Status of the APEX Project at LBNL

STATUS OF THE APEX PROJECT AT LBNL* F. Sannibale, B. Bailey, K. Baptiste, J. Byrd, C. Cork, J. Corlett, S. De Santis, S. Dimaggio, L. Doolittle, J. Doyle, P. Emma, J. Feng, D. Filippetto, D. Garcia Quintas, G. Huang, H. Huang, T. Kramasz, S. Kwiatkoswski, W.E. Norum, H. Padmore, C.F. Papadopoulos, C. Pappas, G. Portmann, J. Qiang, J. Staples, T. Vecchione, M. Venturini, M. Vinco, W. Wan, R. Wells, M. Zolotorev, F. Zucca, LBNL, Berkeley, CA, USA M. Prantil, M. Messerly, LLNL, Livermore, CA, USA C. Pogue, Naval Postgraduate School, Monterey, CA, USA Abstract The Advanced Photo-injector Experiment (APEX) at the Lawrence Berkeley National Laboratory is focused on the development of a high-brightness high-repetition rate (MHz-class) electron injector for X-ray FEL applications. The injector is based on a new concept gun, utilizing a normal conducting 186 MHz RF cavity operating in CW mode in conjunction with high quantum efficiency photocathodes capable of delivering the required repetition rates with available laser technology. The APEX activities are staged in 3 main phases. In Phases 0 and I, the gun is tested at its nominal energy of 750 keV and several different photocathodes are tested at full repetition rate. In Phase II, a pulsed linac is added for accelerating the beam at several tens of MeV to reduce space charge effects and measure the high-brightness performance of the gun when integrated in an injector scheme. At Phase II energies, the radiation shielding configuration of the bunker where APEX is located limits the repetition rate to a maximum of several Hz. Phase 0 is under commissioning, Phase I components under fabrication, and initial activities for Phase II are underway. This paper presents an update on the status of all these activities. APEX electron photogun is based on reliable and mature mechanical and RF technologies. The core of the gun, see Figure 1, is a NC copper RF cavity operating in CW mode in the VHF band at 186 MHz (7th sub- harmonic of 1.3 GHz). The cavity, when fed by ~ 100 kW of RF power, creates a field on the cathode of ~19.5 MV/m that accelerates the beam through the 4 cm gap at ~750 keV. The two major goals targeted by the gun design were the CW operation capability, and the low vacuum pressure performance (10 -11 – 10 -9 Torr) necessary to operate the sensitive high quantum efficiency (QE) semiconductor photo-cathodes required at those repetition rates. The relatively low RF frequency choice allowed addressing both of these needs. Indeed, the larger resonating structure associated with the VHF frequency decreased the heat load on the cavity wall at a level small enough to permit CW operation with conventional cooling techniques. Additionally, the long wavelength allowed opening significantly large slots on the cavity walls with negligible field distortion and creating the high conductance vacuum path required by the low pressure operation. INTRODUCTION APEX (the Advanced Photo-injector EXperiment) is an electron injector based on a normal-conducting (NC) continuous-wave (CW) RF photo-gun under construction at the Lawrence Berkeley National Laboratory (LBNL) [1-3]. The project is part of the R&D activities promoting the development of the Next Generation Light Source (NGLS), a soft x-ray light source based on an array of up to ten independently tunable free electron laser (FEL) beamlines [4, 5]. The NGLS design addresses the interest of a large scientific community in the XUV and soft x- rays requiring extremely high brightness sources with photon energies ranging from about few hundreds eV to few keV at repetition rates as high as ~ 100 kHz per beamline [6, 7]. Particularly challenging are the requirements for the electron injector to operate in a facility such as the NGLS [8]. Indeed, it must deliver beams at MHz repetition rate with the required high brightness over a broad range of charges per bunch. Such an injector presently does not exist, and APEX has been designed to address such a need. *Work supported by the Director of the Office of Science of the US Department of Energy under Contract no. DEAC02-05CH11231. FSannibale@lbl.gov Figure 1: The VHF gun cross-section. APEX is staged in 3 phases. In Phase 0 the VHF gun and a diagnostic beamline for cathode characterization are installed. Different photocathodes are tested at MHz repetition rate at the gun energy to define the best choice for a high repetition rate X-ray FEL. Before testing cathodes, several important milestones need to be demonstrated in this phase, including full RF condition of the gun cavity, measurement of the beam energy at the gun exit, and vacuum performance demonstration. In Phase I, an electron beam diagnostic suite is added to the Phase 0 layout, to allow a full 6D characterization at