New beamline for electron bunch diagnostics

New beamline for electron bunch diagnostics

These copper disks are going to become transverse deflecting cavities for the new diagnostic beamline

A new diagnostic beamline connected directly to the MAX IV linear accelerator is under construction. It will enable time-resolved characterization of primarily the ultrashort electron bunches for the FemtoMAX beamline but will also be useful for other time-resolved experiments. The design of the highly specialized beamline components is to a large part done in-house. 

Head up and tail down
The linear accelerator accelerates electrons up to high energies. Short bunches containing 109 electrons are delivered from the linear accelerator to make X-ray pulses for the FemtoMAX beamline. The duration of the bunches is in the femtosecond (10-15 s) regime to enable high temporal-resolution measurements at the beamline. The short duration makes the bunches very challenging to characterize with time resolution as conventional detection devices are too slow.

In the new setup, two so-called transverse deflecting cavities (TDC) will make the acquisition of time-resolved data possible. They will in principle add an electromagnetic field that deflects the head of the electron bunch upwards and the tail down so that the first electrons hitting the beam profile analyzer will end up at the top of the screen and the last ones will end up at the bottom. The resulting streak gives a time-resolved measurement of the shape of the bunch but the method will also be used to characterize for example how emittance and energy vary as a function of time.

– Today we rely on calculations and relative measurements for the bunch length delivered to FemtoMAX says project leader Erik Mansten, the TDC is a way for us to verify what we deliver. It also helps us preparing the linac for a possible free electron laser in the future.

The TDC will introduce a time resolution on the beam position monitor

Collaboration and skill development
Some of the most challenging parts for the new diagnostics beamline are fabricated in-house and all the MAX IV engineering teams are involved. One example is the magnets which have been specially developed to make the beamline versatile and prepared for possible future add-ons. Another example is the copper radio-frequency cells that are the building blocks of the TDC structures.

– We tried to find a way to join the cells together without the traditional brazing, explains one of the mechanical designers Karl Åhnberg, as avoiding heat treatment keeps the properties of the copper more intact. We have tried different solutions, and it has been a bit of a trial-and-error process.

In the end, the team found a method where the radio frequency cavities are joined using screws. The method has the extra advantage of making the cavities easy to detach and replace if needed.

The next step is for the team in the mechanical workshop to machine and connect around five cells of the final design. If all goes well, the manufacturing will then continue and the rest of the in total 182 cells will be produced and installed.

The MAX IV Engineering unit is divided into five teams

  • Vacuum
  • Mechanical Design and Workshop
  • Stability, Alignment and Metrology (SAM)
  • Electrical Systems and PLC Automation
  • Technical Infrastructure

In total, the engineering unit has 31 employees