title: First Realization of a Multi-Turn Energy-Recovery Mode at the S-DALINAC
Creators: name: Schließmann, Felix
Publisher: name: UNSPECIFIED
Year: 2024

description: The main topic of this work is the first realization of a multi-turn energy-recovery mode at the superconducting Darmstadt electron linear accelerator (S-DALINAC). In such an operation mode, electrons first pass the main linear accelerator (LINAC) multiple times to accumulate the energy gain per pass, which – for a given main-LINAC capability per pass – enables higher electron energies compared to a one-pass operation. During this acceleration, energy is withdrawn from the alternating electric fields located inside the cavities of the main LINAC and supplied to the electrons. After this multiple acceleration, the electrons pass the main LINAC for a multiple deceleration, while the number of passes for the acceleration and the number of passes for the deceleration are equal. Deceleration is possible by passing the alternating electric fields with a corresponding phase offset. During the deceleration, the process is reversed, that is, energy is withdrawn from the electrons and restored to the alternating electric fields and thus recovered in the main LINAC. Due to the use of superconducting cavities, the recovered energy is temporarily stored almost losslessly and can thus be almost completely recycled during the acceleration of subsequent electrons. Compared to the corresponding conventional operation, operating the main LINAC as an energy-recovery LINAC (ERL) therefore requires less externally provided radio-frequency power to accelerate a given number of electrons per time, that is, a given beam current. Consequently, operating the main LINAC as an ERL enables the acceleration of higher beam currents for a given externally provided radio-frequency power compared to the corresponding conventional operation. In summary, a multiple acceleration with an ERL enables both high electron energies and high beam currents, and thus high beam powers.

Multiple passing of the main LINAC is possible since its exit is connected to its entrance via recirculation beamlines. In a multi-turn energy-recovery mode at the S-DALINAC, beams are superimposed in at least one recirculation beamline. Such a common recirculation transport provides less degrees of freedom compared to a separate recirculation transport; however, the former transport scheme is more cost-efficient. Furthermore, a high machine efficiency can be achieved if the electron energy when leaving the injector LINAC (injector energy) is as low as possible since energy delivered to the electrons in upstream located sections cannot be recovered at the S-DALINAC and therefore cannot be recycled. However, a low injector energy results in an electron speed that differs significantly from speed of light, which leads to significant phase slippage during a main-LINAC pass, which in turn affects the acceleration and the deceleration, respectively. The occurring phase slippage in combination with the common recirculation transport makes beam-dynamics simulations necessary to determine a suitable working point.

This work covers the necessary preparations to realize a two-turn energy-recovery mode at the S-DALINAC, in particular beam-dynamics simulations and beamline modifications, as well as the successful realization of the operation mode itself. Within the scope of this work, wire scanners were constructed, installed and utilized for quasi--non-destructive beam-diagnostics while operating in the two-turn energy-recovery mode. Furthermore, beam-dynamics simulations have been conducted to study a potential three-turn energy-recovery mode at the S-DALINAC.