Beam Based Monitoring of the Rf Photo Gun Stability at Pitz

The stability of the photo injector is a key issue for the successful operation of linac based free electron lasers. Several types of jitter can impact the stability of a laser driven RF gun. Fluctuations of the RF launch phase and the cathode laser energy have significant influence on the performance of a high brightness electron source. Bunch charge measurements are used to monitor the stability of the RF gun phase and the cathode laser energy. A basic measurement is the so called phase scan: the accelerated charge downstream of the gun is measured as a function of the launch phase, the relative phase of the laser pulses with respect to the RF. We describe a method which provides simultaneous information on RMS jitters from phase scans at different cathode laser energies. Fluctuations of the RF gun phase together with cathode laser energy jitter have been measured at the Photo Injector test facility at DESY in Zeuthen (PITZ). Obtained results will be presented in comparison with direct independent measurements of corresponding instability factors. Dedicated beam dynamics simulations have been done in order to optimize the method performance. INTRODUCTION The stability of the phase in the RF photo gun is one of the most important specifications for the linac based FELs. The requirements on the RF phase stability are derived from the desired electron beam parameters such as bunch-to-bunch and pulse-to-pulse energy spread, the bunch compression in the injector, and the arrival-time of the beam at the undulators. The RF systems in the injector of the XFEL require tight control of the RF field in the gun. The RF launch phase stability is expected to be in the order of 0.1 deg for the phase [1]. The shot-to-shot stability in energy of the cathode laser pulses is expected to be 2% (RMS) for single pulses and 1% (RMS) averaged over a pulse train [1]. This determines the stability of the bunch charge, which could be slightly better than the cathode laser one due to space charge related effects. The photo injector test facility at DESY in Zeuthen (PITZ) develops electron sources for FELs like FLASH and the European XFEL at DESY in Hamburg. The stability of the electron source is one of the central issues of the research program at PITZ. This paper presents a method for precise monitoring of the gun stability, including RF phase and the cathode laser energy. PHOTO INJECTOR IN ZEUTHEN The PITZ photo injector consists of an L-band RF gun supplied with a cathode load-lock system and solenoids for space charge compensation. The cathode laser system is able to generate trains of electron pulses including temporal and transverse laser beam shaping. Further on the electron beam line contains a booster cavity and a big variety of beam diagnostics systems for the characterization of the electron beam at different energies. The RF gun cavity is a 11⁄2-cell normal conducting copper cavity, operated at a resonance frequency of 1.3 GHz with a peak power of up to ~7 MW. The RF power to the gun is supplied by a 10 MW multibeam klystron through two equal output ports. In front of the gun the RF pulses from both waveguides are combined using a custom T-shape combiner. No field pickups are available for the current gun cavity design. Before 2010 the control of the RF feed to the gun was realized via two directional couplers installed before the T-combiner. Cross-talking of both directional couplers under not wellknown resonance conditions of the gun cavity made the control of the RF field in the gun practically impossible. So, no routine feed back was available and only the feed forward had been used. After the facility upgrade in spring 2010 a 10 MW in-vacuum directional coupler has been installed after the T-combiner. Measurements of the combined RF pulses should provide a possibility for better control on the field in the gun closing a feedback loop for the amplitude and phase stabilization. The PITZ photo cathode laser system is developed by the Max-Born Institute (MBI, Berlin) and is capable to generate trains of flat-top pulses with up to 800 micropulses with 1 MHz frequency at 10 Hz repetition rate. An individual micropulse with a typical duration of ~20 ps (FWHM) and very short rise and fall time (~2 ps) has a wavelength of 257 nm, the pulse energy provides the possibility to emit high charge electron bunches (up to several nC) from Cs2Te cathodes. The master oscillator (MO) is one of the major components of the timing system at PITZ. Its fundamental frequency of 9.027775 MHz is used for timing and diagnostics and to generate harmonics for the synchronization of the low-level RF (144 harmonics – 1.3 GHz) and the photo cathode laser system (3 , 6 and 144 harmonics – 27, 54 MHz and 1.3 GHz correspondingly). A detailed description of the diagnostics available at PITZ can be found in [2, 3]. Most related to the subject of this work are bunch charge measurements, monitoring of the RF phase and amplitude in the gun and the laser pulse energy diagnostics. The bunch charge at PITZ can be measured using Faraday Cups (FCs) and Integrating ___________________________________________ [email protected] Current Transformers (ICTs) [4]. Whereas the FC being a ground-insulated copper absorber intercepts the whole electron beam, an ICT monitors the bunch charge without interception. The 70-ns output signals from ICTs give the charge with a precision of ~30 pC [2] and are more suitable to measure the bunch charge in the range from 100 pC to several nC. Faraday cups can measure much lower charges down to several pC with a precision of ~ 2 pC [2]. Recently more reliable RF measurements became available at PITZ. They are based on signals of forward and reflected waves obtained from antennas of the 10 MW in-vacuum directional coupler. The vector sum phase can be used to estimate the phase jitter in the RF gun. To monitor the cathode laser pulse energy an industrial energy meter is integrated in the cathode laser diagnostics system before the laser beam enters the vacuum beam line. The laser pulse energy fluctuations within the pulse train and shot-to-shot jitter is monitored by a photomultiplier tube (PMT). PHASE SCAN The RF gun phase scan for a given laser energy – measurement of the accelerated charge downstream of the gun as a function of the cathode laser launch phase – is one of the basic measurements to characterize emission properties of an RF photo gun. A phase scan measured at PITZ is shown in Fig.1. This measurement has been performed using the first FC (~0.8 m from the cathode). RF peak power in the gun is plotted at the right axis in Fig.1a, its mean value is ~1.09 MW. The main solenoid current of 210 A has been applied to focus beams with high energy at the FC location. The shape of the phase scan is impacted from many parameters of the gun. The space charge density at the cathode due to the laser temporal and transverse profiles at the given laser pulse energy determines the charge dependence in the phase range from the field zerocrossing to the phase of the maximum beam energy gain from the gun (-90 deg to -40 deg in Fig.1a) [2]. A Schottky-like effect – a charge production enhancement due to the presence of a high electric field at the cathode – contributes in an additional slope in the phase scan for the phases corresponding higher RF field during the photo emission (-40 deg to -20 deg in Fig.1a) [5]. The control of the cathode laser pulse energy is realized at PITZ by means of a polarizer based attenuator. By its rotation the laser transmission (LT) can be tuned in order to adjust the energy of the laser pulse hitting the photo cathode. The laser transmission scans for selected RF phases are shown in Fig.1b. Initial (linear) parts of these curves are typically used for the quantum efficiency (QE) determination, their further (nonlinear) behaviour is strongly influenced by the space charge effects during emission. PHASE SCAN FOR GUN STABILITY MEASUREMENTS Fig.2a shows a simulated phase scan. Standard PITZ gun conditions have been applied: electric field of 60 MV/m at the cathode, a flat-top cathode laser temporal profile with 20 ps FWHM and 2 ps rise and fall time, ~0.4 mm RMS laser spot size. These and all other fixed parameters were taken from a simulation setup optimized to minimize the beam emittance in the PITZ photo injector. Additionally a Schottky constant of 0.005 nC/(MV/m) has been used in these ASTRA simulations [6]. A 2D phase scan – simulated accelerated charge as a function of the RF phase and the laser pulse energy – is shown in Fig.2b. The value E qe ⋅ used for one of horizontal axis can be treated as a charge which could be extracted from the cathode if no space charge or Schottky-like effects would be applied. Zero RF phase on these plots refers to the gun phase with maximum mean energy gain (the beam energy is plotted at the right axis in Fig.2a).