ABSTRACT
Electron beam quality is paramount for X-ray pulse production in free-electron-lasers (FELs). State-of-the-art linear accelerators (linacs) can deliver multi-GeV electron beams with sufficient quality for hard X-ray-FELs, albeit requiring km-scale setups, whereas plasma-based accelerators can produce multi-GeV electron beams on metre-scale distances, and begin to reach beam qualities sufficient for EUV FELs. Here we show, that electron beams from plasma photocathodes many orders of magnitude brighter than state-of-the-art can be generated in plasma wakefield accelerators (PWFAs), and then extracted, captured, transported and injected into undulators without significant quality loss. These ultrabright, sub-femtosecond electron beams can drive hard X-FELs near the cold beam limit to generate coherent X-ray pulses of attosecond-Angstrom class, reaching saturation after only 10 metres of undulator. This plasma-X-FEL opens pathways for advanced photon science capabilities, such as unperturbed observation of electronic motion inside atoms at their natural time and length scale, and towards higher photon energies.
Subject(s)
Electrons , Particle Accelerators , X-Rays , Lasers , PhotonsABSTRACT
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'.
ABSTRACT
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.
ABSTRACT
OBJECTIVE: To aid in detection of osteoarthritis (OA) progression in serial magnetic resonance (MR) scans, we assessed feasibility and accuracy of rapid 3D image registration of the tibial plateau in normal and arthritic subjects, and inter-scan reliability of semi-automated cartilage volume measurement from these images. DESIGN: Two T1 fat-suppressed knee MR scans were obtained 2 weeks apart in healthy adults (n = 9, age 23-48 years). Four scans of each of three patients with established OA were obtained over 2 years. At baseline, the tibial surface was digitized by semi-automated edge detection and medial tibial plateau cartilage volume was calculated from high-intensity voxels within a manually drawn region of interest (ROI). In subsequent scans, the digitized tibial surface was registered to the baseline location by photogrammetric 3D coordinate transformation, and cartilage volume was automatically recalculated by reuse of the ROI. We measured registration accuracy by root mean square (RMS) distance between registered tibial surfaces. RESULTS: In normals, RMS distance between tibial surfaces in baseline and subsequent scans was 1/3 voxel length (0.121 mm), and medial tibial plateau cartilage volumes varied by 1.4+/-3.2%. Despite change in cartilage volumes by up to 20% over 2 years in arthritic patients, surface registration accuracy was unaffected (0.122 mm). User-supervised processing time was 15 min at baseline and 7 min in subsequent scans. CONCLUSION: Tibial surfaces on magnetic resonance imaging (MRI) can be rapidly and accurately co-registered, even in arthritic knees, allowing direct visualization of changes over time. Compared to most current methods, cartilage volume measurement in registered images is faster and has equivalent inter-scan reliability in initially normal subjects.
