Single Helicity Research Articles

Revisiting the shadow stress tensor in celestial CFT

We revisit the standard construction of the celestial stress tensor as a shadow of the subleading conformally soft graviton. In its original formulation there is an obstruction to reproducing the expected overline{TT} OPE in the double soft limit. We propose a modification to the definition which circumvents this obstruction and then extend this change of basis beyond the conformally soft and single helicity sectors. In the process we investigate how (non)-commutativity of double soft limits is tied to the decoupling of primary descendants, and how our choice of celestial basis determines which symmetries are manifest at the level of the OPE beyond the MHV sector.

Combined Capacitor–Resistor Energy Transfer System to Increase Plasma Current in RFX-Mod2

RFX-mod is an experimental fusion device, which contributes to plasma physics studies both in reversed field pinch (RFP) and tokamak configurations. Its high flexibility, due to an active magnetohydrodynamic (MHD) control system and the modular coils power supply (CPS) system, allowed operating RFX-mod in a wide range of experimental conditions with a plasma current up to 2 MA. Experiments with such high plasma current allowed the study on new promising confinement regimes, dominated by a self-organization process with the generation of a helical structure in the plasma core. Presently, RFX-mod is under a significant upgrade (RFX-mod2) to extend the operational scenarios increasing the proximity between the conductive shell and the plasma. This main modification of the magnetic front-end, together with other main improvements, is expected to increase the performance of the machine in both magnetic configurations. In the frame of the studies to exploit the new potential of RFX-mod2 achieving a higher plasma current and longer flat-top duration, a solution based on additional magnetic energy storage has already been proposed. The study of RFP physics at higher plasma current could be crucial to confirm positive trends, such as electron temperature and persistence of quasi single helicity states with the plasma current, and to explore and achieve improved confinement states. This article presents an alternative reconfiguration of the poloidal power supply system of RFX-mod2 based on a combined resistor–capacitor energy transfer system. This system allows to store energy in capacitor banks during the first phase of the plasma current ramp-up and release it to the plasma when the magnetizing current changes polarity, driving the plasma current over 2.5 MA. The proposed upgrade does not involve radical poloidal CPS modifications, maintaining the present converters number and ratings and remaining within the power limits of the main power transformers (300 MVA).

Sizable time-reversal violating effects in bottom baryon decays

We study time-reversal (T) asymmetries in the charmless two-body decays of antitriplet bottom baryons. We find that in Bb → BnP with Bb = ( {Xi}_b^{-} , {Xi}_b^0 , Λb) and Bn(P) a octet baryon (pseudoscalar meson), the positive and negative helicity amplitudes are both sizable, resulting in the large T-violating effects. Particularly, the T-odd parameters, βw, for the color-enhanced channels are expected to be around −40% and 25% in b → s and b → d transitions for the standard model, which can be measured by the experiments at LHCb, respectively. On the other hand, in Bb → BnV(γ) with V(γ) a vector meson (photon), the decays are predominated by a single helicity amplitude, and the T violating effects by the correlations among spins are suppressed. We also explore the T violating observables for the Ξb decays based on the angular distributions.

Bright, single helicity, high harmonics driven by mid-infrared bicircular laser fields.

High-harmonic generation (HHG) is a unique tabletop light source with femtosecond-to-attosecond pulse duration and tailorable polarization and beam shape. Here, we use counter-rotating femtosecond laser pulses of 0.8 µm and 2.0 μm to extend the photon energy range of circularly polarized high-harmonics and also generate single-helicity HHG spectra. By driving HHG in helium, we produce circularly polarized soft x-ray harmonics beyond 170 eV-the highest photon energy of circularly polarized HHG achieved to date. In an Ar medium, dense spectra at photon energies well beyond the Cooper minimum are generated, with regions composed of a single helicity-consistent with the generation of a train of circularly polarized attosecond pulses. Finally, we show theoretically that circularly polarized HHG photon energies can extend beyond the carbon K edge, extending the range of molecular and materials systems that can be accessed using dynamic HHG chiral spectro-microscopies.

