Advanced linear accelerator design relies on Optical Transition Radiation (OTR) screens to measure beam spot size; for instance, such screens are foreseen in plasma based accelerators (EuSPARC or EuPRAXIA) or Compton machines (Gamma Beam Source at ELI-NP).
OTR angular distribution strongly depends on beam energy. Since OTR screens are typically placed in several positions along the Linac to monitor beam envelope, one may perform a distributed energy measurement along the machine. Such feature could be very useful to quantify the effect of beam loading when bunch trains are traveling in the accelerating structures (e.g. in Compton Sources). Furthermore, a single shot energy measurement can be useful in plasma accelerators to measure shot to shot energy variations after the plasma interaction.
This research aims to investigate the use of such technique in the above-mentioned machines to propose a cheap, reliable diagnostic tool applicable in next generation particle accelerators. It will start from the description of OTR radiation reported in literature; then we will design a set-up that can be hosted in a typical accelerator environment. Bench measurements of the designed optical lines are foreseen as well.
The innovation of our research is to apply the OTR based energy measurement to plasma based accelerator and to Compton multi-bunch machines.
For plasma accelerators, the shot to shot detection possibility of this technique can offer a reliable tool to tune the energy gain of the plasma channel and to monitor the stability of the process. Conventional spectrometer magnets require some time to find the proper current feeding the magnet and their operation is tedious when the beam energy is completely unknown or instable.
Moreover, all plasma based accelerators are nowadays facing the problem of staging, i.e. feeding a plasma channel with electron beams accelerated or generated with plasma as well. Such issue is fundamental for the application of this technology to high energy physics colliders, where the necessary high energy cannot be reached with a single plasma channel. A shot to shot diagnostics, occupying small space in the machine and distributed along the accelerating stages is surely recommended.
Concerning the Compton sources Linacs and others multi-bunch high intensity Linac, our research aims to asses a cheap and simple energy measurement distributed along the accelerator. For GBS it can be the only way to locate the accelerating sections preventing the achievement of the 0.2% energy spread along the bunch train required by the specifications. The usage of the gated camera to measure the energy spread along the bunch train also peculiar and it has never been tested elsewhere.
Another output of this research will be a detailed study of all the implementation issues; it will be done highlighting the limiting factor giving a quantitative estimation on the measurement quality parameters such as resolution, uncertainty, sensitivity.
The direct application to state-of-the-art machines (GBS Linac) or design study (EUPRAXIA) is an added value. It will allow comparison with other solutions and discussion within the international collaboration working on them.
Our results will be also interesting for the plasma acceleration experiment ongoing at SPARC_LAB. It can allow a direct implementation of the ideas (or part of them) that we will develop; preliminary measurement data will be available as a side result of the single shot emittance measurements planned.