High-speed videos in an optical cryostat, with frame rates up to 5x10(6) fps, are used to study the dynamics of laser-induced cavitation in helium near the critical point and in the supercritical region. The propagation of strong shock waves are observed in both regimes. The time dependence of the cavitation bubble radius as well as the acoustic pressure field outside the bubble are described by standard compressible flow models. In the temperature range 4K < T < 5.2K, a symmetric cloud of micron-scale bubbles are observed outside the main cavitation bubble as it approaches its maximum radius which is due to homogeneous nucleation and spinodal decomposition in the low-pressure fluid outside the bubble. Nucleation of secondary bubbles is also observed far below the critical point, but this requires large negative pressures that can be generated by shock waves that reflect from the primary bubble.

A newly uncovered class of plasmons in the strongly excited limit opens access to unprecedented Petavolts per meter electromagnetic fields with wide-ranging, transformative impact. Unlike conventional plasmons, such plasmons are constituted by non-perturbative, large-amplitude oscillations of the ultradense, delocalized free electron Fermi gas inherent in conductive media. Here structured semiconductors doped to have an appropriate conduction electron density are introduced to tune the properties of the Fermi gas for matched excitation of large-amplitude plasmons using readily available electron beams which enables immediate experimental validation. Specifically, an electrostatic, surface “crunch-in” plasmon is collisionlessly excited by the beam launched inside a tube. Strong excitation due to matching results in relativistic oscillations of the electron gas and unravels unique phenomena. Relativistically induced ballistic electron transport comes about due to relativistic multifold increase in the mean free path and also leads to unconventional heat deposition beyond Ohm’s law. This explains the absence of observed damage or solid-plasma formation in past experiments on conductive samples interacting with electron bunches shorter than 10−13 seconds. Furthermore, relativistic momentum leads to copious tunneling of electron gas across the surface, which then crunches inside the tube. Relativistic effects along with large, localized electron density variations underlying these modes necessitate kinetic approach to theoretical and computational modeling. Kinetic model presented here demonstrates experimental viability of observing tens of gigavolts per meter plasmonic fields excited by matching readily available electron beams to plasmons in semiconductors with 1018cm−3 free electron density, and paves the way for Petavolts per meter plasmonics.

Plasma wakefield acceleration (PWFA) has shown illustrious progress and resulted in an impressive demonstration of tens of GeV particle acceleration in meter-long single structures. To reach even higher energies in the 1 TeV to 10 TeV range, a promising scheme is channeling acceleration in solid-density plasmas within crystals or nanostructures. The E336 experiment studies the beam-nanotarget interaction with the highly compressed electron bunches available at the FACET-II accelerator. These studies furthermore involve an in-depth research on dynamics of beam-plasma instabilities in ultra-dense plasma, its development and suppression in structured media like carbon nanotubes and crystals, and its potential use to transversely modulate the electron bunch.

Ultra-compact electron beam technology based on laser wakefield acceleration (LWFA) could have a significant impact on radiotherapy treatments. Recent developments in LWFA high-density regime (HD-LWFA) and low-intensity fiber optically transmitted laser beams could allow for cancer treatments with electron beams from a miniature electronic source. Moreover, an electron beam emitted from a tip of a fiber optic channel could lead to new endoscopy-based radiotherapy, which is not currently available. Low-energy (10 keV–1 MeV) LWFA electron beams can be produced by irradiating high-density nano-materials with a low-intensity laser in the range of ~1014 W/cm2. This energy range could be useful in radiotherapy and, specifically, brachytherapy for treating superficial, interstitial, intravascular, and intracavitary tumors. Furthermore, it could unveil the next generation of high-dose-rate brachytherapy systems that are not dependent on radioactive sources, do not require specially designed radiation-shielded rooms for treatment, could be portable, could provide a selection of treatment energies, and would significantly reduce operating costs to a radiation oncology clinic.

