Nondestructive Beam Diagnostics Research Articles

Boosting the Efficiency of Smith–Purcell Radiators Using Nanophotonic Inverse Design

The generation of radiation from free electrons passing a grating, known as Smith–Purcell radiation, finds various applications, including nondestructive beam diagnostics and tunable light sources, ranging from terahertz toward X-rays. So far, the gratings used for this purpose have been designed manually, based on human intuition and simple geometric shapes. Here we apply the computer-based technique of nanophotonic inverse design to build a 1400 nm Smith–Purcell radiator for subrelativistic 30 keV electrons. We demonstrate that the resulting silicon nanostructure radiates with a 3× higher efficiency and 2.2× higher overall power than previously used rectangular gratings. With better fabrication accuracy and for the same electron–structure distance, simulations suggest a superiority by a factor of 96 in peak efficiency. While increasing the efficiency is a key step needed for practical applications of free-electron radiators, inverse design also allows to shape the spectral and spatial emission in ways inaccessible with the human mind.

Review of Transverse Beam Profile Measurements Using Synchrotron Radiation

Abstract –Synchrotron radiation (SR) is a tool for non-destructive beam diagnostics since its characters are substantially related to those of the source beam. The spectrum of SR is extremely intense and extends over a broad energy range from the infrared through the visible and ultraviolet, into the soft and hard X-ray regions of the electromagnetic spectrum. The visible light (400 – 800 nm) and X-ray (0.05 – 0.3 nm) regions are used in the beam instrumentation. In the visible light region, transverse beam profile or size diagnostics can be done by an interferometer (light is observed as a wave). Meanwhile, in the submicron beam size measurements, the X-ray SR monitor is commonly used. This paper reports the review of transverse beam profile measurements using SR covering principles and practical experiences with the technique at some accelerator facilities such as Photon Factory, Diamond Light Source, CesrTA, and SuperKEKB. Key words: accelerator, beam instrumentation, transverse beam profile, synchrotron radiation, X-ray, visible light

Parametric X-ray radiation in the Smith-Purcell geometry for non-destructive beam diagnostics

We investigate parametric X-ray radiation (PXR) under condition of the extremely asymmetric diffraction, when the ultra-relativistic electron bunch is moving in vacuum parallel to the crystal-vacuum interface, close to the crystal surface. This type of geometry coincides with the well known mechanism of generation of radiation, when the self-field of the particle beam interacts with the reflecting metal grating, namely the Smith-Purcell effect. We demonstrate that in this geometry the main contribution is given via a tail region of the beam distribution, which penetrates the crystal and X-rays are radiated along the normal to the crystal surface. We determine the electron beam characteristics, when this phenomenon can be observed. It is essential that in this geometry the majority of electrons does not undergo multiple scattering and consequently the characteristics of the particle beam are not changed, thus allowing the usage of the emitted X-rays for the purpose of non-destructive beam diagnostics, which can complement the traditional knife-edge method.

Feasibility of resonant diffraction radiation from inclined gratings for a nondestructive beam diagnostics

Characteristics of the resonant diffraction radiation (RDR) generated by ultrarelativistic particles passing near a tilted grating have been considered. Angular distribution of RDR can be used for diagnostics of a beam divergence. Such a technique is noninvasive practically.

Exact theory of diffraction radiation for arbitrary geometry and superradiation of charged particles beam for nondestructive beam diagnostics

In this paper we represent the exact solution for diffraction radiation by a charged particle passing above a semi-infinite ideally conducting screen at the arbitrary geometry, and definition of coherent diffraction radiation—superradiation for a particle beam by the exact theory. The obtained analytical results are applied to the nondestructive beam diagnostics to determine the particle beam parameters via exact formulas for spectral and angular distributions of the beam diffraction superradiation, which lies in the terahertz domain of the spectrum for the actual beam parameters.

Use of a charge coupled device-camera system for optical diagnostics of intense ion beams

Among the various possibilities for investigating the properties of ion beams, two of the most important principles for determining the beam quality are profile measurements using wires or slit apertures, and different kinds of emittance measurement devices. However, most of the commonly used systems have the disadvantage of being either destructive to the beam, or at least to have a considerable influence on the further beam propagation downstream of the point of measurement. Also, for the investigation of intense ion beams, thermal problems on the scanners themselves might arise. Observation of particles emitted from the ion beam in radial direction allow an indirect but nondestructive observation of the beam. Of the different optical methods that exist to accomplish this, we recently conducted an investigation on the possibilities to employ a charge coupled device-camera system for such nondestructive beam diagnostics in our space-charge compensation test stand. First, experimental results on optical profile measurements are presented and compared to results obtained by a slit scanner and a wire scanner. Additionally, to gain a better understanding of the measured data, optically obtained sectional profiles behind a slit and a circular aperture were analyzed. Possibilities for gaining information on the two-dimensional beam profile and the beam emittance are explored.

Developments in Non-Destructive Beam Diagnostics

With the large average beam currents being achieved in accelerators and storage rings, there is an increasing need for non-destructive beam diagnostic devices. For continuous beams, position monitors of the capacitive pick-up type are replaced by resonant devices that respond to the transverse displacement of the beam centroid. Bunch length monitors of the SLAC type using resonant cavities operating in the TM0l0 mode can be used for continuous beams. The more detailed information derivable from beam profile scanners requires development of improved non-destructive devices. Profile monitors which scan the visible light produced by high current beams may be more reliable than ones using the residual ionization if the light intensity from gas molecules following nonionizing collisions with beam particles gives a measure of the beam current density independent of the local electron density. The intense Balmer series lines from neutral hydrogen beams have been used successfully to measure beam profiles. At CRNL and at LASL, beam light profile monitors are being developed for high average current accelerators. Three or more projections will be recorded to allow tomographic reconstruction of the two-dimensional beam current density. Light detection is either by intensified Reticons or ISIT vidicons. The use of three or more beam light monitors on a beam transport line will also permit estimates of the transverse emittance to be made through the reconstruction technique.