24 research outputs found
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Spatiotemporal dynamics of ultrarelativistic beam-plasma instabilities
An electron or electron-positron beam streaming through a plasma is notoriously prone to microinstabilities. For a dilute ultrarelativistic infinite beam, the dominant instability is a mixed mode between longitudinal two-stream and transverse filamentation modes, with a phase velocity oblique to the beam velocity. A spatiotemporal theory describing the linear growth of this oblique mixed instability is proposed which predicts that spatiotemporal effects generally prevail for finite-length beams, leading to a significantly slower instability evolution than in the usually assumed purely temporal regime. These results are accurately supported by particle-in-cell (PIC) simulations. Furthermore, we show that the self-focusing dynamics caused by the plasma wakefields driven by finite-width beams can compete with the oblique instability. Analyzed through PIC simulations, the interplay of these two processes in realistic systems bears important implications for upcoming accelerator experiments on ultrarelativistic beam-plasma interactions.
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Chromatic Dynamics of an Electron Beam in a Plasma Based Accelerator
We present a theoretical investigation of the chromatic dynamics of the
witness beam within a plasma based accelerator. We derive the single particle
motion of an electron in an ion column within a nonlinear, blowout wake
including adiabatic dampening and adiabatic variations in plasma density. Using
this, we calculate the evolution of the beam moments and emittance for an
electron beam. Our model can handle near arbitrary longitudinal phase space
distributions. We include the effects of energy change in the beam, imperfect
wake loading, initial transverse offsets of the beam, and mismatch between the
beam and plasma. We use our model to derive analytic saturation lengths for the
projected, longitudinal slice, and energy slice emittance under different beam
loading conditions. Further, we show that the centroid oscillations and spot
sizes vary between the slices and the variation depends strongly on the beam
loading. Next, we show how a beam evolves in a full plasma source with density
ramps and show that the integral of the plasma density along the ramp
determines the impact on the beam. Finally, we derive several simple scaling
laws that show how to design a plasma based injector to produce a target beam
energy and energy spread.Comment: 17 pages, 10 figure
Wakefield Generation in Hydrogen and Lithium Plasmas at FACET-II: Diagnostics and First Beam-Plasma Interaction Results
Plasma Wakefield Acceleration (PWFA) provides ultrahigh acceleration
gradients of 10s of GeV/m, providing a novel path towards efficient, compact,
TeV-scale linear colliders and high brightness free electron lasers. Critical
to the success of these applications is demonstrating simultaneously high
gradient acceleration, high energy transfer efficiency, and preservation of
emittance, charge, and energy spread. Experiments at the FACET-II National User
Facility at SLAC National Accelerator Laboratory aim to achieve all of these
milestones in a single stage plasma wakefield accelerator, providing a 10 GeV
energy gain in a <1 m plasma with high energy transfer efficiency. Such a
demonstration depends critically on diagnostics able to measure emittance with
mm-mrad accuracy, energy spectra to determine both %-level energy spread and
broadband energy gain and loss, incoming longitudinal phase space, and matching
dynamics. This paper discusses the experimental setup at FACET-II, including
the incoming beam parameters from the FACET-II linac, plasma sources, and
diagnostics developed to meet this challenge. Initial progress on the
generation of beam ionized wakes in meter-scale hydrogen gas is discussed, as
well as commissioning of the plasma sources and diagnostics
Generation of meter-scale hydrogen plasmas and efficient, pump-depletion-limited wakefield excitation using 10 GeV electron bunches
High repetition rates and efficient energy transfer to the accelerating beam
are important for a future linear collider based on the beam-driven plasma
wakefield acceleration scheme (PWFA-LC). This paper reports the first results
from the Plasma Wakefield Acceleration Collaboration (E300) that are beginning
to address both of these issues using the recently commissioned FACET-II
facility at SLAC. We have generated meter-scale hydrogen plasmas using
time-structured 10 GeV electron bunches from FACET-II, which hold the promise
of dramatically increasing the repetition rate of PWFA by rapidly replenishing
the gas between each shot compared to the hitherto used lithium plasmas that
operate at 1-10 Hz. Furthermore, we have excited wakes in such plasmas that are
suitable for high gradient particle acceleration with high drive-bunch to wake
energy transfer efficiency -- a first step in achieving a high overall energy
transfer efficiency. We have done this by using time-structured electron drive
bunches that typically have one or more ultra-high current (>30 kA) femtosecond
spike(s) superimposed on a longer (~0.4 ps) lower current (<10 kA) bunch
structure. The first spike effectively field-ionizes the gas and produces a
meter-scale (30-160 cm) plasma, whereas the subsequent beam charge creates a
wake. The length and amplitude of the wake depends on the longitudinal current
profile of the bunch and plasma density. We find that the onset of pump
depletion, when some of the drive beam electrons are nearly fully depleted of
their energy, occurs for hydrogen pressure >1.5 Torr. We also show that some
electrons in the rear of the bunch can gain several GeV energies from the wake.
