Relative bunch length measurements at the MAX IV Linac

The MAX IV Linac is capable of delivering short, below 100 fs, electron bunches to the Short Pulse Facility and the FemtoMAX beamline.

Detector responseThe electron bunches are compressed in two bunch compressors, both reducing the duration with up to a factor of 10. To tune the compression, we are developing a simple relative measurement technique.

The measurement depends on the fact that the form factor of the electron bunch reaches higher frequencies when the bunch is compressed. Our detectors act as band pass filters. The radiation spectrum of a short bunch has a higher amplitude in the band pass range. Thus the shorter the bunch the higher the signal on our detector until the response saturates, see figure on the left.

Results Phase scanfrom measurements after BC1 are presented in the figure to the right. On crest acceleration corresponds to 340 degrees. The electron energy was kept constant by varying the SLED filling time and phase. The results indicate maximum compression at 370 degrees.

We are currently installing a relative bunch length measurement setup downstream of our second bunch compressor. We want to find a good trade-off between detector sensitivity and band pass frequency. The bunches can be much shorter downstream of the second compressor compared to the first, hence the higher frequency. We will keep you posted.


Last updated: June 21, 2016