Ir-192 radioisotope replacement with a hand-portable 1 MeV Ku-band electron linear accelerator.

Although linear accelerators are used in many security, industrial and medical applications, the existing technologies are too large and expensive for several critical applications such as radioactive source replacement, field radiography and mobile cargo scanners. One of the main requirements for these sources is to be highly portable to allow field operation. In response to this problem, RadiaBeam has designed a hand-portable 1 MeV X-ray source, scalable to higher energies, based on Ku-band split electron linac, that can be used for Ir-192 radioisotope replacement. In this paper, we present its multiphysics and engineering design studies, as well as an accelerating structure prototype along with RF measurements.

Advanced schemes for underdense plasma photocathode wakefield accelerators: pathways towards ultrahigh brightness electron beams.

The 'Trojan Horse' underdense plasma photocathode scheme applied to electron beam-driven plasma wakefield acceleration has opened up a path which promises high controllability and tunability and to reach extremely good quality as regards emittance and five-dimensional beam brightness. This combination has the potential to improve the state-of-the-art in accelerator technology significantly. In this paper, we review the basic concepts of the Trojan Horse scheme and present advanced methods for tailoring both the injector laser pulses and the witness electron bunches and combine them with the Trojan Horse scheme. These new approaches will further enhance the beam qualities, such as transverse emittance and longitudinal energy spread, and may allow, for the first time, to produce ultrahigh six-dimensional brightness electron bunches, which is a necessary requirement for driving advanced radiation sources. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Single-stage plasma-based correlated energy spread compensation for ultrahigh 6D brightness electron beams.

Plasma photocathode wakefield acceleration combines energy gains of tens of GeV m-1 with generation of ultralow emittance electron bunches, and opens a path towards 5D-brightness orders of magnitude larger than state-of-the-art. This holds great promise for compact accelerator building blocks and advanced light sources. However, an intrinsic by-product of the enormous electric field gradients inherent to plasma accelerators is substantial correlated energy spread-an obstacle for key applications such as free-electron-lasers. Here we show that by releasing an additional tailored escort electron beam at a later phase of the acceleration, when the witness bunch is relativistically stable, the plasma wave can be locally overloaded without compromising the witness bunch normalized emittance. This reverses the effective accelerating gradient, and counter-rotates the accumulated negative longitudinal phase space chirp of the witness bunch. Thereby, the energy spread is reduced by an order of magnitude, thus enabling the production of ultrahigh 6D-brightness beams.

Functionalization of arrays of silica nanochannels by post-condensation.

Functionalized arrays of silica nanochannels (ASNCs) were prepared by post-condensation of amino-functional alkoxysilanes (NH-silanes). The method of post-condensation combines postsynthetic grafting and co-condensation - the classical pathways for the functionalization of mesoporous silica - to afford a convenient one-pot reaction in aqueous medium. The structure of the NH-silanes determines the distribution of the surface-bound groups in the ASNCs. Accumulation at the channel entrances was observed by confocal laser scanning microscopy in the case of sterically hindered NH-silanes, whereas smaller NH-silanes led to a more uniform distribution. In all cases, the channels remained accessible for additional postsynthetic treatment or introduction of guests after extraction of the structure-directing agent. The particle size and morphology, as well as the pore size and the degree of ordering of the amino-functionalized ASNCs were comparable to pristine ASNCs.

Bimodal mesoporous silica with bottleneck pores.

Bimodal mesoporous silica consisting of two sets of well-defined mesopores is synthesized by a partial pseudomorphic transformation of an ordered mesoporous starting material (SBA-15 type). The introduction of a second set of smaller mesopores (MCM-41 type) establishes a pore system with bottlenecks that restricts the access to the core of the bimodal mesoporous silica particles. The particle size and shape of the starting material are retained, but micropores present in the starting material disappear during the transformation, leading to a true bimodal mesoporous product. A varying degree of transformation allows the adjustment of the pore volume contribution of the two mesopore domains. Information on the accessibility of the mesopores is obtained by the adsorption of fluorescence-labeled poly(amidoamine) dendrimers and imaging by confocal laser scanning microscopy. This information is correlated with nitrogen sorption data to provide insights regarding the spatial distribution of the two mesopore domains. The bimodal mesoporous materials are excellent model systems for the investigation of cavitation effects in nitrogen desorption isotherms.

Planar-dielectric-wakefield accelerator structure using Bragg-reflector boundaries.

