137 research outputs found
Single-shot, transverse self-wakefield reconstruction from screen images
A single-shot method to reconstruct the transverse self-wakefields acting on
a beam, based only on screen images, is introduced. By employing numerical
optimization with certain approximations, a relatively high-dimensional
parameter space is efficiently explored to determine the multipole components
of the transverse-wakefield topology up to desired order. The reconstruction
technique complements simulations, which are able to directly describe the
wakefield composition based on experimental conditions. The technique is
applied to representative simulation results as a benchmark, and also to
experimental data on wakefield observations driven in dielectric-lined
structures.Comment: 10 pages, 8 figure
Shear-Wave Elastography Assessments of Quadriceps Stiffness Changes prior to, during and after Prolonged Exercise: A Longitudinal Study during an Extreme Mountain Ultra-Marathon.
In sports medicine, there is increasing interest in quantifying the elastic properties of skeletal muscle, especially during extreme muscular stimulation, to improve our understanding of the impact of alterations in skeletal muscle stiffness on resulting pain or injuries, as well as the mechanisms underlying the relationships between these parameters. Our main objective was to determine whether real-time shear-wave elastography (SWE) can monitor changes in quadriceps muscle elasticity during an extreme mountain ultra-marathon, a powerful mechanical stress model. Our study involved 50 volunteers participating in an extreme mountain marathon (distance: 330 km, elevation: +24,000 m). Quantitative SWE velocity and shear modulus measurements were performed in most superficial quadriceps muscle heads at the following 4 time points: before the race, halfway through the race, upon finishing the race and after recovery (+48 h). Blood biomarker levels were also measured. A significant decrease in the quadriceps shear modulus was observed upon finishing the race (3.31±0.61 kPa) (p<0.001) compared to baseline (3.56±0.63 kPa), followed by a partial recovery +48 h after the race (3.45±0.6 kPa) (p = 0.002) across all muscle heads, as well as for each of the following three muscle heads: the rectus femoris (p = 0.003), the vastus medialis (p = 0.033) and the vastus lateralis (p = 0.001). Our study is the first to assess changes in muscle stiffness during prolonged extreme physical endurance exercises based on shear modulus measurements using non-invasive SWE. We concluded that decreases in stiffness, which may have resulted from quadriceps overuse in the setting of supra-physiological stress caused by the extreme distance and unique elevation of the race, may have been responsible for the development of inflammation and muscle swelling. SWE may hence represent a promising tool for monitoring physiologic or pathological variations in muscle stiffness and may be useful for diagnosing and monitoring muscle changes
Approaching Petavolts per meter plasmonics using structured semiconductors
A new class of strongly excited plasmonic modes that open access to
unprecedented Petavolts per meter electromagnetic fields promise wide-ranging,
transformative impact. These modes are constituted by large amplitude
oscillations of the ultradense, delocalized free electron Fermi gas which is
inherent in conductive media. Here structured semiconductors with appropriate
concentration of n-type dopant are introduced to tune the properties of the
Fermi gas for matched excitation of an electrostatic, surface "crunch-in"
plasmon using readily available electron beams of ten micron overall dimensions
and hundreds of picoCoulomb charge launched inside a tube. Strong excitation
made possible by matching results in relativistic oscillations of the Fermi
electron gas and uncovers unique phenomena. Relativistically induced ballistic
electron transport comes about due to relativistic multifold increase in the
mean free path. Acquired ballistic transport also leads to unconventional heat
deposition beyond the Ohm's law. This explains the absence of observed damage
or solid-plasma formation in experiments on interaction of conductive samples
with electron bunches shorter than . Furthermore,
relativistic momentum leads to copious tunneling of electron gas allowing it to
traverse the surface and crunch inside the tube. Relativistic effects along
with large, localized variation of Fermi gas density underlying these modes
necessitate the kinetic approach coupled with particle-in-cell simulations.
