VSR Demo project: Setting the stage towards variable pulse-length in storage rings

A novel approach to simultaneously create both long and short photon pulses for all beam lines through adding higher harmonic superconducting cavities (3rd and 3,5 harmonics) was proposed by HZB back in 2013. These cavities when combined with the existing 500MHz base cavities in the ring will generate the required RF beating enabling proper bunch compression for half of the buckets while keeping the other half unaltered. By making use of pulsepicking  echniques each individual user would be enabled to freely switch between high average flux for X-ray spectroscopy, microscopy and scattering or picosecond pulses up to 500 MHz repetition rate for dynamic studies. All these ideas crystallized into the so called BESSY VSR approach (variable pulse-length storage ring) which would be able to maintain the present average brilliance of BESSY II while opening and enhancing new capabilities for user accessible picosecond pulses at high repetition rate. This is described in the TDS report for BESSY VSR.

The extremely challenging nature of the project is recognized in the combination of several operation parameters such as high beam currents (300mA) and high cavity gradients (20MV/m) in a non pulsed operation regime (CW). As a result of such interaction effective damping of the beam excited high order modes energy and avoidance of coupled bunch instabilities represent the major challenge for such a machine. Unfortunately there is currently no world-wide precedent nor state of the art sound SRF technology capable of enabling such goals.

HZB is taking the lead in developing such “tools” and as a consequence the VSR DEMO project is created. The aim of this project is to develop, test and validate the new SRF technology required to enable VSR-like operation in a synchrotron ring. To this end new “High current HOM waveguide-damped” SRF cavities are being developed together with all the necessary ancillaries (high power couplers, HOM loads, ... ) required to build and commission a 2-cavity cryomodule in the SUPRALAB and bERLinPro facilities at HZB. The success of VSR DEMO will set the basis for a new frontier in SRF cavity operation and pave the way towards successful implementation of variable pulse lengths in storage rings.