Ultra-high gradients which are critical for future advances in high-energy physics, have so far relied on plasma and dielectric accelerating structures. While bulk crystals were predicted to offer unparalleled TV/m gradients that are at least two orders of magnitude higher than gaseous plasmas, crystal-based acceleration has not been realized in practice. We have developed the concept of nanoplasmonic crunch-in surface modes which utilizes the tunability of collective oscillations in nanomaterials to open up unprecedented tens of TV/m gradients. Particle beams interacting with nanomaterials that have vacuum-like core regions, experience minimal disruptive effects such as filamentation and collisions, while the beam driven crunch-in modes sustain tens of TV/m gradients. Moreover, as the effective apertures for transverse and longitudinal crunch-in wakes are different, the limitation of traditional scaling of structure wakefields to smaller dimensions is significantly relaxed. The SLAC FACET-II experiment of the nano2WA collaboration will utilize ultra-short, high-current electron beams to excite nonlinear plasmonic modes and demonstrate this possibility. NANOPLASMONIC ACCELERATOR The quest for using crystals to access many TeraVolts per meter (TV/m) acceleration gradients was set forth in 1968 by experimental nuclear physicist and Nobel laureate, R. Hofstadter [1]. It was hypothesized that single-atom excited states across the length of the ionic lattice of a crystal would upon stimulation individually transfer their energy and accelerate the interacting particles. In 1980s, this quest was revived in theory using the mechanism of channeling of particles between the planes of a metallic lattice [2]. However, given several fundamental limitations such as uncontrolled energy loss and emittance growth due to beam collision with the ionic-lattice besides complete disruption of the beam due to filamentation, hosing etc., crystal-based acceleration has remained unrealizable in practice. Consequently, over the past few decades great strides have been made towards tens of GV/m gradients using significantly different media of plasmas [3] and dielectrics [4], although these gradients are well below a TV/m. Our work [5–8] has introduced and demonstrated a new concept on using plasmonic modes [9] in nanomaterials for experimental realizability of unprecedented tens of TV/m ∗ Work supported by Dept. of Electrical Engineering at CU Denver † aakash.sahai@ucdenver.edu electromagnetic (EM) fields. This new mechanism can overcome several fundamental limitations and thereby holds the promise to open up an entirely new extreme field frontier. Specifically, a new class of nonlinear surface plasmons [10] in the crunch-in regime wherein the oscillating surface electrons collectively cross over in to the core vacuum region (as shown in Fig.1) are excited in nanomaterials [11–13] that have tunable structural and material properties. sustained by hollow, bea(Wk) ⌧ a(W?) and a(Wk) ! 0 become evident. of nano2WA sustained by hollowpurely electromagnetic, bea(Wk) ⌧ a(W?) and a(Wk) ! 0 become evident. of nano2WA sustained by hollowpurely ele tromagnetic, bea(Wk) ⌧ a(W?) and a become evident.

We have used video imaging and interferometric techniques to investigate the dynamics of spreading of drops of He-4 on a solid surface for temperatures ranging from 5.2 K (near the critical point) to 2.2 K (near T-lambda). After an initial transient, the drops become pancake-shaped with a radius that grows as R(t) approximate to t(alpha), with alpha = 0.149 +/- 0.002. The drops eventually begin to shrink due to evaporation driven by gravitational and curvature effects, which limits their lifetime to about 1000 s. Although helium completely wets the substrate, and the spreading takes place over a pre-existing adsorbed film, a distinct contact line with a contact angle of order one degree is visible throughout this process.

Superfluid helium droplets impacting on a solid surface behave much differently than any other fluid. After a short period of initial spreading that is similar to classical fluids, superfluid helium drops quickly shrink and disappear as the superfluid drains out through a thin adsorbed layer of helium on the surface. The lifetime and contact angle of these drops is strongly temperature-dependent, and colder drops (with high superfluid fractions) maintain a constant contact angle throughout the contraction. Above ${T}_{l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}a}$, helium spreads slowly like other classical fluids.

Thin films of quantum fluids, i.e., 4He, 3He, and H2, have played an important role in understanding the phenomenology of quantum fluids and the role of spatial dimension on the development of long-range order in condensed matter. Standard experimental probes used to study these systems include heat capacity measurements, torsional oscillators, third sound, quartz crystal microbalances, and x-ray and neutron scattering. We describe the historical development of important models and experiments in quantum films which underpin our understanding of superfluid onset in helium, phases, and phase transitions in adsorbed films, and wetting and growth of bulk phases.

The breakup of jets of superfluid and normal liquid ${}^{4}$He is studied between 1.2 K and the liquid-vapor critical point at 5.2 K. Both gas and liquid properties vary widely over this small temperature range, creating a unique parameter space with variations of several orders of magnitude for the Ohnesorge number, Reynolds number, and gas-liquid density ratio. The five breakup regimes seen previously, and transitions between them, are described in detail and shown pictorially. New criteria are proposed for the Rayleigh to 1st wind, 1st wind to sinuous, and sinuous to 2nd wind transitions.

