High-energy Particle Accelerators Research Articles

FIRST DETECTION OF >100 MeV GAMMA-RAYS ASSOCIATED WITH A BEHIND-THE-LIMB SOLAR FLARE

We report the first detection of >100 MeV gamma rays associated with a behind-the-limb solar flare, which presents a unique opportunity to probe the underlying physics of high-energy flare emission and particle acceleration. On 2013 October 11 a GOES M1.5 class solar flare occurred ~ 9.9 degrees behind the solar limb as observed by STEREO-B. RHESSI observed hard X-ray emission above the limb, most likely from the flare loop-top, as the footpoints were occulted. Surprisingly, the Fermi Large Area Telescope (LAT) detected >100 MeV gamma-rays for ~30 minutes with energies up to GeV. The LAT emission centroid is consistent with the RHESSI hard X-ray source, but its uncertainty does not constrain the source to be located there. The gamma-ray spectra can be adequately described by bremsstrahlung radiation from relativistic electrons having a relatively hard power-law spectrum with a high-energy exponential cutoff, or by the decay of pions produced by accelerated protons and ions with an isotropic pitch-angle distribution and a power-law spectrum with a number index of ~3.8. We show that high optical depths rule out the gamma rays originating from the flare site and a high-corona trap model requires very unusual conditions, so a scenario in which some of the particles accelerated by the CME shock travel to the visible side of the Sun to produce the observed gamma rays may be at work.

Power Supplies for High Energy Particle Accelerators

The on-going research and the development projects with Large Hadron Collider at CERN, Geneva, Switzerland has generated enormous enthusiasm and interest amongst all to know about the ultimate findings on ‘God’s Particle’. This paper has made an attempt to unfold the power supply requirements and the methodology adopted to provide the stringent demand of such high energy particle accelerators during the initial stages of the search for the ultimate particles. An attempt has also been made to highlight the present status on the requirement of power supplies in some high energy accelerators with a view that, precautionary measures can be drawn during design and development from earlier experience which will be of help for the proposed third generation synchrotron to be installed in India at a huge cost.

Spontaneous and induced radiation by electrons/positrons in natural and photonic crystals. Volume free electron lasers (VFELs): From microwave and optical to X-ray range

Spontaneous and induced radiation produced by relativistic particles passing through natural and photonic crystals has enhanced capabilities for achieving the radiation sources operating in different wavelength ranges. Use of a non-one-dimensional distributed feedback, arising through Bragg diffraction in spatially periodic systems (natural and artificial (electromagnetic, photonic) crystals), establishes the foundation for the development of volume free electron lasers/masers (VFELs/VFEMs) as well as high-energy charged particle accelerators. The analysis of basic principles of VFEL theory demonstrates the promising potential of VFELs as the basis for the development of high-power microwave and optical sources.

Thermomechanical assessment of the effects of a jaw-beam angle during beam impact on Large Hadron Collider collimators

The correct functioning of a collimation system is crucial to safely and successfully operate high-energy particle accelerators, such as the Large Hadron Collider (LHC). However, the requirements to handle high-intensity beams can be demanding, and accident scenarios must be well studied in order to assess if the collimator design is robust against possible error scenarios. One of the catastrophic, though not very probable, accident scenarios identified within the LHC is an asynchronous beam dump. In this case, one (or more) of the 15 precharged kicker circuits fires out of time with the abort gap, spraying beam pulses onto LHC machine elements before the machine protection system can fire the remaining kicker circuits and bring the beam to the dump. If a proton bunch directly hits a collimator during such an event, severe beam-induced damage such as magnet quenches and other equipment damage might result, with consequent downtime for the machine. This study investigates a number of newly defined jaw error cases, which include angular misalignment errors of the collimator jaw. A numerical finite element method approach is presented in order to precisely evaluate the thermomechanical response of tertiary collimators to beam impact. We identify the most critical and interesting cases, and show that a tilt of the jaw can actually mitigate the effect of an asynchronous dump on the collimators. Relevant collimator damage limits are taken into account, with the aim to identify optimal operational conditions for the LHC.

Angular momentum transport and particle acceleration during magnetorotational instability in a kinetic accretion disk.

Angular momentum transport and particle acceleration during the magnetorotational instability (MRI) in a collisionless accretion disk are investigated using three-dimensional particle-in-cell simulation. We show that the kinetic MRI can provide not only high-energy particle acceleration but also enhancement of angular momentum transport. We find that the plasma pressure anisotropy inside the channel flow with p(∥)>p(⊥) induced by active magnetic reconnection suppresses the onset of subsequent reconnection, which, in turn, leads to high-magnetic-field saturation and enhancement of the Maxwell stress tensor of angular momentum transport. Meanwhile, during the quiescent stage of reconnection, the plasma isotropization progresses in the channel flow and the anisotropic plasma with p(⊥)>p(∥) due to the dynamo action of MRI outside the channel flow contribute to rapid reconnection and strong particle acceleration. This efficient particle acceleration and enhanced angular momentum transport in a collisionless accretion disk may explain the origin of high-energy particles observed around massive black holes.