The isotope effects in RFP magnetic configuration

The comparison between deuterium and hydrogen plasmas at high current in the RFX-mod experiment proves that the reversed field pinch configuration experiences the isotope effect in a way analogous to tokamak experiments. Both energy and particle confinement exhibit a favourable scaling with the isotope mass M i , respectively, as and as . This effect is mainly due to the mitigation of transport at the edge, which is driven by electrostatic fluctuations, although a mitigation of the stochastic transport in the plasma core is also observed. As in tokamaks, the MHD properties are modified when the gas mass is changed. The most evident modification regards the spectrum of tearing modes that are more stable in deuterium, so that the duration and the purity of quasi single helicity (QSH) phases are enhanced. The thermal structures that develop during the QSH phases enclose a plasma volume that is wider in deuterium discharges than in hydrogen ones.

(Nanocarbons Division Robert C. Haddon Research Award) Chemical Functionalization of Synthetic Carbon Allotropes

Chemical functionalization of new C-allotropes is of fundamental interest and opens the door to unprecedented materials applications. In principle, the physical and chemical properties of fullerenes, carbon nanotubes (CNTs) and graphene are related to each other, although their levels of development vary considerably. In order to efficiently explore the reactivity of the less developed CNTs and especially that of graphene it is our goal to provide a unifying approach for the chemistry of all three new carbon allotropes. The fullerenes present the first family of synthetic carbon allotropes. Since their availability in macroscopic quantities numerous investigations with respect to their physical characterization and chemical functionalization have been carried out. CNTs - the second new family of C-allotropes - exhibit at least the same potential for unprecedented applications. Their chemistry, however, is much less developed. Although many protocols for covalent and non-covalent CNT functionalization have been published there are still many fundamental problems to be solved. This includes inter alia the highly selective functionalization of metallic- or semiconducting SWNTs, the high throughput isolation of SWNTs with single helicity, the development of general concepts allowing for tunable doping of individualized tubes with single helicity or least defined transport characteristics (metallic – semiconductive). Finally, the youngest representative in the list of new C-allotropes is graphene and its chemical functionalization opens exciting opportunities towards the development of precisely structured high performance materials. We will talk on the systematic investigation of the chemical properties of graphene both in homogenous solution and deposited on solid substrates. We have revealed fundamental reactivity principles, such as role of defects, charging and substrate. Moreover, we have disclosed for the first time basic fundamental topicity concepts. This allows us to tailor highly integrated graphene based 2D-materials. An example is the introduction of reversible covalent 2D-structuring providing the basis for chemical data storage.

Control of multi-helicity neo-classical tearing modes by electron cyclotron current drive in tokamak plasmas

The effect of electron cyclotron current drive (ECCD) on the multi-helicity neo-classical tearing modes (2/1 and 3/2 NTMs) in tokamak plasmas is numerically studied by adopting a reduced magnetohydrodynamic (MHD) model. It is found that the control process of certain single helicity NTM by ECCD can be influenced by nonlinear multi-helicity interaction. In the case without plasma rotation, the ‘phase flip’ of 2/1 NTM, which happens in the control of single helicity situation, has been modified by multi-helicity interaction and consequently behaves periodicity. And the ‘phase flip’ of 3/2 NTM disappears. When plasma rotation is considered, the direct control of 3/2 NTM becomes difficult. Moreover, a dual role of 5/3 mode is found in the multi-helicity interaction between 2/1 and 3/2 modes. The nonlinear mode coupling among 2/1 and 3/2 modes through the 5/3 mode will increase the difficulty of the suppression of 3/2 mode by ECCD, while the competition effect between multi-helicity NTMs leads to the relatively weak amplitude of 2/1 mode. In addition, investigations of multiple ECCDs are also carried out. The numerical results indicate that the control strategy for controlling multi-helicity NTMs by ECCD in experiments may require well-designed aiming to gain high effectiveness and efficiency.

Deterministic all-optical magnetization writing facilitated by non-local transfer of spin angular momentum

Ever since its discovery around a decade ago, all-optical magnetization switching (AOS) using femtosecond laser pulses has shown potential for future data storage and logic devices. In particular, single pulse helicity independent AOS in certain ferrimagnetic alloys and multilayers is highly efficient and ultrafast. However, in most cases it is a toggle mechanism, which is not desirable for applications. Here we experimentally demonstrate conversion from toggle switching to a deterministic mechanism by biasing AOS in a Co/Gd bilayer with a spin polarized current which is optically generated in an adjacent ferromagnetic reference layer. We show deterministic writing of an ‘up’ and ‘down’ state using a sequence of one or two pulses, respectively, and demonstrate the non-local origin by varying the magnitude of the generated spin current. Our demonstration of deterministic magnetization writing could provide an essential step towards the implementation of future optically addressable spintronic memory devices.

