On Wednesday, 9th of August 2017 first ERL measurements have been conducted successfully at the S-DALINAC, making it the first accelerator in Germany running in this mode.

Figure 1 shows the path of the electron beam for this special setting. The beam is produced by a thermionic gun and pre-accelerated in the injector LINAC to a kinetic energy of 2.5 MeV (energy chosen for this setting). The beam then gains additional 20 MeV (energy chosen for this setting) during the first main LINAC passage before travelling through the second recirculation beam line. After being accelerated a second time by the main LINAC the beam is stopped with a final energy of approx. 42.5 MeV in a beam dump (blue). To run in ERL mode the electron beam has to be decelerated during its second passage of the main LINAC. Most of its energy is recovered to the cavities making it possible to dump the beam at injection energy in the “ERL-Cup” (green).

Figure 1: This diagram shows the beam path in a once recirculating setting (marked in red). Depending on the mode, the end of the beam path is either highlighted in green (ERL operation) or blue (twice accelerating mode). Elements needed for the ERL measurements (first main LINAC cavity in yellow, both cups in green and blue) are mentioned.

The change from a twice accelerated mode to ERL operation is conducted actively in the second recirculation. An increase or decrease of the distance travelled by the bunches shift the phase of the beam relatively to the RF phase by 180°. Figure 2 illustrates the principle of an ERL operation applied to the S-DALINAC layout.

Figure 2: Principle of ERL operation applied to the S-DALINAC. The first beam in the main accelerator is accelerated (red particles) while the second beam gets decelerated (green particles) due to a phase shift of 180°.

Figure 3 shows first results of our measurements. We determined the power consumption of our first main LINAC cavity (A1SC01, see Fig. 1) in four different settings:

  • ERL operation (green area)
  • No beam (red area)
  • One main LINAC passage (gray area)
  • Two main LINAC passages – twice accelerating operation (blue area)

In parallel the beam current was measured in two beam dumps (ERL-Cup and E0F1-Cup, see Fig. 1) to monitor the beam positions. As clearly shown in Fig. 3 the power consumption of both beams nearly cancels out in ERL operation while one or two accelerated beams need significantly more RF power

Figure 3: Comparison of four different settings in terms of power consumption of first main LINAC cavity A1SC01 and beam current on dedicated beam dumps. The power consumption of the beams nearly cancels out during ERL operation. In case of one or two accelerated beams significantly more RF power is needed.