Commissioning Status of the Fritz Haber Institute THz FEL

The IR and THz Free-Electron Laser (FEL) at the Fritz Haber Institute (FHI) of the Max Planck Society in Berlin [1] is designed to deliver radiation from 4 to 500 microns using a single-plane-focusing mid-IR undulator and a two-plane-focusing far-IR undulator that acts as a waveguide for the optical mode. A key aspect of the accelerator performance is the low longitudinal emittance, 200 Micropulse Length psec 1 5 1 10 Micropulse Repetition Rate GHz 1 1 & 3 Micropulse Jitter psec 0.5 0.1 Macropulse Length μsec 1 8 1 15 Macropulse Repitition Rate Hz 10 20 Normalized rms Transverse Emittance mm-mrad 20 20 Figure 2: Layout of the FHI accelerator section from the gridded gun (left) to the linac 2 output (right). The final design optimized the specifications of the linac that are most relevant for the IR and THz FEL performance. For instance, the maximum bunch charge of the micro-pulses, which are repeated at rate of up to 1 GHz, has been increased to 300 pC. In addition, the length of the electron macro-pulses has been increased to 15 μsec. The 3 GHz operation of Table 1 is not implemented at this time. Figure 3: Completed accelerator structures after brazing. Fabrication took place through the early months of 2011. The completed linac structures and sub-harmonic buncher are shown in Figure 3, following brazing. The design energy of the IR-output is more than 10 μJ per micro-pulse and more than 100 mJ per macro-pulse. This corresponds to an optimized output in terms of millijoule per micro-second, which is the figure of merit for many gas-phase spectroscopy experiments.