Large Superconducting linacs such as the European XFEL and the International Linear Collider will operate with long bunch trains and heavily beam-loaded accelerating cavities. This workshop will address the special challenges of operating such machines. A central feature of the program will be results and lessons learnt from the '9mA studies' at FLASH during September 2009, with one goal of the workshop being to develop plans for future studies at FLASH. The 9mA program was initiated jointly by DESY and ILC-GDE in order to achieve the following objectives:

• Demonstrate bunch-to-bunch beam energy uniformity and pulse to pulse beam energy stability with long pulses at full beam loading
• Characterise performance limitations at gradients approaching quench, including Control requirements for gradient overhead and RF power overhead
• Measure cryogenic loads (HOM absorber studies)

WORKSHOP OBJECTIVES

The workshop will address special issues associated with running long bunch trains in the FLASH linac at DESY. Central to the workshop will be the joint DESY-ILC '9mA studies program' and long bunch-train operation for the FLASH photon users. XFEL-related studies at FLASH will also be considered. Goals for the workshop can be broadly summarized as follows:

1. Review results and lessons learnt from the most recent 9mA studies period in Sept 09.
2. Explore strategies for efficiently re-establishing stable long bunch-train operation with currents from 1mA to 9mA
3. Explore strategies for characterizing LLRF stabilization of beam energy at the limits of RF power and cavity gradient
4. Develop proposals for future studies at FLASH and identify associated preparatory work

The program will include a review of FLASH operation with long bunch-trains for ILC- and XFEL-related accelerator studies and for photon science experiments. A major part of the workshop will be dedicated to working sessions developed around four thematic Working Groups, described below.

Working Groups

A full day of the workshop will be dedicated to working sessions developed around four thematic Working Groups. The working group themes are briefly described below. There will be joint working sessions of the various groups to cover overlapping topics.

Working Group #1: FLASH setup, tuning, and operation (Leaders: B. Faatz, J. Carwardine)

During the September 2009 studies, it took more than 10 shifts of expert tuning to achieve low-loss transmission of several hundred 3nC bunches. To make efficient use of future studies periods, it will be necessary to tune the machine reliably and consistently much more rapidly. This group will consider what is necessary to accomplish that, such as: identifying ‘golden’ working points using detailed information archived in the FLASH DAQ; tuning strategies; procedures and automation; parameter reproducibility and stability; diagnostics; accelerator modeling; extrapolation to XFEL. Differences between 1nC and 3nC bunch charges should be considered.

Working Group #2: FLASH feedback and control (Leaders: H. Schlarb, V. Ayvazyan)

Robust feedback systems are crucial achieving stable and repeatable performance. The LLRF systems regulate the cavity vector sums using feedback within the bunch train and by adjusting the RF drive waveforms pulse-to-pulse (‘adaptive feed forward’). Beam-based feedbacks are used to stabilize the bunch arrival times and compression factors at the end of the bunch compressor chicanes and the beam orbits from injector to beam dump. Topics for this group include: examining the performance and robustness of these feedback systems and considering possible improvements to algorithms and implementation; exception and error handling.

Working Group #3: ILC studies at FLASH (Leaders: N. Solyak, S. Michizono)

The FLASH 9mA study program was jointly initiated by DESY and the ILC Global Design Effort in order to demonstrate and study operation of the Tesla/ILC superconducting accelerating cavities with full beam loading and long bunch trains. The study program is focused on characterizing operations limitations to achieving ILC energy stability requirements for the main linac and bunch compressors: LLRF performance; accelerating cavity gradient operating margins; RF power operating margins. The group will evaluate data from the September studies and develop proposals for future future studies.

Working Group #4: DAQ and data analysis (Leaders: T. Wilksen, N. Arnold)

This working group will focus on tools and techniques for analyzing accelerator data from the FLASH DAQ. The emphasis will be on usage of the DAQ data rather than on detailed DAQ system implementation. Specific analysis use-cases will be developed ahead of time and then refined in collaboration with the other working groups. The group will explore implementation of these use-cases. Requirements for online and operations tools and offline analysis toolkits will be explored.