BEAM DYNAMICS OPTIMIZATION OF A NORMAL-CONDUCTING GUN BASED CW INJECTOR FOR THE EUROPEAN XFEL* H. Shaker † , H. Qian, S. Lal, G. Shu, F. Stephan, DESY, Zeuthen, Germany Abstract The European XFEL is operating up to 17.5 GeV elec- tron energy with maximum 0.65% duty cycle. There is a prospect for continuous wave and long pulse mode (CW/LP) operation of the European XFEL, which enables more flexible bunch pattern time structure for experi- ments, higher average brightness and better stability. Due to engineering limitations, the maximum electron beam energy in the CW/LP mode is about 8.6/12.8 GeV [1], which puts more pressure on the injector beam quality for lasing at the shortest wavelength. This paper optimizes the beam dynamics of an injector based on a normal- conducting VHF gun. INTRODUCTION The European XFEL operates in the pulsed mode at the moment. The duration of each RF pulse is max. 650 µs and the repetition rate is 10 Hz. The time interval between bunches inside a pulse is 220 ns. This result to 27000 bunches per second and 0.65% duty cycle. For future development, the CW/LP mode operation is considered. Recent study shows a maximum 25 μA average current capability of SRF cavities in the European XFEL [1]. Based on this, the optimal value for the CW mode is se- lected to be 100 pC bunch charge with 4us bunch time interval. The maximum achievable energy is about 8.6 GeV for the CW mode. For the LP mode the energy can be increased to about 12.8 GeV [1]. More pressure on the beam quality is at the lower energy of the CW mode for lasing at short wavelengths. This requires better beam quality in comparison to the pulsed mode. The pulsed mode injector is based on a normal-conducting photo- cathode RF gun which is located in the XTIN1 tunnel [2]. To have the ability to work in dual modes, a new injector is required which will be placed in the XTIN2 tunnel on top of the XTIN1 tunnel. The second injector should have more or less the same energy as the first injector which is above 120 MeV [2,3]. To have a complete super- conducting machine, the first choice is a super-conducting gun. This kind of injector is under study and test [4]. As a backup solution a normal-conducting gun based CW injector is studied at PITZ [5,6]. CW Normal conducting photo-cathode RF guns were developed at LBNL in last decade based on mature room temperature RF technology [7], and the main limitation is the high average heat loss which limits both the achievable cathode gradient and the gap voltage. From DC to gigahertz frequency range, VHF band frequencies give us a reasonable cathode gradient, gap voltage and surface heat power. The successful CW APEX gun operates at 187 MHz, 20MV/m cathode gradi- ent and 750KV gap voltage [7]. This gun is used as the electron source for LCLS-II at SLAC [8]. Based on the APEX experience, a 216.6 MHZ gun is under physics design at PITZ, and the cathode gradient and gun voltage are increased to 28 MV/m and 830 kV respectively[5, 6]. Besides, a 400 kV normal-conducting 1.3GHz buncher was developed [9]. To validate the gun and buncher design, a new injector dynamics optimization is presented in this paper. A multi-objective optimizer parallel processing code[10] based on the NSGA-II [11] algorithm is used to drive ASTRA [12] simulations for optimizing the injector. MAIN BEAMLINE ELEMENTS Figure 1 shows the conceptual beamline layout. To find the right amount of input parameters for the optimization algorithm we will talk about each component in the beam- line. The initial laser beam has a flat-top longitudinal distribution with 2ps rise/fall time. The transverse distri- bution is Gaussian truncated at 1σ. The laser duration and radius are variable in the optimizer. Two cathode thermal emittances are assumed for injector optimizations: 1mm.mrad/mm for conservative case and 0.5mm.mrad/mm for optimistic case. The first case is close to the Cs 2 Te cathode [13], and the second case is close to the multi-alkali cathode, such as K 2 CsSb and NaKSb [14]. Figure 1: Beamline Layout Gun and Main Solenoid To minimize the emittance growth due to space charge, the electrons should be accelerated into the relativistic regime after photoemission as fast as possible. Then the maximum cathode gradient and gap voltage is desirable, but the average heat loss and the peak electric field on cavity surface limits the cathode gradient. Based on APEX gun experience, a maximum 100 kW heat loss and 30 MV/m peak surface field are considered as the base- line parameters. Due to these limitations a gun with 28 MV/m cathode gradient, 832 kV gap voltage and 3cm gap length was designed [5, 6]. Only the gun phase is varia- ble during the beam dynamics optimization. The cathode position is assumed to be zero (z=0). The gun solenoid ___________________________________________ * Work supported by the European XFEL † hamed.shaker@desy.de 39th Free Electron Laser Conf. FEL2019, Hamburg, Germany JACoW Publishing ISBN: 978-3-95450-210-3 doi:10.18429/JACoW-FEL2019-WEP054 WEP054 452 Content from this work may be used under the terms of the CC BY 3.0 licence (© 2019). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI Electron Sources