Single-shot coherent Smith-Purcell monitor: preliminary concepts H. Harrison, A. J. Lancaster, F. Bakkali Taheri, G. Doucas, I. V. Konoplev John Adams Institute for Accelerator Science, Department of Physics, University of Oxford Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH 10/12/2015Single-shot cSPr concepts – JAIfest 20151

Overview Why are we working on this? What is coherent Smith-Purcell Radiation? Multi-shot system  proof of concept Single-shot proposal: – Grating layout – Background reduction – Other modifications Summary 10/12/2015Single-shot cSPr concepts – JAIfest 20152

Why? Many applications require (or provide) short bunch lengths: – Particle colliders – Plasma wakefield acceleration – Free-electron lasers Demand for non-destructive diagnostics Complex interactions can be difficult to model Better to simply measure the beam 10/12/2015Single-shot cSPr concepts – JAIfest 20153

Smith-Purcell Radiation 10/12/2015Single-shot cSPr concepts – JAIfest Coherent radiation emitted from a periodic structure Induced by a charged bunch travelling above the structure Radiation has angular dispersion – frequency spectrum can be measured Longitudinal bunch profile is encoded within the frequency spectrum

Multi-shot system 10/12/2015Single-shot cSPr concepts – JAIfest The multi-shot monitor demonstrated that cSPr can be used to measure the longitudinal profile [1] Successful reconstruction of sub-ps bunch profiles The properties of cSPr were studied ≈20 GeV electrons x electrons per bunch Normalized emittance 60 mm-mrad Bunches at 10 Hz [1] H.L. Andrews et. al., “Reconstruction of the time profile of GeV, subpicosecond long electron bunches by means of coherent Smith-Purcell radiation,” Phys. Rev. ST Accel. Beams, 17, , 2014.

Schematic layout 10/12/2015Single-shot cSPr concepts – JAIfest 20156

Limitations 10/12/2015Single-shot cSPr concepts – JAIfest High background, low signal – Requires significant averaging Requires 6 sets of measurements: – Three different gratings on a carousel – One “blank” measurement for each Mechanically complex: – Carousel rotation – Carousel translation – Changing filters Components must be λ-independent Geometry changes are required

Multi-grating layout 10/12/2015Single-shot cSPr concepts – JAIfest Gratings will be longitudinally spaced to avoid geometry problems Rotated around azimuthal angle to reduce length of the apparatus Number of gratings / detectors required is being investigated Diffraction radiation is expected from each grating

Grating development How many gratings do we need? Grating size – width and length Grating shape Distance from the beam How good is the surface-current model? 10/12/2015Single-shot cSPr concepts – JAIfest mm 3mm 5mm

Background signal 10/12/2015Single-shot cSPr concepts – JAIfest Low signal-to-noise ratio expected – Current system uses blank measurements – Provides background estimate New system would require 3 blanks – Extra detection system for each – Different environment to grating – Increases system length Proposed solution – polarization – Preliminary results show cSPr is polarized – Seen in both simulation and experiment – Background unpolarized (at FACET)

Background signal 10/12/2015Single-shot cSPr concepts – JAIfest Background subtraction not ideal – advantageous to minimise background radiation Optics have limited acceptance – Trace back to last possible reflection – Attempt to attenuate at that point Propose a window / absorber – Window lets radiation exit chamber – Absorber not in vacuum – Polydimethylsiloxane (PDMS) [2] – Surface structures reduce reflection Only affects reflected radiation – Direct radiation inevitable – Background subtraction system is higher priority [2] DH. Kim, DS. Kim, S. Hwang, and JH. Jang, "Surface relief structures for a flexible broadband terahertz absorber," Opt. Express 20, , 2012.

Other components 10/12/2015Single-shot cSPr concepts – JAIfest

Filters Use waveguide array plate filters (WAPs) – Well understood (e.g. Winnewisser [3]) – Predictable geometry defined pass-band limits – Polarization independent Simulations in CST Microwave studio – Angular dependence – Depth studies etc Not expecting to change filter style 10/12/2015Single-shot cSPr concepts – JAIfest [3] C. Winnewisser, F. Lewen and H. Helm, “Transmission characteristics of dichroic filters measured by THz time-domain spectroscopy,” Appl. Phys. A, 66, , Increasing depth (l) With thanks to A. J. Lancaster for the simulation results

Proposed update Larger filters Square layout Add adjustable masks – Similar slit used at SLAC Two main benefits: – Selection of angular acceptance – Allows study of cSPr distribution Adds mechanical complexity 10/12/2015Single-shot cSPr concepts – JAIfest

Masked filters 10/12/2015Single-shot cSPr concepts – JAIfest

Winston cones Non-imaging concentrators [4] Wavelength independent – Important for multi-grating system – Not necessary for single-shot layout Looking at horn antennae – Waveguide coupled collection Two benefits: – Transmission away from accelerator – Possibility of additional filtering 10/12/2015Single-shot cSPr concepts – JAIfest [4] A. Rabl and R. Winston, "Ideal concentrators for finite sources and restricted exit angles," Appl. Opt. 15, , 1976.

“Final” geometry 10/12/2015Single-shot cSPr concepts – JAIfest

Summary Outline of a single-shot cSPr beam profile monitor Early stages of development Revision of almost all subsystems of the current experiment Aim to have a design by March /12/2015Single-shot cSPr concepts – JAIfest This work was supported (in parts) by the: UK Science and Technology Facilities Council (STFC UK) through grant ST/M003590/1 and The Leverhulme Trust through the International Network Grant (IN – 2015 – 012). F. Bakkali Taheri would like to thank STFC UK and H. Harrison to thank STFC UK and JAI University of Oxford for supporting their DPhil projects. WAP simulations and 3D diagrams produced using CST Studio Suite 2015.

Grating Layout 10/12/2015Single-shot cSPr concepts – JAIfest

Polarisation 10/12/2015Single-shot cSPr concepts – JAIfest