These results are reproduced by particle-in-cell simulations using the QPAD
code. At a pressure of ~2 Torr, simulations results and experimental data show
that the beam transfers about 60% of its energy to the wake
Spatiotemporal dynamics of ultrarelativistic beam-plasma instabilities
An electron or electron-positron beam streaming through a plasma is
notoriously prone to micro-instabilities. For a dilute ultrarelativistic
infinite beam, the dominant instability is a mixed mode between longitudinal
two-stream and transverse filamentation modes, with a phase velocity oblique to
the beam velocity. A spatiotemporal theory describing the linear growth of this
oblique mixed instability is proposed, which predicts that spatiotemporal
effects generally prevail for finite-length beams, leading to a significantly
slower instability evolution than in the usually assumed purely temporal
regime. These results are accurately supported by particle-in-cell (PIC)
simulations. Furthermore, we show that the self-focusing dynamics caused by the
plasma wakefields driven by finite-width beams can compete with the oblique
instability. Analyzed through PIC simulations, the interplay of these two
processes in realistic systems bears important implications for upcoming
accelerator experiments on ultrarelativistic beam-plasma interactions
Underdense plasma lens with a transverse density gradient
We explore the implications of a transverse density gradient on the performance of an underdense plasma lens and nonlinear plasma-based accelerator. Transverse density gradients are unavoidable in plasma sources formed in the outflow of standard gas jets, which are used heavily in plasma accelerator communities. These density gradients lead to longitudinal variations in the transverse wakefields, which can transversely deflect an electron beam within the blowout wake. We present a theoretical model of the fields within the plasma blowout cavity based on empirical analysis of 3D particle-in-cell (PIC) simulations. Using this model, the transverse beam dynamics may be studied analytically, allowing for an estimation of the net kick of a witness electron bunch from an underdense plasma lens and for density uniformity tolerance studies in plasma accelerators and plasma lenses. This model is compared to PIC simulations with a single electron bunch and constant density profile, and to PIC simulations with two bunches and a thin, underdense plasma lens density profile with density ramps
Laughter and humor as complementary and alternative medicines for dementia patients
<p>Abstract</p> <p>Background</p> <p>The number of dementia patients has increased worldwide, with an estimated 13.7 million dementia patients in the Asia Pacific region alone. This number is expected to increase to 64.6 million by the year 2050.</p> <p>Discussion</p> <p>As a result of advances in research, there several pharmacological therapies available for the treatment of dementia patients. However, current treatments do not suppress the disease process and cannot prevent dementia, and it will be some time before these goals are realized. In the meantime, complementary and alternative medicine (CAM) is an important aspect in the treatment of dementia patients to improve their quality of life throughout the long course of the disease. Considering the individuality of dementia patients, applicability of laughter and humor therapy is discussed. Even though there are many things that need to be elucidated regarding the mechanisms underlying the beneficial effects of laughter and humor, both may be good CAM for dementia patients if they are applied carefully and properly.</p> <p>Summary</p> <p>In this debate article, the physiological basis and actual application of laughter and humor in the treatment of dementia patients are presented for discussion on the applicability to dementia patients.</p