the transverse RMS size of the bunch and I 0 the peak current of the charge distribution. There- fore, a small beam size (or emittance) and a high peak cur- rent are required to obtain a short gain length necessary for high-gain FELs. In order to obtain the high peak current, the electron bunch needs to be compressed. For a proper analysis of ∗ Corresponding author, e-mail: arik.willner@desy.de Figure 1: The infrared undulator installed between the FEL and the electron dump. the FEL process and operation, longitudinal bunch diag- nostics with high resolution for sub-picoseconds bunches are required. Since summer 2007 there is a new tool for longitudinal bunch diagnostics at FLASH. Behind the FEL, an electro- magnetic infrared (IR) undulator has been installed produc- ing radiation from 1-200 μm for an electron beam energy of 500 MeV [2]. The aim is to reconstruct the longitudi- nal bunch profile by tuning the undulator to certain wave- lengths and measuring the intensity at these points. The advantage compared to the broad band techniques is that the radiation intensity produced within the IR undulator at a certain wavelength is emitted within a small band width. THE EXPERIMENT The first step towards the usage of the new infrared un- dulator as bunch diagnostic tool has to be the spectral inves- tigation of the source which is presented in this paper (for detailed information see [3]). For this purpose a spectrom- eter has been designed and implemented in an experimental station at the new infrared beamline of FLASH. The Undulator The infrared undulator is implemented in the accelerator tunnel between the FEL undulators and the electron dump (see fig. 1). It is an electromagnetic device with 44 poles, a gap of 40 mm and a period length of 40 cm. With a maxi- mum current of 435 A a magnetic field of 1.2 T can be pro- duced. The wavelength of the emitted radiation can be calcu- lated by the undulator equation λ 1 = λ U 2γ 2  1+ K 2 2 + γ 2 Θ 2  (1) TUPC110 Proceedings of EPAC08, Genoa, Italy 06 Instrumentation, Controls, Feedback & Operational Aspects 1320 T03 Beam Diagnostics and Instrumentation