FLUTE, A LINAC BASED THz SOURCE

We propose a versatile THz source named FLUTE (Ferninfrarot LinacUnd Test-Experiment) based on a 30 50 MeV S-band linac with bunch compressor, that aims at not only producing high field THz pulses but also at serving as a test facility to study accelerator physics issues. This source is an important step towards the planned ultra-broadband THz to mid infrared user facility TBONE. Special emphasis is put on studies of bunch compression as a function of bunch charge (0.1-5 nC) and of different generation mechanisms of coherent radiation (CSR, CER, CTR). This paper describes the design and layout of the proposed FLUTE machine and presents results of beam dynamic calculations with the tracking programs ASTRA and CSRtrack. In addition, calculations for the achievable peak electrical field and spectral characteristics for one version of the FLUTE layout are shown. THE CONCEPT OF FLUTE KIT (Karlsruhe Institute of Technology) has a long tradition in generating [1] and using [2] coherent IR and THz radiation. The coherent radiation is produced with the storage ring ANKA operating in the so-called low alpha mode: sub-picosecond electron bunches are produced by reducing the momentum compaction factor α of the stored beam. The sub-picosecond long bunches emit coherent synchrotron radiation. Recently, PSI and KIT discussed the possibility to build a compact linac-based coherent THz source named FLUTE. Built at KIT, it could eventually be used later for pump-probe experiments at the SwissFEL [3] while KIT develops TBONE [4], a THz source based on a superconducting linac. The general parameter list for FLUTE is max. repetition rate: 100 Hz pulse length: 0.2-1 ps max. pulse charge: 5 nC max. beam energy: 50 MeV The general layout of FLUTE is shown in fig. 1. With an RF laser gun operating at 3 GHz a single bunch with a bunch charge of up to 5 nC is produced. The bunch is accelerated in the 2 1/2 cell gun to 7 MeV. After a beam diagnostic area the beam enters a 3 GHz linac and is accelerated to about 45 MeV out of crest. The ∗[email protected] Figure 1: The concept of FLUTE with a 21/2 cell RF photogun and 3 GHz linear accelerator followed by a bunch compressor. The total length of FLUTE is about 12 m. bunch, originally several ps long, is afterward compressed in a bunch-compressor to about 300 fs. Different compressor layouts, e.g. with a two or a four magnet chicane, will be investigated. In the following, the four magnet version will be discussed. The source of the coherent radiation can be either the synchrotron radiation at the last bending magnet, edge radiation or transition radiation from a foil. The different source mechanisms will be studied for the FLUTE beam in theory and experiment. TECHNICAL LAYOUT OF FLUTE The photo-injector foreseen for FLUTE is the CTF3 gun developed for the CLIC test facility [5]. The electron beam is accelerated in the electron gun to 7 MeV. The gun cavity is powered by a 2μs long pulse from a klystron. The estimated 45 MW power from this klystron is split: 22 MW are sent to the gun and 20 MW to the 5.2 m long S-band linac with 156 cells. The maximum energy of the linac is 50 MeV. For the gun a Ti:Sapphire laser is foreseen with a pulse length of 1 to 4 ps, an energy per pulse of 200μJ, and a repetition rate of up to 100 Hz. With these parameters the CTF3 gun reaches gradients of up to 100 MV/m. The bunch charge can be as high as 5 nC, using large laser spot size of up to 10 mm full width.