Michaela Arnold

First ERL Operation of S-DALINAC and Commissioning of a Path Length Adjustment System

The S-DALINAC is a thrice-recirculating electron accelerator, capable of being operated in energy-recovery linac (ERL) mode. The change between conventional, accelerating operation and ERL mode is done by changing the distance traveled by the bunches in the second recirculation beam line. This distance is increased or decreased by moving dedicated magnets on a rail system. These so called pathlength adjustment systems exist in all recirculation beam lines but only the stroke in the second recirculation beam line enables a phase shift of up to 360° of the RF phase. After having proven the functionality of the new systems the once-recirculating ERL operation was demonstrated successfully in August 2017. This contribution will introduce the path-length adjustment systems and present the commissioning of them. Results of the operation of the first ERL in Germany will be discussed.

ERL Mode of S-DALINAC: Design and Status

Recently, the S-DALINAC was extended by an additional recirculation beam line to a thrice-recirculating linear accelerator. This upgrade enables an increase of the maximum achievable energy close to its design value of 130 MeV as well as an operation as an ERL. The new beam line features a path-length adjustment system which is capable of changing the phase of the beam by a full RF phase and, thus, allowing to shift the timing of the electron bunches to the decelerating phase. The project comprises different aspects concerning the design (magnets, beam dynamics, lattice, etc.) and the construction work including the alignment done at the accelerator. This contribution presents a rough overview on the design, installation and status.

Beam Break-up Measurements at the Recirculating Electron Accelerator S-DALINAC

Beam break-up (BBU) instability is an important limitation to the beam current which can be accelerated in a superconducting linac. In particular recirculating machines and Energy Recovery Linacs (ERLs) have to deal with that problem. Therefore, it is important to find strategies for increasing the threshold currents of these machines. At the recirculating superconducting accelerator S-DALINAC at the Technische Universität Darmstadt BBU was observed in different recirculation schemes. As expected the threshold current increased for a higher beam momentum. But also an improvement by mixing the transverse phase space advance with three skew quadrupole magnets was measured. Furthermore measurements in a new ERL scheme at the S-DALINAC are planned for the future. INTRODUCTION In recirculating linear accelerators transverse beam break-up BBU can be a crucial limitation of the maximum beam current [1,2]. Also Energy Recovery Linacs (ERL), as a special type of recirculating machines, have to deal with that limitation [3]. BBU occurs when electron bunches travelling through an accelerating cavity excite a higher order dipole mode (HOM) in it. These modes can have a large quality factor and thus a long lifetime in superconducting cavities. The electro-magnetic field of such a mode deflects the beam and then the bunches start to oscillate around the design orbit of a recirculation loop. When they re-enter the cavity with that offset the same HOM can be excited even more and thus the deflection becomes larger. As the excitation strength of the HOM also depends on the bunch charge the beam current is limited. The formula, taken and modified from [4], shows the maximum beam current for an ERL with one HOM and one recirculation: