P-3: Betratron Radiation

Principle Investigator:
Stuart Mangles (JAI)

Contact in absentia:
Richard D'Arcy

Description and goals:
In wakefield accelerators the focusing forces due to the transverse profile of the wake can lead to transverse oscillations of the electron bunch known as “betatron oscillations”.  These oscillations produce radiation in the form of X-rays, in an analogous manner to X-rays produced in a magnetic wiggler or undulator. The X-ray radiation is usually characterised by a synchrotron spectrum, characterised by a critical energy that increases with increasing plasma density, electron beam energy and oscillation amplitude.  Betatron radiation can therefore be used as a diagnostic of the transverse behaviour of the beam inside the wakefield.

Betatron radiation is expected to be very significant in FLASHForward in the case of the wake being excited by a beam with a “head-to-tail tilt” as the rear of the bunch is then subject to a hosing instability.   Betatron radiation will also be produced in witness bunch injection experiments if the witness bunch is injected away from the drive beam propagation axis.  

The FLASHForward P-3 experiment intends to diagnose the betatron radiation produced in beam driven wakefield experiments and will provide important information about the accelerator performance. Deliberate excitation of betatron oscillations could also prove to be a source of useful X-rays which have unique characteristics – namely that they are both ultra-short and broadband in nature and are therefore ideally suited to X-ray absorption spectroscopy of rapidly evolving systems.