Development of a Novel Method for the Exploration of the Thermal Response of Superfluid Helium Cooled Superconducting Cables to Pulse Heat Loads

Management of transient heat deposition in superconducting magnets and its extraction from the aforementioned is becoming increasingly important to bring high energy particle accelerator performance to higher beam energies and intensities. Precise knowledge of transient heat deposition phenomena in the magnet cables will permit to push the operation of these magnets as close as possible to their current sharing limit, without unduly provoking magnet quenches. With the prospect of operating the Large Hadron Collider at CERN at higher beam energies and intensities an investigation into the response to transient heat loads of LHC magnets, operating in pressurized superfluid helium, is being performed. The more frequently used approach mimics the cable geometry by resistive wires and uses Joule-heating to deposit energy. Instead, to approximate as closely as possible the real magnet conditions, a novel method for depositing heat in cable stacks made out of superconducting magnet-cables has been developed. The goal is to measure the temperature difference as a function of time between the cable stack and the superfluid helium bath depending on heat load and heat pulse length. The heat generation in the superconducting cable and precise measurement of small temperature differences are major challenges. The functional principle and experimental set-up are presented together with proof of principle measurements.

Low secondary electron yield engineered surface for electron cloud mitigation

Secondary electron yield (SEY or δ) limits the performance of a number of devices. Particularly, in high-energy charged particle accelerators, the beam-induced electron multipacting is one of the main sources of electron cloud (e-cloud) build up on the beam path; in radio frequency wave guides, the electron multipacting limits their lifetime and causes power loss; and in detectors, the secondary electrons define the signal background and reduce the sensitivity. The best solution would be a material with a low SEY coating and for many applications δ < 1 would be sufficient. We report on an alternative surface preparation to the ones that are currently advocated. Three commonly used materials in accelerator vacuum chambers (stainless steel, copper, and aluminium) were laser processed to create a highly regular surface topography. It is shown that this treatment reduces the SEY of the copper, aluminium, and stainless steel from δmax of 1.90, 2.55, and 2.25 to 1.12, 1.45, and 1.12, respectively. The δmax further reduced to 0.76–0.78 for all three treated metals after bombardment with 500 eV electrons to a dose between 3.5 × 10−3 and 2.0 × 10−2 C·mm−2.

An Interpretation of a Possible Mechanism for the First Ground-Level Enhancement of Solar Cycle 24

It is well known that solar flares and shocks driven by coronal mass ejections (CMEs) are high-energy particle acceleration processes that might cause a high-energy particle event known as a ground-level enhancement (GLE). In this context, we have attempted to understand the processes responsible for the first GLE event (GLE71 17 May 2012 01:50 UT) of Solar Cycle 24. We studied the spatial and spectral data from the Solar Dynamics Observatory (SDO) the Culgoora radio-heliograph, and Wind/WAVES instrument, and analyzed the temporal data of the solar-flare components, the solar radio-flux density, and the electron fluxes from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), the Geostationary Operational Environmental Satellite (GOES), the Radio Solar Telescope Network (RSTN), and Wind spacecraft. The flare had two ribbons separated by the neutral line between negative and positive magnetic polarity. Their structure was also almost consistent with the contours of some flare components, which were almost saturated during the flare-peak time. As indicated by the metric–kilometric Type-II burst, and because it extended over a wide heliolongitude (> ≈ 41∘) range, the CME-driven shock was fast enough to cause high-energy particle acceleration at a high altitude in the solar corona. Moreover, the CME and flare-flash phases were aligned along the same direction, which implies that if the CME-driven shock played the leading role in causing the GLE, preceding flare components may have contributed to the shock.

The triple GEM detector as stray neutron monitor

A triple GEM detector for low energy neutrons and high rejection of gamma background was tested at the CERF facility at CERN as stray neutron monitor. The detector was exposed to a wide neutron spectrum generated by a 120 GeV/c positively charged hadron beam hitting a copper target. The beam particle rate on target ranged over three orders of magnitude. The neutron count rate measured with the GEM could be linearly correlated with the beam intensity and also compared well with Monte Carlo simulations. The detector performance suggests that it can be used as an independent low energy neutron monitor at high-energy particle accelerator facilities.

