Riccardo Bartolini

Single shot transverse emittance measurement from OTR screens in a drift transport section

Single shot transverse emittance measurement is essential to assess the beam quality and performance of new generation light sources such as linac based X-ray Free Electron Lasers (FELs) or laser plasma wakefield accelerators (LPWA). To this end, we have developed a single shot transverse emittance measurement using at least 3 screens inserted in the beam at the same time, measuring the beam size at different positions in a drift space in one single shot. In this paper, we firstly present the theoretical aspects to perform the measurement. We secondly show experimental results obtained at Diamond for a 3 GeV electron beam in the transfer line from the Booster to the Storage Ring, using this thin OTR screens method. Finally, we discuss the results showing the strength of the measurement in comparison with more standard and established emittance measurement, like the quadrupole scan method.

Microbunch Instability Observations from a THz Detector at Diamond Light Source

Diamond Light source is a third generation synchrotron facility dedicated to producing radiation of outstanding brightness, ranging from infra-red to x-rays. The short electron bunches that are accelerated around the storage ring are susceptible to the phenomenon of microbunching instabilities when the bunch charge exceeds a threshold. The primary feature of the microbunch instabilities is the onset of bursts of radiation in the THz range. The high frequencies involved in the emissions make detection and analysis challenging. A 60-90 GHz Schottky Barrier Diode detector was installed to investigate turn by turn evolution of the instabilities.

A Novel Approach in the One-Dimensional Phase Retrieval Problem and its Application to the Time Profile Reconstruction

Phase retrieval problem occurs in a number of areas in physics and is the subject of continuing investigation [115]. One dimensional case, for example, an electron bunch temporal profile reconstruction, is particularly challenging. Frequently applied methods, are reliable if the Blaschke phase [10-12] contribution is negligible. This, however, is neither known a priori nor can it be assumed for an arbitrary profile. In this work we present a novel algorithm with additional constraints which gives reproducible, stable solutions for profiles, both artificial and experimental, otherwise unresolved by existing techniques. INTRODUCTION Accurate knowledge of the longitudinal (time) profile of an electron bunch is important in the context of linear colliders and X-ray FELs, but it is a parameter that becomes progressively more difficult to determine for fslong bunches [1-7]. These, however, are the bunch lengths expected from the next generation of high-gradient particle accelerators which will be based on laser-plasma or wake-field acceleration. Apart from the desirability of determining the profile in a non-destructive manner, and because of the low repetition rate of these accelerators, it is equally desirable to be able to achieve this in a single shot. In this paper we will discuss the technique to recover phase information from power spectrum which one can measure using different spectroscopic technique. The problem of retrieving the phase of a signal from a measurement of its power spectrum alone is well known and has been under investigation for a few decades. Some information about the missing phase can be retrieved from the so-called ‘minimal’ phase calculated by the Kramers-Kronig (KK) method under the assumption that the signal is a holomorphic function. There are number of iterative techniques [8, 9, 13-15] which also allowing recovering missing phase and reconstructing the bunch profile. In this work we discuss the method, which combine KK and iterative techniques. The KK feeds iterative method information about initial (minimal) phase and it is also used as a boundary condition limiting possibilities of generating profile with unphysical phase. The conditions which indicates that algorithm is converging to a solution and the second that this solution is likely (i.e. most probable) to be the correct one will be also discussed. ALGORITHM DESCRIPTION The new algorithm is based on a combination of the minimal phase θm and an iterative procedure (with repeated iterations between the frequency and time domains). The Kramers-Kronig (KK) method is used to calculate the minimal phase. We assume for clarity reason that the power spectrum is known (measured) on the whole frequency domain. It is important to note that the Blaschke contribution (if it exists) is a monotonically increasing function of frequency and that the minimal phase θm provides a lower limit for the possible phase values. The new phase-constrained iterative (PCI) algorithm is named to indicate this new phase boundary condition. In addition to the minimal phase, the integral value F (bunch charge) is also used as a limiting condition and to monitor the convergence (as discussed below) to the solution. The profile recovered from the minimal phase also provides an initial estimate of the support function (see below). These points listed above are essential for the new algorithm operation. We note that displacements along the time axis, mirror imaging of the temporal profile or equivalently, the sign of the spectral phase cannot be determined unambiguously. We will not be addressing those issues as it is only for few cases these variations have physical significance. The features of the PCI algorithm are summarised by (2). The algorithm iterative part is similar to the Gerchberg-Saxton (GS) and Hybrid Input-Output (HIO) group of algorithms and can be summarized as follows: + � = { ′ � � ′ � Υ � − � ′ � � ′ � Υ � = �′ � �′ > � � = � � �′ ≤ � (1) where Υ is the set of constrains of function f(t). In this letter the abbreviation FT and FT -1 are used to denote the forward and the inverse Fourier Transforms respectively. The above expressions define the result of the n+1 iterative step where fn and n denote the modulus and phase respectively of the time profile function calculated by FT from the frequency to time domain, while m is the minimal phase. 1. First, the measured amplitude spectrum and � are used for the initial FT -1 e.g. from the frequency into the time domain to derive the zeroth order approximation ′ � to the unknown profile f(t). Any values of that function inside the defined window () which do not Proceedings of IPAC2016, Busan, Korea MOPOY048 06 Beam Instrumentation, Controls, Feedback and Operational Aspects T03 Beam Diagnostics and Instrumentation ISBN 978-3-95450-147-2 955 C op yr ig ht

Longitudinal Beam Profile Measurements of the Microbunching Instability

The microbunching instability is a phenomenon characterized by the onset of radiation bursts above a threshold bunch current. These bursts consist of coherent emissions with wavelengths comparable to the bunch length and shorter. The instability has recently been observed at Diamond Light Source, a 3rd generation synchrotron. The operating conditions for triggering the instability at Diamond Light Source are well known, however measuring the spectral content of the resulting emissions is a more challenging investigation. A Michelson interferometer has been installed with the aim of recording the coherent spectrum from the bunches, using ultra-fast response Schottky Barrier Diode detectors. The longitudinal profile of the bunches can be estimated with subsequent analysis.

Long-term Stability of the Diamond Light Source Storage Ring

The Diamond Storage Ring (SR) has been in operation since January 2007. This paper summarises a number of measurementsthat have been made over that periodto monitor the SR stability in height and position including general survey, Hydrostatic Levelling System (HLS), horizontal and vertical magnet corrector strengths as well as Radio Frequency(RF)measurementsthathavegivenanindication of changing circumference.

Numerical analysis of space charge effects in electron bunches at laser-driven plasma accelerators

Laser-driven Plasma Accelerators (LPA) have successfully generated high energy, high charge electron bunches which can reach many kA peak current, over short distances. Space charge issues, even in transport lines as simple as a drift section, have to be carefully taken into account since they can degrade the beam quality, preventing any further application of such electron beams. We analyse the space charge effects within an electron bunch with numerical simulations in order to assess their effect on the beam. We use LPA beam parameters published in previous experimental studies. These studies can give an indication of the working point where space charge can dominate the beam dynamics and has to be taken into account in the application of such beams.