JCJC SIMI 5 - JCJC - SIMI 5 - Physique subatomique et théories associées, astrophysique, astronomie et planétologie

Longitudinal profile measurement of ultra-short electron bunches at a plasma accelerator – SP

Longitudinal profile measurements of electron bunches

How to use Smith-Purcell radiation to measure and monitor the length of electron bunches

A new diagnostic for novel accelerators

State of the art accelerators such as free electron lasers and beyond state of the art accelerators such as plasma accelerators require a carefull monitoring of the bunch length so that they can deliver their best performances. We are developping a diagnostic capable of doing that without destroying the beam.

Coherent Smith-Purcell radiation emission is a phenomena that occurs when a bunch of electrons passes a grating with a sufficiently short period. This radiation encodes the beam longitudial profile. A set of detectors around the grating can detect this radiation and use it to extract information about the profile of the beam.

During our data taking campaign in April we asked for 3 different accelerator settings with 3 different longitudinal settings et clearly saw different profiles for each of them. We also compared our technique with another longitudinal diagnostic available (but intercepting the beam) The profiles obtained we very similar.

Build a single shot longitudinal profile measurement système.

1 seminar, 4 conference contributions, 1 publication being written.

Smith-Purcell radiation offers the possibility to measure components similar to the Fourier transform of the longitudinal profile of ultra-short electron bunches. This information can be used to reconstruct the actual longitudinal profile of electron bunches. The measurement of such profile will be critical in the development of new particle accelerators such as drivers for Free Electron Lasers or plasma-wakefield accelerators. Indeed, at the latest conference, the community received our recent results with strong interest.

Building on our recent results, we propose to perform a systematic experimental study of Smith-Purcell radiation to validate theoretical predictions and numerical simulations in a real accelerator environment. These experimental studies will allow us to build a well understood Smith-Purcell radiation monitor to measure components in wavelengths of the longitudinal profile of ultra-short electron bunches. Another task will focus on converting these components in a reconstruction of the actual longitudinal profile.

To maximize our chances of success we will proceed by steps from the easiest to the most difficult environment where to perform such measurement. We will start by a systematic study at the end of the SOLEIL LINAC where we will easily get access to a large number of electron pulses. Theses pulses will be several picoseconds long and therefore the Smith-Purcell radiation expected from these pulses will be in a wavelength range (millimetric waves) where “optical” components can be built rather easily in a mechanical workshop such as the one available at LAL. The results of this study will allow us to build a single shot longitudinal profile monitor for these electron bunches in the picosecond range that will be tested at the same location.

This will be followed by a study at SPARC in Frascati (Italy) where we will have access to shorter pulses (and a tuneable length). The tests at SPARC will allow us to study in depth Smith-Purcell radiation in the hundreds femtoseconds range, leading to the construction of a single shot profile monitor suitable for that pulse length range.

In parallel we will continue tests on FACET at SLAC (USA). These tests which have already started will allow us to explore the low hundreds femtoseconds pulse length range and at a much higher energy. Theses tests together with the knowledge accumulated at SPARC and SOLEIL will allow us to build a single shot profile monitor suitable for FACET.

Gathering all our experience from the tests at SOLEIL, SPARC and FACET we will be able to move to the much more challenging environment of laser-driven plasma wakefield accelerators. There we will build a single shot longitudinal profile monitor to achieve our ultimate goal of measuring the longitudinal profile of the electron pulses produced by a laser-driven plasma accelerator. Unlike previous measurements performed on conventional accelerators, this measurement will have to deal with large variation from shot to shot and this is where the single shot nature of the monitor will be critical. It is important to stress that such longitudinal measurement in a single shot has never been attempted at a laser-driven plasma-wakefield accelerator. It will therefore provide a new observable dimension in the study of the performances of these accelerators.

Over the course of the project we will have used an easy range of wavelength to build firm foundations for this monitor and then moved by steps to more difficult wavelength ranges and settings to make very challenging measurements. The main deliverable will be a single shot longitudinal profile monitor for plasma-wakefield accelerators but as by-products we will also have produced designs of monitors interesting our partners at SOLEIL (for LUNEX5) and SPARC (for their FEL driver and their plasma accelerator) and we will have established an internationally recognised group at LAL working on diagnostics for plasma accelerators.

Project coordinator

Monsieur Nicolas Delerue (Laboratoire de l'Accélérateur Linéaire, CNRS/IN2P3 et Université Paris-Sud) – delerue@lal.in2p3.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LAL Laboratoire de l'Accélérateur Linéaire, CNRS/IN2P3 et Université Paris-Sud

Help of the ANR 429,936 euros
Beginning and duration of the scientific project: November 2012 - 36 Months

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