Proton activation history on the Vulcan high-intensity petawatt laser facility

AbstractHigh-intensity lasers are an effective source for the acceleration of high-energy particles. Using different interaction configurations, such facilities can be optimized for the acceleration of electrons, protons, heavy ions, high-energy photons, or neutrons. The shielding of these facilities to ensure the safety of personnel has always been a critical requirement and is a fundamental step within the design phase. The knowledge of radiation source terms through both experiments and modelling is now well understood and for the most part can be dealt with through the use of shielding and specialized beam dumps. Unlike most other particle accelerators most high-power laser facilities are still accessed by personnel post shot with little or no remote handling capabilities. As a result, the secondary activation and control of components that lie around the interaction is of great importance to safety. In this paper, we present a 10 year history of activation data on the Vulcan petawatt facility and discuss the primary sources of activation and the potential impact on future laser facilities.

Scientists and Students from Hong Kong Join a Top Particle Physics Experiment

A team of physicists from Hong Kong has now formally joined one of the most prestigious physics experiments in the world. Following a unanimous vote of approval today by its Collaboration Board, ATLAS has admitted the Hong Kong team as a member. The ATLAS Collaboration operates one of the largest particle detectors in the world, located at the Large Hadron Collider (LHC), the world's highest energy particle accelerator at CERN, Switzerland. In 2012, the ATLAS team — along with the CMS Collaboration — co-discovered the Higgs boson, or so-called 'God Particle'. The gigantic but sensitive and precise ATLAS detector, together with the unprecedentedly high collision energy and luminosity of the LHC, make it possible to search for fundamentally new physics, such as dark matter, hidden extra dimensions, and supersymmetry — a proposed symmetry among elementary particles. The LHC is currently undergoing an upgrade, targeting a substantial increase in beam energy and intensity in a year's time. It is widely expected that the discovery of the Higgs boson is only the beginning of an era of new breakthroughs in fundamental physics. All these exciting opportunities are now opened up to scientists and students from Hong Kong.

A phenomenological cost model for high energy particle accelerators

Accelerator-based facilities have enabled forefront research in high-energy physics for more than half a century. The accelerator technology of colliders has progressed immensely, while beam energy, luminosity, facility size, and cost have grown by several orders of magnitude. Themethod of colliding beams has not fully exhausted its potential but has slowed down considerably in its progress. In this paper we derive a simple scaling model for the cost of large accelerators and colliding beam facilities based on costs of 17 big facilities which have been either built or carefully estimated. Although this approach cannot replace an actual cost estimate based on an engineering design, this parameterization is to indicate a somewhat realistic cost range for consideration of what future frontier accelerator facilities might be fiscally realizable.

Filaments in the southern giant lobe of Centaurus A: constraints on nature and origin from modelling and GMRT observations

We present results from imaging of the radio filaments in the southern giant lobe of Centaurus A using data from GMRT observations at 325 and 235 MHz, and outcomes from filament modelling. The observations reveal a rich filamentary structure, largely matching the morphology at 1.4 GHz. We find no clear connection of the filaments to the jet. We seek to constrain the nature and origin of the vertex and vortex filaments associated with the lobe and their role in high-energy particle acceleration. We deduce that these filaments are at most mildly overpressured with respect to the global lobe plasma showing no evidence of large-scale efficient Fermi I-type particle acceleration, and persist for ~ 2-3 Myr. We demonstrate that the dwarf galaxy KK 196 (AM 1318-444) cannot account for the features, and that surface plasma instabilities, the internal sausage mode and radiative instabilities are highly unlikely. An internal tearing instability and the kink mode are allowed within the observational and growth time constraints and could develop in parallel on different physical scales. We interpret the origin of the vertex and vortex filaments in terms of weak shocks from transonic MHD turbulence or from a moderately recent jet activity of the parent AGN, or an interplay of both.

Augmented reality aiding collimator exchange at the LHC

Novel Augmented Reality techniques have the potential to have a large positive impact on the way remote maintenance operations are carried out in hazardous areas, e.g. areas where radiation doses that imply careful planning and optimization of maintenance operations are present. This paper describes an Augmented Reality strategy, system and implementation for aiding the remote collimator exchange in the LHC, currently the world׳s largest and highest-energy particle accelerator. The proposed system relies on marker detection and multi-modal augmentation in real-time. A database system has been used to ensure flexibility. The system has been tested in a mock-up facility, showing real time performance and great potential for future use in the LHC. The technical-scientific difficulties identified during the development of the system and the proposed solutions described in this paper may help the development of future Augmented Reality systems for remote handling in scientific facilities.

Top Quarks Go Solo in Rare Events

The subatomic elementary particles known as quarks are among the building blocks of the standard model (SM) of particle physics. Protons and neutrons are made of the lightest, up and down quarks, but the unstable strange, charm, bottom, and top quarks can also be produced in high-energy particle accelerators. The enigmatic top quark—which is the heaviest elementary particle, weighing as much as a gold nucleus—has only been studied at Fermilab’s Tevatron collider near Chicago and at the Large Hadron Collider at CERN, Geneva. It is most often produced with its antiparticle (antitop) through the strong interaction, but it also appears singly through the electroweak interaction, as was demonstrated in 2009 [1, 2]. Two new papers in Physical Review Letters [3, 4] report the first observations of single top quarks coming out of less common electroweak reactions, or “channels.” The results fall within SM predictions, but further studies of the relative frequency of these rare channels may reveal new physics, such as a hidden generation of quarks or unexpected top-quark couplings.