Experimental verification of acceleration and focusing of electron beams
modeled here using tens of Gigavolts per meter fields excited in semiconductors
with free electron density will pave the way for Petavolts
per meter plasmonics.Comment: 16 pages, 10 figure
Machine learning-based analysis of experimental electron beams and gamma energy distributions
The photon flux resulting from high-energy electron beam interactions with
high field systems, such as in the upcoming FACET-II experiments at SLAC
National Accelerator Laboratory, may give deep insight into the electron beam's
underlying dynamics at the interaction point. Extraction of this information is
an intricate process, however. To demonstrate how to approach this challenge
with modern methods, this paper utilizes data from simulated plasma wakefield
acceleration-derived betatron radiation experiments and high-field
laser-electron-based radiation production to determine reliable methods of
reconstructing key beam and interaction properties. For these measurements,
recovering the emitted 200 keV to 10 GeV photon energy spectra from two
advanced spectrometers now being commissioned requires testing multiple methods
to finalize a pipeline from their responses to incident electron beam
information. In each case, we compare the performance of: neural networks,
which detect patterns between data sets through repeated training; maximum
likelihood estimation (MLE), a statistical technique used to determine unknown
parameters from the distribution of observed data; and a hybrid approach
combining the two. Further, in the case of photons with energies above 30 MeV,
we also examine the efficacy of QR decomposition, a matrix decomposition
method. The betatron radiation and the high-energy photon cases demonstrate the
effectiveness of a hybrid ML-MLE approach, while the high-field electrodynamics
interaction and the low-energy photon cases showcased the machine learning (ML)
model's efficiency in the presence of noise. As such, while there is utility in
all the methods, the ML-MLE hybrid approach proves to be the most
generalizable.Comment: 23 pages, 30 figure
Sextupole Correction of the Longitudinal Transport of Relativistic Beams in Dispersionless Translating Sections
Abstract We examine the use of sextupole magnets to correct nonlinearities in the longitudinal phase space transformation of a relativistic beam of charged particles in a dispersionless translating section, or dogleg. Through heuristic analytical arguments and examples derived from recent experimental efforts, augmented by simulations using the particle tracking codes PARMELA and ELEGANT, sextupole corrections are found to be effective in optimizing the use of such structures for beam compression or for shaping the current profile of the beam, by manipulation of the second-order longitudinal dispersion. Recent experimental evidence of the use of sextupoles to manipulate second-order horizontal and longitudinal dispersion of the beam is presented. The theoretical and experimental results indicate that these manipulations can be used to create an electron bunch with a current profile having a long ramp followed by a sharp cut-off, which is optimal for driving large amplitude wake fields in a plasma wake field accelerator
All-optical density downramp injection in electron-driven plasma wakefield accelerators
Injection of well-defined, high-quality electron populations into plasma waves is a key challenge of plasma wakefield accelerators. Here, we report on the first experimental demonstration of plasma density downramp injection in an electron-driven plasma wakefield accelerator, which can be controlled and tuned in all-optical fashion by mJ-level laser pulses. The laser pulse is directed across the path of the plasma wave before its arrival, where it generates a local plasma density spike in addition to the background plasma by tunnelling ionization of a high ionization threshold gas component. This density spike distorts the plasma wave during the density downramp, causing plasma electrons to be injected into the plasma wave. By tuning the laser pulse energy and shape, highly flexible plasma density spike profiles can be designed, enabling dark current free, versatile production of high-quality electron beams. This in turn permits creation of unique injected beam configurations such as counter-oscillating twin beamlets
An Ultra-Compact X-Ray Free-Electron Laser
In the field of beam physics, two frontier topics have taken center stage due
to their potential to enable new approaches to discovery in a wide swath of
science. These areas are: advanced, high gradient acceleration techniques, and
x-ray free electron lasers (XFELs). Further, there is intense interest in the
marriage of these two fields, with the goal of producing a very compact XFEL.
In this context, recent advances in high gradient radio-frequency cryogenic
copper structure research have opened the door to the use of surface electric
fields between 250 and 500 MV/m. Such an approach is foreseen to enable a new
generation of photoinjectors with six-dimensional beam brightness beyond the
current state-of-the-art by well over an order of magnitude. This advance is an
essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one
may accelerate these bright beams to GeV scale in less than 10 meters. Such an
injector, when combined with inverse free electron laser-based bunching
techniques can produce multi-kA beams with unprecedented beam quality,
quantified by ~50 nm-rad normalized emittances. These beams, when injected into
innovative, short-period (1-10 mm) undulators uniquely enable UC-XFELs having
footprints consistent with university-scale laboratories. We describe the
architecture and predicted performance of this novel light source, which
promises photon production per pulse of a few percent of existing XFEL sources.
We review implementation issues including collective beam effects, compact
x-ray optics systems, and other relevant technical challenges. To illustrate
the potential of such a light source to fundamentally change the current
paradigm of XFELs with their limited access, we examine possible applications
in biology, chemistry, materials, atomic physics, industry, and medicine which
may profit from this new model of performing XFEL science.Comment: 80 pages, 24 figure
Generation and acceleration of electron bunches from a plasma photocathode
Plasma waves generated in the wake of intense, relativistic laser1,2 or particle beams3,4 can accelerate electron bunches to gigaelectronvolt energies in centimetre-scale distances. This allows the realization of compact accelerators with emerging applications ranging from modern light sources such as the free-electron laser to energy frontier lepton colliders. In a plasma wakefield accelerator, such multi-gigavolt-per-metre wakefields can accelerate witness electron bunches that are either externally injected5,6 or captured from the background plasma7,8. Here we demonstrate optically triggered injection9–11 and acceleration of electron bunches, generated in a multi-component hydrogen and helium plasma employing a spatially aligned and synchronized laser pulse. This ‘plasma photocathode’ decouples injection from wake excitation by liberating tunnel-ionized helium electrons directly inside the plasma cavity, where these cold electrons are then rapidly boosted to relativistic velocities. The injection regime can be accessed via optical11 density down-ramp injection12–16 and is an important step towards the generation of electron beams with unprecedented low transverse emittance, high current and 6D-brightness17. This experimental path opens numerous prospects for transformative plasma wakefield accelerator applications based on ultrahigh-brightness beams
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