Driven by the demand to suppress transverse beam instabilities and develop novel short pulse operation modes in the SPEAR3 storage ring, a wide-band transverse bunchby-bunch feedback system was recently commissioned for SPEAR3. The system was demonstrated to be sufficient to suppress the transverse coupled bunch instabilities caused by trapped RF modes in one of the in vacuum insertion devices. A new function of beam instability interlock was developed and is part of the machine protection system for the in vacuum insertion devices. In addition, the bunch-bybunch feedback system serves as an indispensable diagnostic tool that enables us to measure machine parameters, beam impedance, and characteristics of beam instability modes. In this paper, we describe commissioning and performance of the bunch-by-bunch feedback system at SPEAR3.

Establishing good initial quantum efficiency (QE) and reliable in-situ cleaning for copper cathode in the RF gun is of critical importance for the RF gun operations. Recent studies on the SLAC RF gun test bed indicated that the pre-cleaning (plasma cleaning) in the test chamber followed by copper cathode exposure to air for cathode change leads to a very low initial QE in the RF gun, and also demonstrated that without the pre-cleaning good initial QE >4×10−5 can be routinely achieved in the RF gun with the cathodes of QE <1×10−7 measured in the test chamber. QE can decay over the time in the RF gun. The in-situ laser cleaning technique for copper cathodes in the RF gun is established and refined in comparison to previous cleaning at the linac coherent light source, resulting in an improved QE and emittance evolutions. The physics of the laser cleaning process is discussed. It is believed that the reflectivity change is one of the major factors for the QE boost with the laser cleaning.