We report experimental measurements of narrow-band, single-mode excitation, and drive beam energy modulation, in a dielectric wakefield accelerating structure with planar geometry and Bragg-reflector boundaries. A short, relativistic electron beam (∼1 ps) with moderate charge (∼100 pC) is used to drive the wakefields in the structure. The fundamental mode of the structure is reinforced by constructive interference in the alternating dielectric layers at the boundary, and is characterized by the spectral analysis of the emitted coherent Cherenkov radiation signal. Data analysis shows a narrow-band peak at 210 GHz corresponding to the fundamental mode of the structure. Simulations in both 2D and 3D provide insight into the propagating fields and reproduction of the electron beams dynamics observables and emitted radiation characteristics.

Ultracold electron bunch generation via plasma photocathode emission and acceleration in a beam-driven plasma blowout.

Beam-driven plasma wakefield acceleration using low-ionization-threshold gas such as Li is combined with laser-controlled electron injection via ionization of high-ionization-threshold gas such as He. The He electrons are released with low transverse momentum in the focus of the copropagating, nonrelativistic-intensity laser pulse directly inside the accelerating or focusing phase of the Li blowout. This concept paves the way for the generation of sub-µm-size, ultralow-emittance, highly tunable electron bunches, thus enabling a flexible new class of an advanced free electron laser capable high-field accelerator.

Probing molecular order in zeolite L inclusion compounds using two-photon fluorescence polarimetric microscopy.

We investigate the local static molecular orientational behavior in zeolite L inclusion compounds by polarimetric two-photon fluorescence microscopy. This technique, based on the polarized read-out of the signal under a tunable incident polarization state, provides refined information on molecular disorder that is not achievable using traditional fluorescence anisotropy. Moreover, the polarimetric microscopy imaging scheme permits a spatial investigation of possible heterogeneities, with a submicrometric resolution. The study performed on different fluorescent molecules inserted in zeolite L channels evidence a degree of disorder for either small or flexible structures.

High-quality electron beams from a laser wakefield accelerator using plasma-channel guiding.

Laser-driven accelerators, in which particles are accelerated by the electric field of a plasma wave (the wakefield) driven by an intense laser, have demonstrated accelerating electric fields of hundreds of GV m(-1) (refs 1-3). These fields are thousands of times greater than those achievable in conventional radio-frequency accelerators, spurring interest in laser accelerators as compact next-generation sources of energetic electrons and radiation. To date, however, acceleration distances have been severely limited by the lack of a controllable method for extending the propagation distance of the focused laser pulse. The ensuing short acceleration distance results in low-energy beams with 100 per cent electron energy spread, which limits potential applications. Here we demonstrate a laser accelerator that produces electron beams with an energy spread of a few per cent, low emittance and increased energy (more than 10(9) electrons above 80 MeV). Our technique involves the use of a preformed plasma density channel to guide a relativistically intense laser, resulting in a longer propagation distance. The results open the way for compact and tunable high-brightness sources of electrons and radiation.

Reinvestigation of the aromatic side-chains in the basic pancreatic trypsin inhibitor by heteronuclear two-dimensional nuclear magnetic resonance.

The 1H nuclear magnetic resonance (n.m.r.) assignments for the aromatic spin systems of the four tyrosines and four phenylalanines in the basic pancreatic trypsin inhibitor (BPTI) were reinvestigated using novel 13C-1H heteronuclear two-dimensional experiments. Resonance lines which are degenerate in homonuclear 1H n.m.r. spectra could thus be resolved. Based on this new evidence the previous assignments for Phe22 and Phe33 had to be corrected. This affects the earlier conclusions on aromatic ring flips in BPTI in that Phe22 is rotating rapidly on the n.m.r. time scale at 36 degrees C, rather than being immobilized up to 80 degrees C.

Toward the complete assignment of the carbon nuclear magnetic resonance spectrum of the basic pancreatic trypsin inhibitor.

A total of 54 of the 58 alpha-carbon resonances and numerous side-chain carbon signals were individually assigned in the basic pancreatic trypsin inhibitor by using two-dimensional heteronuclear correlated and relayed coherence transfer spectroscopy with proton detection. No isotope enrichment was used, and the spectra were recorded in 5-mm sample tubes. The pulse sequences were optimized to eliminate, prior to phase cycling, the signals of protons attached to 12C. We have concentrated on assignments of carbons bearing a single hydrogen in view of a relatively easy interpretation of carbon relaxation times, and most of these carbon resonances could be assigned. Furthermore, we demonstrate that two-dimensional heteronuclear correlated and relayed coherence transfer spectra can be used to elucidate connectivities between degenerate resonances within proton spin systems that often occur in threonines and aromatic side chains.