TERRAHERTZ RADIATION SOURCE

1. LINAC CAVITY 1 LINAC CAVITY 2 PROGRESS ON THE COMMISSIONING OF ALICE, THE ENERGY RECOVERY LINAC-BASED LIGHT SOURCE AT DARESBURY LABORATORY C.Beard, S.Buckley, P.Corlett, D.Dunning, P.Goudket, S.Hill, F.Jackson, S.Jamison, J.Jones, L.Jones, P.McIntosh, J.McKenzie, K.Middleman, B.Militsyn, A.Moss, B.Muratori, J.Orrett, P.Phillips, Y.Saveliev, D.Scott, B.Shepherd, S.Smith, M.Surman, N.Thompson, A.Wheelhouse, P.Williams (STFC Daresbury Laboratory), D.Holder, P.Weightman (Liverpool Univ.), K.Harada (KEK) ENERGY RECOVERY ACHIEVED Full energy recovery has been established at 21MeV beam energy and several bunch charges up to 20pC. Higher bunch charges were not attempted because of the beam loading effects in the injector SC booster cavities Main linac RF power demand signals: without (left) and with (right) energy recovery TERRAHERTZ RADIATION SOURCE ALICE provides a THz source with coherent enhancement due to sub-picosecond bunch length. The final dipole in the compression chicane is the source of THz radiation A linear dependence on THz detector signal on the bunch train length was observed at constant bunch charge, and a clear quadratic dependence on bunch charge was observed at constant train length EMITTANCE MEASUREMENTS Only a limited number of emittance measurements were made in the injector beamline using slit scans. No attempts were made to minimise the emittance for each bunch charge. This and the existence of the field emission current probably accounts for significantly larger emittance values compared to that expected from the ASTRA model (~3mm at 80pC). Quadratic dependence of the THz signal amplitude on the bunch charge. Typical signal from THz detector (bolometer) The latest observations (August 2009) of the THz intensity at the bunch charge of up to 40pC indicate that the THz pulse energy can reach several tens of mJ. BEAM LOADING PHOTO-CATHODE PERFORMANCE A field emitter was found at the centre of cathode, its effect can be mitigated by first solenoid next to gun. This field emitter is likely to be responsible for a hole in the quantum efficiency map of the cathode, which disappears after the cathode re-caesiation PHOTO-GUN COMMISSIONING Beam loading in the booster was visible on the LLRF signals at train lengths of a few tens of microseconds and above 10pC. The effect is an energy droop of a few percent over the bunch train. Beam loading was observed on the Faraday cup in a dispersive section of the injector. The Faraday cup current is not constant due to beam loading. The gun operating voltage of 350kV was initially used for gun commissioning but there were several failures of the high voltage insulating ceramic joint. It was necessary to install a more robust but smaller inner diameter ceramic that reduced the maximum gun operating voltage to ~230kV LATEST NEWS Tweaking the LLRF system and manipulating the external quality factors of the booster cavities allowed the operation of the machine at ~40pC bunch charge and up to 100ms train lengths FE SOL-01 =2.6A FE SOL-01 =3.3A 20 pC beam SOL-01 =3.3A Faraday cup traces with the energy droop (left) and after RF tweaking was done (namely, increasing “Grad Loop Gain” on BC1) to alleviate the problem. 20pC; 4.8MeV. Images on YAG screen after booster at various SOL-01. The FE after the booster becomes acceptable at SOL-01 setting above ~3.3A. FUTURE DEVELOPMENTS IN 2009 INCLUDE IR-FEL FFAG CBS X-ray Source THz Studies An oscillator type IR-FEL will be commissioned later in 2009. The undulator (originally from JLAB) will have variable gap, allowing FEL tuning in the range 4-12 MeV for beam energies in the range 24-35 MeV. The FEL will be used to test energy recovery with a disrupted beam and provide FEL output for experiments. The facility could also be used as a test bed for now FEL concepts. Gun HV conditioning voltage as a function of the number of shifts Compton Backscattering X-ray source with photon energy of 15-30keV. The CBS source is powered by a terawatt IR femtosecond laser that can also be used as a stand-alone light source for a variety of experiments. A programme of THz studies is planned including the first experiments at the Tissue Culture Facility to determine the safe limits of human exposure to THz radiation Towards the end of 2009 experiments with EMMA, the first non-scaling FFAG, will commence and continue throughout 2010. See WEPC39 for further details www.astec.ac.uk