The bunch length has been compressed in two different ways so far. One relies on the velocity control of the beam so that the electrons in the tail can overtake the electrons in the head. The other way is introducing anisochronous beamline such as chicane so that the electrons having different energy take different paths. Although these methods show amazingly good performances, they have clear limitations in terms of energy control requirement, profile control capability, and energy control capability.
AWA suggested a new bunch compression method using EEX and its feasibility is experimentally demonstrated. Based on the transverse-to-longitudinal mapping process in EEX, transverse focusing is converted to the longitudinal compression. Contrary to the velocity bunching or the path length difference, the EEX based compression relies on the exchange. This scheme requires a transverse focusing from a quadrupole magnet instead of the energy control. At the same time, since all other transverse manipulations can be mapped into the longitudinal space, this compression method can control the bunch length, profile and the chirp simultaneously. AWA demonstrated this concept using the beam dynamics simulation and the experimental demonstration is planned in 2019 using double EEX beamline.
CSR Studies and Suppression
CSR is a well-known limitation for dispersive beamlines such as chicane. Obviously, it is a critical issue in double EEX beamline due to eight dipole magnets consisting the beamline. While the CSR impact on the beam manipulations can be significantly reduced by the asymmetric dogleg method suggested by AWA (simulation work), there was intolerable emittance growth in double EEX beamline.
AWA is actively working on development and experimental demonstration of new CSR mitigation methods. The experiment is planned in 2018–2019 to explore the CSR impact on the beam in the EEX beamline instead of a single dipole magnet. This experimental study would provide thorough understanding of beam quality degradation mechanism in the EEX beamline. AWA also have developed several different CSR mitigation methods such as asymmetric dogleg, shielding with the beam matching and leak exchange. These methods and its effectiveness will be experimentally demonstrated.
Double Emittance Exchange
An electron beam’s longitudinal emittance is usually higher than its transverse emittances.
While a single EEX beamline provides a high precision manipulation, EEX increases the transverse emittance after this beamline due to the aforementioned nature. The simplest way to avoid it is using two EEX beamlines as a manipulation section. Here, the first exchanger converts longitudinal phase space to transverse phase space. The middle section in between EEX beamlines allows transverse manipulations, and the second exchanger maps it back into the longitudinal phase space. In this way, we can keep the strength of EEX based manipulation while we bring the low transverse emittance back into the transverse phase space.
AWA is currently upgrading the installed single EEX beamline to a double EEX beamline. This beamline will be installed and commissioned in 2018. AWA plans to demonstrate double EEX based longitudinal profile tailoring, bunch compression, frequency downshift THz generation etc. Also, AWA is going to study CSR and its impact on the EEX beamline. Double EEX beamline and the AWA drive injector can only meet the widest range of user experiment requirements in terms of beam manipulation and charge level.
Longitudinal Bunch Shaping
When the electron beam interacts with its surrounding environment, it generates electromagnetic field such as wakefield for high gradient acceleration, synchrotron radiation for light sources, etc. Temporal properties of these electromagnetic fields are characterized by the temporal distribution of the electron beam. Therefore, the precise control of the temporal distribution is highly demanded.
AWA reported the first experimental demonstration of arbitrary temporal distribution shaping and the mitigation of aberrations in early 2017. The single EEXbeamline installed at AWA drive beamline was used to shape the temporal distribution by means of EEX. Several different masks were applied to control the transverse distribution and they were successfully mapped into the longitudinal space. As a continued work, AWA reported world highest transformer ratio (~5) from ramped bunch in early 2018. AWA plans to demonstrate an extreme transformer ratio (~10) with an advanced temporal distribution, high frequency temporal modulations for THz and double horn suppressed bunch compressions keeping up with the double EEX upgrade project.