Solid-state or crystal acceleration has for long been regarded as an attractive frontier in advanced particle acceleration. However, experimental investigations of solid-state acceleration mechanisms which offer [Formula: see text] acceleration gradients have been hampered by several technological constraints. The primary constraint has been the unavailability of attosecond particle or photon sources suitable for excitation of collective modes in bulk crystals. Secondly, there are significant difficulties with direct high-intensity irradiation of bulk solids, such as beam instabilities due to crystal imperfections and collisions etc. Recent advances in ultrafast technology with the advent of submicron long electron bunches and thin-film compressed attosecond x-ray pulses have now made accessible ultrafast sources that are nearly the same order of magnitude in dimensions and energy density as the scales of collective electron oscillations in crystals. Moreover, nanotechnology enabled growth of crystal tube structures not only mitigates the direct high-intensity irradiation of materials, with the most intense part of the ultrafast source propagating within the tube but also enables a high degree of control over the crystal properties. In this work, we model an experimentally practicable solid-state acceleration mechanism using collective electron oscillations in crystals that sustain propagating surface waves. These surface waves are driven in the wake of a submicron long particle beam, ideally also of submicron transverse dimensions, in tube shaped nanostructured crystals with tube wall densities, [Formula: see text]. Particle-In-Cell (PIC) simulations carried out under experimental constraints demonstrate the possibility of accessing average acceleration gradients of several [Formula: see text] using the solid-state tube wakefield acceleration regime. Furthermore, our modeling demonstrates the possibility that as the surface oscillations and resultantly the surface wave transitions into a nonlinear or “crunch-in” regime under [Formula: see text], not only does the average gradient increase but strong transverse focusing fields extend down to the tube axis. This work thus demonstrates the near-term experimental realizability of Solid-State Tube Wakefield Accelerator (SOTWA). The ongoing progress in nanoengineering and attosecond source technology thereby now offers the potential to experimentally realize the promise of solid-state or crystal acceleration, opening up unprecedented pathways in miniaturization of accelerators.

Plasma-based accelerator technology enables compact particle accelerators. In Laser Wakefield Acceleration, with an ultrafast high-intensity optical laser driver, energy gain of electrons is greater if the electron density is reduced. This is because the energy gain of electrons is proportional to the ratio of laser's critical density to electron density. However, an alternative path for higher energy electrons is increasing the critical density via going to shorter wavelengths. With the advent of Thin Film Compression, we now see a path to a single cycle coherent X-ray beam. Using this X-ray pulse allows us to increase the plasma density to solid density nanotube (carbon or porous alumina) regime and still be under-dense for a Laser Wakefield Acceleration technique. We will discuss some implications of this below.

The advent of the path to a single cycle X-ray laser pulse via thin film compression and the relativistic compression enables laser wakefield acceleration in solid materials. We study the collective interaction of the X-ray laser pulse with the solid-state plasma, including ultrafast polariton effects, giving rise to TeV/cm wakefields with highly increased critical density. Our particle-in-cell computational analysis delineates wakefield effects and polariton dynamics. We show that a good quality wakefield can be excited even in the presence of the lattice force and the electron acceleration process is not influenced by polaritons. The applications and implications of the ultrafast wakefield and ultrafast plasmonics are discussed.

Author(s): Wallace, Matthew | Advisor(s): Taborek, Peter | Abstract: We present the results of our investigation of superfluid and normal fluid helium droplets impacting on a solid surface in an optical cryostat at temperatures between 1.2 K and 5.1 K at saturated vapor pressure. We use high-speed video to image the impacting drops over a large range of Reynolds and Weber numbers. We also use high-speed interferometry to measure the thickness and curvature of the droplets. We find that the initial impact stages for both normal and superfluid helium droplets are similar to the results for conventional fluids. We observe that at longer spread times, the normal helium droplets do not completely wet the surface and maintain a small but finite contact angle indefinitely. This result is surprising because helium is expected to fully wet nearly all surfaces. The spreading dynamics of normal fluid and superfluid droplets are temperature-dependent, and the lifetime of normal fluid drops on the substrate is substantially longer than superfluid drop lifetime. The normal fluid long-term spreading follows a power law of spreading diameter vs. time with an exponent very close to 1/7, while the superfluid drops dissipate before long-term spreading can occur. Unexpectedly, we observe the Leidenfrost effect for normal fluid helium at high temperatures near the critical point.

Deformability, the extent to which a particle changes shape under applied stresses, has recently gained attention as an important mechanical marker for cells. Cell deformability is determined by physiological properties such as cytoskeletal structure, and can be related to the cell's phase in its mitotic cycle and can be used as a marker to differentiate cancerous and non-cancerous cells. Given the wide range of information gained from deformability measurements, a deformability-based cell detector is highly desirable. One recent method for measuring cell deformability is flow deformability cytometry, where cells are driven through microconstrictions at fast velocities and deform under the large hydrodynamic loads. Cell deformation is usually measured optically using a microscope and high-speed camera; however, while optical detection is accurate, it is financially and computationally expensive. Here we report deformability cytometry experiments performed with simultaneous optical and electrical detection using the resistive pulse technique. By synchronizing the optical and electrical signals in time, both recorded with 200,000 Hz, we are able to directly observe the effect of deformation of the cells on the resistive pulse signal, leading to the eventual possibility for replacing optical detection with resistive pulse entirely. The experiments were performed using a non-traditional channel design characterized by narrow entrances along with a relatively larger central cavity. The channel design induces bidirectional deformation as the cells pass through the channel, which was confirmed through experimental observation and finite element analysis modeling.