BASIC BELL-MHD TURBULENCE

Nonresonant current instability was identified by Bell (2004) as an important mechanism for magnetic field amplification in supernova remnants. In this paper we focus on studying the nonlinear stage of this instability using the incompressible MHD formulation. We demonstrate that the evolution of magnetic turbulence driven by the Bell instability resembles turbulence driven on large scales. More importantly, we demonstrate that the energy-containing scale for magnetic fields is proportional to the square root of magnetic energy density. Given the observational constraints of the possible field amplification, this new relation allows us to directly estimate the maximum energy of particles scattered by such fields and this estimate is normally below the average particle energy. This implies that, without taking into account the feedback to cosmic rays, the typical scales of Bell fields, in either linear or nonlinear regime, will be too small to affect high energy particle acceleration. We mention several scenarios of back-reaction to cosmic rays that could be important.

A multi-scale point of view on the structure–property relationships of A15 superconductors

Nb3Sn and other A15 members have been widely applied in nuclear power, nuclear magnetic resonance, and high-energy particle accelerators for their high critical current density (Jc) and upper critical field (Bc2). There have been comprehensive and intensive studies on the applications, the fundamental lattice dynamic and electronic properties, etc., of A15 superconductors. Various reviews on the preparations, structures, and properties have already been written in the last few years. Nevertheless, on account of the large amount of existing facts and views, a coherent view on the relations between the structures and properties has not appeared to unify the facts. This article sketches a multi-scale point of view on the relations between the multi-scale structures and the corresponding properties.

Loss cone evolution and particle escape in collapsing magnetic trap models in solar flares

Collapsing magnetic traps (CMTs) have been suggested as one possible mechanism responsible for the acceleration of high-energy particles during solar flares. An important question regarding the CMT acceleration mechanism is which particle orbits escape and which are trapped during the time evolution of a CMT. While some models predict the escape of the majority of particle orbits, other more sophisticated CMT models show that, in particular, the highest-energy particles remain trapped at all times. The exact prediction is not straightforward because both the loss cone angle and the particle orbit pitch angle evolve in time in a CMT. Our aim is to gain a better understanding of the conditions leading to either particle orbit escape or trapping in CMTs.

On the maximum energy of shock-accelerated cosmic rays at ultra-relativistic shocks

The maximum energy to which cosmic rays can be accelerated at weakly magnetised ultra-relativistic shocks is investigated. We demonstrate that for such shocks, in which the scattering of energetic particles is mediated exclusively by ion skin-depth scale structures, as might be expected for a Weibel-mediated shock, there is an intrinsic limit on the maximum energy to which particles can be accelerated. This maximum energy is determined from the requirement that particles must be isotropized in the downstream plasma frame before the mean field transports them far downstream, and falls considerably short of what is required to produce ultra-high-energy cosmic rays. To circumvent this limit, a highly disorganized field is required on larger scales. The growth of cosmic ray-induced instabilities on wavelengths much longer than the ion-plasma skin depth, both upstream and downstream of the shock, is considered. While these instabilities may play an important role in magnetic field amplification at relativistic shocks, on scales comparable to the gyroradius of the most energetic particles, the calculated growth rates have insufficient time to modify the scattering. Since strong modification is a necessary condition for particles in the downstream region to re-cross the shock, in the absence of an alternative scattering mechanism, these results imply that acceleration to higher energies is ruled out. If weakly magnetized ultra-relativistic shocks are disfavoured as high-energy particle accelerators in general, the search for potential sources of ultra-high-energy cosmic rays can be narrowed.

New Materials for Vacuum Chambers in High Energy Physics

Vacuum chambers must fulfil ultra-high vacuum requirements while withstanding thermo-mechanical loads. This is particularly true in high energy particle accelerator where interactions of particles with matter may induce thermal load, material activation, background… The choice of the material of the vacuum chamber is crucial for the final application. Metals such as stainless steel, copper and aluminium are usually used. Even with outstanding mechanical and physical properties, beryllium is used for very specific applications because of its cost and toxicity.Ceramics such as alumina are usually used for fast magnet vacuum chambers. With the next generation of high energy physics accelerator generation such as CLIC and TLEP, the problematic of high cyclic thermal load induced by synchrotron radiation is raised. This paper aims at defining some figures of merit of different materials with respect to several load scenarios and presents briefly their vacuum compatibility.