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Investigations into Advanced Beam Instrumentation for the Optimization of Particle Accelerators

Final Report Summary - DITA-IIF (Investigations into Advanced Beam Instrumentation for the Optimization of Particle Accelerators)

Accelerators are ubiquitous devices (there are more than 30,000 worldwide) that are used to accelerate charged particles (electrons, protons and ions) to high energies. Both the charged particle beams (CPBs) themselves and the radiation they produce are used for myriad of important applications, e.g. medical therapies, materials processing, food sterilization, etc. as well as to advance our fundamental understanding of the universe. For example, the XFEL accelerator at DESY/Hamburg produces extremely short x-ray pulses that are used to study the structure and motion of complex molecules, e.g. proteins; and the Large Hadron Collider at CERN, the highest energy accelerator in the world, is used to study the ultimate building blocks of matter. To effectively produce, control and transport any CPB to its intended target, it is essential to diagnose the properties of the beam, e.g. its size, divergence, bunch length, energy and brightness.
The goals of the DITAIIF Project are to develop advanced accelerator diagnostics based on optical radiation produced as a CPB interacts with a material object or a magnetic field. The project has four objectives: 1) significantly improve the dynamic range (DR) and resolution of current beam imaging systems; 2) develop an all optical method to measure and map the area in phase (angle-position) space occupied by the beam particles - the density of latter determines the brightness of the beam; 3) develop a non-invasive beam imaging system using optical ‘diffraction’ radiation produced as the beam moves through an aperture; and 4) develop a non-interceptive, single shot bunch length monitor using coherent diffraction radiation produced from an ensemble of charged particles. Accomplishing all of these objectives requires the development of novel experimental and computer simulation tools, as well as performing experiments at international accelerator facilities.
During the two year period of the grant we have modified and reprioritized some of these objectives but have made significant progress in achieving all of them; and we have added and are close to achieving a new goal that is synergistically related to the original objectives. To achieve all our goals we have: 1) developed new computer simulation tools to guide our experiments; 2) developed several specialized optical systems to test and optimize experimental diagnostic systems at the Cockcroft Institute (CI); 3) established important scientific collaborations with a number of international accelerator research institutes and laboratories in the UK (RHUL and DIAMOND) and abroad (CERN, PSI, KEK, SLAC and ANL); 4) executed or planning experiments at many of these laboratories; and 5) proposed a number of beam diagnostics for the AWAKE project at CERN that complement our original research objectives.

Specific Research Accomplishments
1) To meet the first three objectives we have developed a suite of optical systems that optionally employ an optical device called a Digital Light Processor (DLP), an array of electronically controlled micro-mirrors. This device can used as an optical mask to improve the dynamic range of the imaging system and also to spatially filter (select) portions of the light produced by the beam. Several DLP based imaging systems have been built by us at CI to 1- compare the DR achievable with a DLP to that of a highly specially camera being developed by CIDTEC as well as a commercially available SCMOS camera; 2- test the concept of laser beam shaping for CPB source control; and 3- test the performance of optical setups we have been developing for high resolution beam imaging using optical transition and diffraction radiation produced as the beam interacts with a foil or aperture, respectively, at KEK in Japan.
2) To understand the effect of all the components of our prototype diagnostic systems on the resolution and dynamic range of a beam image we employ an optical simulation code (Zemax) to predict the ‘point spread function’ (PSF), i.e. the image produced by a point source of light. This enables us to systematically understand the performance of each element and develop strategies to reduce or control optical aberrations.
3) To compare simulations with measurements we have developed a laboratory ‘point source’ in our optics lab at CI, by a focusing a very high quality laser down to a 0.003 mm diameter spot, and use it to measure the PSF of our optics. This tool has successfully been applied to test and optimize the design of high resolution imaging systems used during 2016 at KEK and continues to be developed in the post grant period. We note that this source and the associated high dynamic range imaging methods we have developed in the course of this grant can be used to measure the PSF of any optical system and therefore the technology we have developed in DITAIIF it is an important accomplishment and contribution to the optical science and optical engineering communities in its own right.
4) We have developed an optimized imaging system to fulfil the third objective of our grant, i.e. to develop a non-interceptive beam imaging diagnostic using ‘optical diffraction radiation’ produced as a CPB passes through an aperture. The first experiments to image both ODR and OTR from a very small (0.001 mm) electron beam were done in June of 2016 at KEK and are continuing beyond the grant period. These highly successful experiments have led to the development a new strategy to characterize and optimize OTR and ODR imaging to reach their theoretical resolution limits. Simulation studies are nearly completed which will annunciate this new strategy and an experiment is being planned for January 2017 at KEK to verify the simulation work. This research has very important applications to the development of future accelerators, where beam sizes are expected to reach submicron dimensions.
5) To meet the fourth objective, i.e. to develop a non-invasive, single shot CPB bunch length monitor, we have performed experiments at the ALICE accelerator in Daresbury and SLAC’s FACET accelerator to image ‘coherent diffraction radiation’ (CDR) from a bunch of charged particles passing through an aperture. The goal of these experiments is to demonstrate that the angular and spatial distributions of CDR can be used directly to measure the CPB bunch length. This technique is much simpler than current methods and can be used over a much wider range of bunch lengths and energies. The angular distribution experiments we performed at SLAC in February 2016 showed quantitative disagreement with theory, most likely due to contamination of the data from upstream sources. However, this result also suggested a new direction in our diagnostic approach, i.e. to the image CDR at its source, i.e. the aperture, to monitor the bunch length. This approach has a number of important advantages over imaging the angular distribution in which the imaging camera is focussed to infinity. In particular, source imaging is much less susceptible to upstream source contamination and can produce a more intense image. Preliminary data from taken in April 2016, just prior to a two year shutdown of SLAC’s FACET accelerator, indicated that the new approach qualitatively works. However, there are still some quantitative discrepancies with theory that need to be resolved with more simulation work and further experiments. The analysis of the April data is still being done at SLAC. We also are preparing and will perform an additional experiment at PSI (SwissFEL) to validate the new method.
6) As the result of an international call to support the AWAKE (advanced plasma wakefield accelerator) project at CERN, we have redirected some of our efforts to experimentally validate a novel method that we had previously proposed. This diagnostic uses OTR interferences, produced as a beam intercepts two thin foils together with a novel algorithm, to measure the phase space density (emittance) of electron beams with high charge. This technique is applicable to AWAKE and many other high brightness accelerators as well. We have already performed several simulation studies and experiments at Argonne National Laboratory’s AWA accelerator to benchmark this new method. These experiments are close to completion and we plan to publish our results early in 2017.
7) Additional studies to fulfill Objective 1 of the original grant, i.e. to extend the dynamic range of beam imaging beyond the current state of the art, have been initiated at DIAMOND. Here we are using a DLP both to measure the PSF of their using their current OSR imaging line with high dynamic range and to create an optical spatial filter to mitigate diffraction and thus improve the PSF of the imaging system. Experiments to meet Objective 2, i.e. optical phase space mapping using a DLP, are also being planned using the DIAMOND OSR beam line as well. Both these studies are expected to be completed in the post grant period, i.e. Nov. 2016 - Sept. 2017.