(Nanocarbons Division Robert C. Haddon Research Award Address) Chemical Functionalization of Synthetic Carbon Allotropes

Chemical functionalization of new C-allotropes is of fundamental interest and opens the door to unprecedented materials applications. In principle, the physical and chemical properties of fullerenes, carbon nanotubes (CNTs) and graphene are related to each other, although their levels of development vary considerably. In order to efficiently explore the reactivity of the less developed CNTs and especially that of graphene it is our goal to provide a unifying approach for the chemistry of all three new carbon allotropes. The fullerenes present the first family of synthetic carbon allotropes. Since their availability in macroscopic quantities numerous investigations with respect to their physical characterization and chemical functionalization have been carried out. CNTs - the second new family of C-allotropes - exhibit at least the same potential for unprecedented applications. Their chemistry, however, is much less developed. Although many protocols for covalent and non-covalent CNT functionalization have been published there are still many fundamental problems to be solved. This includes inter alia the highly selective functionalization of metallic- or semiconducting SWNTs, the high throughput isolation of SWNTs with single helicity, the development of general concepts allowing for tunable doping of individualized tubes with single helicity or least defined transport characteristics (metallic – semiconductive). Finally, the youngest representative in the list of new C-allotropes is graphene and its chemical functionalization opens exciting opportunities towards the development of precisely structured high performance materials. We will talk on the systematic investigation of the chemical properties of graphene both in homogenous solution and deposited on solid substrates. We have revealed fundamental reactivity principles, such as role of defects, charging and substrate. Moreover, we have disclosed for the first time basic fundamental topicity concepts. This allows us to tailor highly integrated graphene based 2D-materials. An example is the introduction of reversible covalent 2D-structuring providing the basis for chemical data storage.

Intermittence and turbulence in fusion devices

In this work, we explore the response of the intermittence parameter C(1) to the magnetic topology of confined fusion plasmas, as well as to spontaneously generated zonal flows. To achieve this, we make extensive use of resistive Magneto-Hydrodynamic calculations. First, we explore single helicity calculations, where we observed clearly that the intermittence is due to the mixing between turbulence and (poloidal) flow fluctuations. The minima of C(1) were correlated with maxima of the mean transfer entropy, , noting that the latter have been identified with ‘trapping zones’ for radial transport in previous work. Thus, these ‘trapping zones’ are closer to monofractality than the surrounding plasma. In more realistic multiple helicity calculations, these results were confirmed, although somewhat less clear, as the mutual interaction between helicities now also contributed to the intermittence. We applied these techniques to experimental Electron Cyclotron Emission data from the W7-X stellarator and found that these observations also appear to hold in an experimental situation. Thus, we conclude that the intermittence parameter C(1) may provide valuable indirect information about the interaction between fluctuations, zonal flows, and the magnetic topology of fusion plasmas.

A new stabilizing regime of tearing mode entrainment in the presence of a static error field

Uncorrected static error fields (EFs) in axisymmetric fusion devices are one of the few remaining serious obstacles for advancing the present tokamak-based approach to a practical reactor. Magnetohydrodynamic tearing modes (TMs) lock to them, causing sudden losses of confinement known as disruptions. Recently, a hypothesis has been proposed that there may exist a self-healing stable regime in which a static resonant EF is effectively shielded by forcing these TMs to slowly rotate inductively by the applied non-axisymmetric field (Inoue et al 2017 Nucl. Fusion 57 116020; Inoue et al 2018 Plasma. Phys. Control. Fusion 60 025003; Inoue et al 2018 Preprint: 2018 IAEA Fusion Energy Conf. TH/P4-24). This is based on non-linear, resistive, reduced magnetohydrodynamic simulations using a cylindrical single helicity model. Proof-of-principle experiments in the DIII-D device showed that the magnetic mode structure on the plasma surface is qualitatively consistent with the simulation prediction. However, radial mode profiles revealed qualitatively different behavior. This led to a revised hypothesis that in actual non-circular toroidal devices, a tearing layer in forced rotation induces a shielding process at other rational surfaces when we take into account multiple resonant Fourier components. The time evolution experiment of the radial penetration is supportive of this hypothesis.

