Accelerator Physics: HF-Couplers for bERLinPro prove resilient

For the measurement campaign, two couplers were mounted in a horizontal test position under a local clean room tent.

For the measurement campaign, two couplers were mounted in a horizontal test position under a local clean room tent. © A. Neumann/HZB

To generate the HF power, a 270 kW klystron is needed, among other things.

To generate the HF power, a 270 kW klystron is needed, among other things. © A. Neumann/HZB

In synchrotron light sources, an electron accelerator brings electron bunches to almost the speed of light so that they can emit the special "synchrotron light". The electron bunches get their enormous energy and their special shape from a standing electromagnetic alternating field in so-called cavities. With high electron currents, as required in the bERLinPro project, the power needed for the stable excitation of this high-frequency alternating field is enormous. The coupling of this high power is achieved with special antennas, so-called couplers, and is considered a great scientific and technical challenge. Now, a first measurement campaign with optimised couplers at bERLinPro shows that the goal can be achieved.

These couplers should supply the cavities with 230 kW in continuous operation at 1.3 GHz. The connection between the ultra-high vacuum of the cavities and the high-frequency transmission link must be guaranteed, both at liquid helium temperature (-269 degrees Celsius or 4 Kelvin) and at room temperature. In addition, clean room conditions must be maintained and particles down to the micrometre range must be removed. The power is to be transferred to the cavity by two couplers each, in order to reduce the individual load, but also to improve the stability of the electron trajectory in the accelerator.

High-performance couplers modified

Now, the team led by Axel Neumann from the HZB Institute SRF (Superconducting Radio Frequency Technologies) has been able to show that this goal is realistic. To do so, they modified the design of the high-performance couplers of a research group from the National Research Centre for High Energy Physics in Japan (KEK).

Measurements up to 45 kW

For the measurement campaign, two of the newly developed couplers were set up in tandem with a test box as a cavity substitute. The measurements started with low power, which was gradually increased up to 45 kW.  Initially, only short pulses were transmitted from the couplers to the cavity at longer intervals, here even up to powers of 100 kW. Then the intervals between the power pulses became shorter and shorter up to continuous operation.

Good News:  heat can be dissipated

The heat development was 0.25 Kelvin per kilowatt of power. At a final power of 120 kW, the material would heat up by about 30 degrees Kelvin. This is good news, because such amounts of heat are technically dissipatable through the planned cooling. "With the original Japanese design, the heat generation was higher by a factor of four than with our adapted form," explains Neumann.

Outlook: 120 kW

"We initially limited the measurements to power levels below 45 kW. Only when all couplers have been successfully tested at these powers will the next steps come. However, we are now very optimistic. If you extrapolate the figures, the coupler should indeed manage 120 kW in continuous operation without any problems," says Prof. Dr. Jens Knobloch, who heads the HZB Institute SRF Science and Technology.

Helmholtz-Programme for Accelerator Physics (ARD)

With its work on high-frequency cavities, HZB is contributing to the Helmholtz Association's research programme for accelerator physics (ARD = "Accelerator Research and Development"). A central topic of ARD is the development of superconducting high-frequency systems for accelerating high currents in continuous wave operation. Just recently, ARD was evaluated by an international panel and received top marks in all categories.

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