Ponderomotive and resonant effects in the acceleration of particles by electromagnetic modes

Funding: U.K. Science and Engineering Research Council under Grant No. EP/N028694/1 (R.A.C.).In the present analysis, we study the dynamics of charged particles under the action of slowly modulated electromagnetic carrier waves. With the use of a high-frequency laser mode along with a modulated static magnetic wiggler, we show that the ensuing total field effectively acts as a slowly modulated high-frequency beat-wave field typical of inverse free-electron laser schemes. This effective resulting field is capable of accelerating particles in much the same way as space-charge wake fields do in plasma accelerators, with the advantage of being more stable than plasma related methods. Acceleration occurs as particles transition from ponderomotive to resonant regimes, so we develop the ponder- omotive formalism needed to examine this problem. The ponderomotive formalism includes terms that, although not discussed in the usual applications of the approximation, are nevertheless of crucial importance in the vicinity of resonant capture. The role of these terms is also briefly discussed in the context of generic laser-plasma interactions.PostprintPeer reviewe

Linear plasma experiment for non-linear microwave interaction experiments

As a non-linear medium, plasma can exhibit diverse dynamics when excited bymultiple EM waves. Electromagnetic waves are vital to the introduction of energyin laser plasma interactions and the heating of magnetically confined fusion reactors.In laser plasma applications Raman coupling via a Langmuir oscillation or Brillouinscattering mediated by ion-acoustic waves are of interest. Signals with normalisedintensities approaching those used in some recent laser plasma interactions can begenerated using powerful and flexible microwave amplifiers, interacting in relativelytenuous, cool and accessible plasma. Other multi-wave interactions are interesting formagnetic confinement fusion plasmas, for example beat-wave interactions betweentwo microwave signals coupling to cyclotron motion of the ions and electrons or thelower hybrid oscillations may be useful in heating the plasmas or for driving currents.A linear plasma experiment is being built to test such multifrequency microwaveinteraction in plasma, based on prior research on geophysical cyclotron wave emissionand propagation [1,2]. The main section of the plasma will be magnetised at up to0.05T, with the plasma created by an RF helicon source to generate a dense, large,cool plasma with a high ionisation fraction. A range of frequency-flexible sources willprovide microwave beams to enable multi-wave coupling experiments. The paper willpresent progress on this apparatus and experiments.The authors gratefully acknowledge support from the EPSRC, MBDA UK Ltd andTMD Technologies Ltd.[1] Ronald K., Speirs D.C., McConville S.L., Phelps A.D.R., Robertson C.W., WhyteC.G., He W., Gillespie K.M., Cross A.W., Bingham R., 2008, Phys. Plasmas, 15,art.056503[2] Speirs, D.C., Bingham, R., Cairns, R.A., Vorgul, I., Kellett, B.J., Phelps, A.D.R.,Ronald, K, 2014, Phys. Rev. Lett., 113, art 15500

Toward an internal gravity wave spectrum in global ocean models

Office of Naval Research, National Science FoundationPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111824/1/grl_internalwavespectrum_2015.pd

Maser radiation from collisionless shocks

Funding: UK Engineering and Physical Sciences Research Council (grant Nos. EP/N014472/1, EP/R004773/1 and EP/N013298/1) and the Science and Technologies Facilities Council of the United Kingdom. F.C. and L.O.S. acknowledge support from the European Research Council (InPairs ERC-2015-AdG 695088) and FCT Portugal (grant No. PD/BD/114307/2016).This paper describes a model of electron energization and cyclotron-maser emission applicable to astrophysical magnetized collisionless shocks. It is motivated by the work of Begelman, Ergun and Rees [Astrophys. J. 625 , 51 (2005)] who argued that the cyclotron-maser instability occurs in localized magnetized collisionless shocks such as those expected in blazar jets. We report on recent research carried out to investigate electron acceleration at collisionless shocks and maser radiation associated with the accelerated electrons. We describe how electrons accelerated by lower-hybrid,waves at collisionless shocks generate cyclotron-maser radiation when the accelerated electrons move into regions of stronger magnetic fields. The electrons are accelerated along the magnetic field and magnetically compressed leading to the formation of an electron velocity distribution having a horseshoe shape due to conservation of the electron magnetic moment. Under certain conditions the horseshoe electron velocity distribution function is unstable to the cyclotron-maser instability [Bingham and Cairns, Phys. Plasmas 7, 3089 (2000); Melrose, Rev. Mod. Plasma Phys. 1 , 5 (2017)].Publisher PDFPeer reviewe

Plasma Dynamics

Contains reports on five research projects.U.S. Air Force - Office of Scientifc Research (Contract AFOSR 84-0026)National Science Foundation (Grant ECS 85-14517)Lawrence Livermore National Laboratory (Subcontract 6264005)National Science Foundation (Grant ECS 85-15032)U.S. Department of Energy (Contract DE-ACO2-78-ET-51013)U.S. Department of Energy (Contract DE-ACO2-ET-51013

The Murchison Widefield Array: The Square Kilometre Array Precursor at Low Radio Frequencies

The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80–300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ~3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised

First spectroscopic imaging observations of the sun at low radio frequencies with the Murchison Widefield Array Prototype

We present the first spectroscopic images of solar radio transients from the prototype for the Murchison Widefield Array, observed on 2010 March 27. Our observations span the instantaneous frequency band 170.9- 201.6 MHz. Though our observing period is characterized as a period of "low" to "medium" activity, one broadband emission feature and numerous short-lived, narrowband, non-thermal emission features are evident. Our data represent a significant advance in low radio frequency solar imaging, enabling us to follow the spatial, spectral, and temporal evolution of events simultaneously and in unprecedented detail. The rich variety of features seen here reaffirms the coronal diagnostic capability of low radio frequency emission and provides an early glimpse of the nature of radio observations that will become available as the next generation of low-frequency radio interferometers come online over the next few years

Plasma Dynamics

Contains reports on four research projects.National Science Foundation (Grant ECS82-13485)University of Maryland (Subcontract A200728)U.S. Air Force - Office of Scientific Research (Grant AFOSR-84-0026B)U.S. Department of Energy (Contract DE-ACO2-78-ET-51013)National Science Foundation (Grant ECS82-13430