New Electron Beam Diagnostics in the Flash Dump Line

Additional beam diagnostics has been installed in the dump line at FLASH in 2009. Its purpose is to prevent damage by long high current electron beam pulses, as happened in autumn 2008, when a vacuum leak occurred near the dump vacuum window. Beam position monitors (BPM), scintillator-based loss monitors and temperature sensors were installed so-far. Additional BPMs and loss monitors have meanwhile been installed. These include a magnetic coupled BPM placed after the vacuum window. Magnetic loops are used in order to prevent the influence of the ions on the pick-up signals. 4 ionization chambers, consisting of air-filled tubes, and 4 glass fibers have been installed parallel to the vacuum pipe, along the last 2 m of the beam pipe. Beam halo monitors were installed next to the magnetic BPM. These consist of 4 diamond and 4 sapphire sensors operating as solid state ionization chambers. The halo monitors are very sensitive to charged particles crossing the detectors. These additional diagnostic monitors have been commissioned in autumn 2009, when they have contributed to the successful run of long pulses with 3 to 9 mA current and up to 800 μs length. Their performance is summarized in this paper. INTRODUCTION FLASH (Free electron LASer in Hamburg) [1] is at the same time a user and a test facility. A laser light tuneable in the range 40 – 6.5 nm is produced for several user beam lines. During special studies periods, various experiments are scheduled, in order to make tests for various accelerator projects, in particular for the European XFEL (X-ray Free Electron Laser) [2] and the ILC [3] (International Linear Collider) study. These have similarities with FLASH, in particular the superconducting technology used for beam acceleration. Also, they all plan to use long bunch trains. The long bunch trains at FLASH have to be safely sent into the dump at the end of the linac. Beam loss monitors (BLM) and charge monitors are primarily used for the machine protection. This paper shows the limitations of the dump diagnostics used so far, describes the modifications made and summarizes the results of the initial commissioning of the new devices.