Flat FLASH – The 3.9 GHz system acting on beam

One of the key components of the recent FLASH upgrade, the newly integrated 3.9 GHz rf system demonstrated for the first time its ability to flatten the energy distribution of the electrons in a bunch, the so called phase space linearization.

This will improve significantly the performance of FLASH by optimizing the creation of ultra short bunches with high peak current and the creation of uniform intensity bunches of adjustable length.

In the FLASH accelerator, ultra short bunches are obtained by a two stage bunch compression scheme based on acceleration of the beam off the rf field crest. That means that the acceleration gradient of each electron in a bunch depends on the position of the particle in the bunch. The superconducting accelerating modules of FLASH are operating at 1.3 GHz. The deviation of the 1.3 GHz rf field’s sine shape from a straight line is visible as a crescent moon in the bunch distribution. It leads to long bunch tails and reduces the peak current. The effect is eliminated by the third harmonic superconducting accelerating system operating at 3.9 GHz to linearize the rf field. The 3.9 GHz system also allows for the creation of uniform intensity bunches of adjustable length that is needed for seeded operation – another main objective of the FLASH upgrade.

The FLASH 3.9 GHz rf system is a joint international effort grouped round the two research centers DESY and Fermilab. A great bulk of the work has been performed by people in the Accelerator and Technical Divisions at Fermilab by constructing the superconducting accelerator module ACC39. Other US members of the TESLA Technology Collaboration like the Thomas Jefferson National Laboratory, the Argonne National Laboratory and the Cornell University contributed to the development of the 3.9 GHz module giving advice and performing production steps. Colleagues from INFN, Milano have provided technical advice as well. DESY contributed to the effort providing substantial subcomponents like the rf power, rf control and other electronics and getting this rf system into operation.

In April 2009, the module was transported from Fermilab to DESY. The first complete system test at DESYs cryomodule test bench in autumn 2009 already showed a very promising performance. Installation in the free-electron laser FLASH followed. Since beginning April the system is being commissioned together with all the other new components inserted in FLASH during the last shutdown.

Now, the commissioning of FLASH and its new components enabled a first examination of the longitudinal phase space distribution comparing the situation when the 3.9 GHz rf system is off and in operation. This demonstration of the action of the 3.9 GHz system on the beam represents a significant milestone in the on going commissioning effort at FLASH.