The Linac Coherent Light Source (LCLS) has used three copper photocathodes since its commissioning in 2007. Two of three copper cathodes had low initial quantum efficiency (QE) (<1×10) in the LCLS radio frequency (RF) gun. The two cathodes were exposed to the plasma cleaning in the cathode test chamber before installation in the RF gun. Recent studies at the SLAC RF gun test bed at the Accelerator Structure Test Area (ASTA) reveals that the pre-cleaning in the test chamber followed by cathode exposure to air for installation in the gun is the major factor leading to the low initial QE. All four cathodes, without the plasma pre-cleaning prior to the installation in the gun, have demonstrated initial QE>4×10 at the ASTA. Systematic studies also demonstrate that high-power RF gun operation provides an initial QE boost. In-situ laser cleaning for three new cathodes in the RF gun is extensively investigated, and a robust laser cleaning procedure is established at the ASTA with improvements of previous cleaning recipe for the LCLS cathode. The QE was shown to reproducibly evolved to >1×10 from about 4×10 immediately following the laser cleaning over ~3 weeks, a time much shorter than a few months for the previous laser cleaning for the present LCLS cathode. The intrinsic emittance of copper cathodes is recovered to the normal value within 1-2 days following the laser cleaning, much shorter than 3 weeks for previous laser cleaning for the present LCLS cathode. The experimental results at the ASTA, including comparison with the previous cleaning for the present LCLS cathode, are presented in the paper. Physics of the laser cleaning process and the evolution of the QE is discussed. INITIAL QE WITHOUT IN-SITU CLEANING IN THE RF GUN An RF gun test bed located at the SLAC’s Accelerator Structure Test Area (ASTA) has been constructed [1] to study photocathodes for the Linac Coherent Light Source (LCLS) injector cathode operations. The beamline of the ASTA gun test bed duplicates the existing LCLS injector gun system [2], consisting of a chirp-pulse-amplifier laser tripled to 253 nm wavelength, LCLS-type RF gun, a solenoid for emittance compensation, one pair of magnet correctors, a Faraday cup to measure the bunch charge, and a YAG screen to measure beam size and intrinsic emittance. Similar to the LCLS injector, the drive laser is configured for normal incident injection to the photocathode surface using a 45 in-vacuum mirror. The final electron beam energy from the RF gun is about 5.5 MeV. The Cause for Low Initial QE in the RF Gun The LCLS located at the SLAC National Accelerator Laboratory has been successfully operated for users for about 5 years. Its copper-based photo-injector has produced an ultra-low emitance and ultra-fast electron beam for the x-ray free electron laser (XFEL). Since its commissioning in 2007, three identical copper cathodes have been used for the LCLS injector operations with different initial quantum efficiency (QE) values. As illustrated in Fig.1, the first and third (present) LCLS cathodes had unexpectedly very low initial QE [3], about 5×10, while the second one had 6.5×10 of initial QE as expected. Lately, it is realized that both the first and third LCLS cathodes were exposed to the plasma cleaning in the test chamber before cathode exposure to air for installation in the RF gun, while the second one did not have this cleaning process. Very low QE measured in the test chamber drives to proceed to plasma cleaning for the cathode prior to the installation in the gun. During the cathode installation in the LCLS RF gun, the cathodes have to be exposed to air for about 3 minutes for the cathode change due to the lack of loadlock system. The recent observations at the ASTA RF gun reveal that the laser-cleaned areas are much more susceptible to the air exposure than the non-cleaned areas do. The ASTA RF gun is vented to nitrogen and then exposed to air for about 3 minutes to mimic the LCLS cathode change before its vacuum starts to be pumped down. Figure 2 (left) and (right) shows the QE maps before and after the RF gun vacuum venting to air, respectively. Before the RF gun vacuum venting, areas A, B, C, D, E, F, G and H on the cathode have been cleaned by the intensive laser, while the circled center area is not exposed to any laser cleaning. In Fig. 2 (left), bunch charge from areas A, B and C have been evolved to about 7500 units for a given laser energy, equivalent to 1×10 of QE, while the cathode center area has about 3500 units of the bunch charge for the same laser energy, equivalent to about 4×10 of QE. After gun vacuum venting, bunch charge productions from the previously cleaned areas A, B, C, D, E, F, G, and H were dropped to 1500-2000 units shown in Fig. 2 (right), equivalent to about 1-2×10 of QE, but the QE of the cathode center area still remains unchanged, at 4×10. The observations indicate that the cleaned or activated surface is susceptible to the air-exposure, revealing the pre-cleaning is the cause for low initial QE measured in the RF gun. Total four cathodes are characterized, which are not exposed to the pre-cleaning prior to the installation in the ASTA RF gun. All four ASTA cathodes have good initial QE in the RF gun ranging from 4×10 to 8×10, as illustrated in Fig. 1. We conclude that the pre-cleaning ____________________________________________ *The work is supported by DOE under grant No. DE-AC02-76SF00515. Proceedings of FEL2014, Basel, Switzerland THP030 Electron Bunch Generation and Manipulation ISBN 978-3-95450-133-5 769 C op yr ig ht © 20 14 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s process activates the cathode surface, extremely susceptible to the contaminations, such as air-exposure. Without plasma cleaning in the test chamber, high initial QE can be routinely achieved in the RF gun. Figure 1: Original QE of cathodes in the LCLS and ASTA RF guns. Figure 2: QE map before (left) and after (right) the gun vacuum venting. Before the gun venting, areas A to H are processed by laser cleaning while the center area is not cleaned by intensive laser. QE Impact from the High-Power RF Processing For LCLS operations all three photocathodes were characterized in a test chamber prior to the installation in the RF gun. The test chamber utilizes a broadband UV light source followed by a narrow-band monochromator to select the desired photon energy. The photoemission from the cathode under 2.6 kV/m of electric field is measured with nanometer. The QE measured in the test chamber is typically on the order of 1×10 at the desired photon energy of 4.91 eV (253 nm). The cathodes with the low QE observed in the test chamber are then directly installed in the ASTA RF gun. Surprisingly, following high-power RF processing all four cathodes in the gun have demonstrated an initial QE >4×10 under normal operation conditions, two orders of magnitude higher than in the test chamber. The Schottky effect enhances QE by less than an order of magnitude for a copper work function between 4.3-4.7 eV. The two orders of magnitude of QE enhancement in the RF gun therefore cannot be explained by the Schottky effect alone. It is noticed that the high-power RF operation boosts QE at the ASTA RF gun, as shown in Fig. 3. For a new cathode installed in the ASTA RF gun, the initial QE is 3×10 for the first day of electron beam operation. Then the QE increases by 50% after a few days of RF operations. It is logical to assume the RF processing conditions the cathode by removing surface contamination. In combination with the Schottky effect, a much higher QE is observed relative to measurements in a test chamber. Figure 3: QE map for a new cathode in the ASTA RF gun: 3×10 of peak QE for first day with turn-on electron beam (left), 4.5×10 for a week later (right). IN-SITU LASER-ASSISTED CLEANING DEVELOPMENTS IN THE ASTA RF GUN Laser-based cleaning techniques have been widely used to clean metal photocathodes, such as copper and Mg, for more than two decades. A high-intensity laser beam, interacting with the metal cathodes, may ablate the cathode surface, removing surface contamination and possibly changing the cathode reflectivity, thereby resulting in a QE increase. However, the laser cleaning may change cathode’s QE uniformity, thereby electron beam emittance, and also generate unwanted dark current or even deteriorate cathode’s crystal quality, if the laser cleaning process is too aggressive. The laser cleaning was performed for the present LCLS cathode in July 2011 [4]. The QE was evolved to 1×10 from 3×10 over a few months following the laser cleaning. Since then, 1×10 of the QE is essentially unchanged for three years to date for 24/7 users operation. The emittance was recovered to the normal value within three weeks following the laser cleaning. Although the previous LCLS laser cleaning was successful, two major concerns still remain. One concern is the reproducibility of the laser cleaning for different spots on the same cathodes and different cathodes. The other is the need to reduce emittance-recovery time and QE-evolution time following the laser cleaning with refinements of laser leaning process. These concerns are particularly important to deliver reliable and high quality electron beam for a high-impact machine for users, like the LCLS, which drives further studies at ASTA. THP030 Proceedings of FEL2014, Basel, Switzerland ISBN 978-3-95450-133-5 770 C op yr ig ht © 20 14 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s Electron Bunch Generation and Manipulation Improvements of Laser Cleaning at the ASTA

Online accelerator optimization is generally a multivariant nonlinear problem with considerable noise which require efficient and robust algorithms. In this study we evaluate the viability of several optimization algorithms and demonstrate the strength of the recently proposed RCDS method for online application with both simulations and experiments.

An accelerator test stand has been constructed at SLAC to characterize laser-assisted photocathode processing, electron beam emission physics and front-end RF gun performance. The objective of the research program is to identify definitive ‘recipes’ for high-reliability photocathode preparation resulting in persistent high quantum efficiency and low beam emittance. In this paper we report on timing, optics and instrumentation for the Ti:Sapphire drive laser and diagnostics for the electron beam.

The SPEAR3 control system nominally operates with the EPICS toolbox on top of VMS hardware. The simultaneous use of Matlab Middlelayer (MML) and Accelerator Toolbox (AT) allow for parallel, high-level machine control and accelerator physics applications that communicate with the control system via EPICS Channel Access (LabCA). While the majority of the MML and AT software is machine independent, site-specific high-level applications are also required to control the accelerator. This paper describes several such high-level application programs that have been developed for control and diagnostics at SPEAR3. Examples include a timedependent waveform display gui, beam steering applications, transport line optics correction, SR beam diagnostics and add-ons to the main MML routines.

A two-slit interferometer has been installed in the SPEAR3 diagnostic beam line to measure vertical beam size at a dipole source point. The unfocused visible light initially passes through a 3.5x6.0mrad aperture and expands to 100mm vertical height at the interferometer slits 17m from the source. For typical emittance coupling factors χ~0.3-0.5%, σy~20μm at the source point and a slit separation of 50mm produces fringe visibility V=0.7. Hence, a significant plot of fringe visibility vs. slit separation can be generated to infer source size via Fourier transform. In this paper we report on interferometer construction, beam size measurements, skew quadrupole coupling compensation and local coupling correction for the BL13 EPU.