Some remarks on dual helicity flag-dipole spinors

In this report we advance into a mapping procedure transmuting a single helicity spinor into a dual helicity spinor. Such a mathematical mechanism reveal us a class of spinor which fits into fourth class within Lounesto classification [1]. The focus of the present manuscript is to expose the algebraic construction of flag-dipole spinors and briefly explore its underlying physical and mathematical contents.

First observation of plasma healing via helical equilibrium in tokamak disruptions

A prompt transition of equilibrium states in a disruptive plasma is observed in the presence of three-dimensional (3D) resonant magnetic fields. A dominantly stochastic field line state heals into a 3D equilibrium with intact flux surfaces, resembling a single helicity magnetic island in the plasma core. On the other hand, the current quench process is bifurcated, i.e. from a completed quench accompanied by a vertically unstable disruption to a rapid termination of current decay followed by a full recovery to its predisruption value. This observation implies prompt stochastic fields healing through the rapid non-linear growth of a magnetic island in the plasma core may provide a plausible path for a soft-landing scenario of tokamak disruption in a high plasma current regime.

Helical plasma-wall interaction in the RFX-mod: effects of high-n mode locking

The purpose of this paper is to analyse higher toroidal harmonics m = 1 with 7 < n < 23 (secondary modes) in the quasi single helicity, reversed field pinch plasma in the RFX-mod device. The quasi single helicity is an improved confinement state characterized by a dominant mode m/n = 1/7, rotating along the toroidal direction. The spectroscopic measurements in the boundary plasma show footprints of a localized plasma deformation called ‘phase locking’, which can be described as an interferential pattern of toroidal Fourier harmonics. In the locking region the magnetic field lines are deformed in large poloidal lobes hitting the plasma facing components, a mechanism similar to the ‘homoclinic lobes’ observed in the Tokamak divertor with RMP application. Correspondingly, the magnetic connection length to the wall presents a strong decrease (‘hole’) with increased plasma-wall interaction, which fortunately is not stationary, but jumps in the toroidal direction thanks to the effective action of the RFX-mod feedback system. The global plasma wall interaction is the superposition of a rotating helix and of the localized toroidal deformation due to the secondary modes, which mirrors the local emission and particle influx from the wall. The local edge perturbation impacts also on the global plasma performance: a threshold to obtain an electron transport barrier has been identified. An improvement in plasma performance is expected in the upgraded device RFX-mod2, where the magnetic boundary will be modified to decrease the edge field deformation by a factor about 2.

Upgrades of the RFX-mod reversed field pinch and expected scenario improvements

RFX-mod is a Reversed Field Pinch device that allowed performing experiments in regimes with a plasma current up to 2 MA, thanks to its MHD active control system. Experiments have shown that improved plasma performances are obtained when in the resonant part of the m = 1 spectrum one dominant tearing mode is much higher than the other secondary ones (quasi single helicity states). Tearing modes play a crucial role in determining energy and particle transport. Based on the present understanding of the interplay between passive conductive boundaries and tearing modes in an RFP, an upgrade of RFX-mod machine assembly has been designed, dubbed RFX-mod2, and it is now being implemented. The highly resistive Inconel vessel will be removed, graphite tiles will be attached to the copper stabilizing shell and the stainless steel support structure will be modified in order to be vacuum tight. In RFX-mod2, the shell-plasma proximity decreases from b/a = 1.11 to b/a = 1.04 and copper, instead of Inconel, will be the continuous conducting structure nearest to the plasma. MHD non-linear simulations show that secondary tearing modes amplitude and the edge bulging due to their phase locking will decrease; moreover the plasma current threshold for tearing modes wall locking will also significantly increase.

Type-4 spinors: transmuting from Elko to single-helicity spinors

In this communication we briefly report an unexpected theoretical discovery which emerge from the mapping of Elko mass-dimension-one spinors into single helicity spinors. Such procedure unveils a class of spinor which is classified as type-4 spinor field within Lounesto classification. In this paper we explore the underlying physical and mathematical contents of the type-4 spinor.

Analytic helicity amplitudes for two-loop five-gluon scattering: the single-minus case

We present a compact analytic expression for the leading colour two-loop five-gluon amplitude in Yang-Mills theory with a single negative helicity and four positive helicities. The analytic result is reconstructed from numerical evaluations over finite fields. The numerical method combines integrand reduction, integration-by-parts identities and Laurent expansion into a basis of pentagon functions to compute the coefficients directly from six-dimensional generalised unitarity cuts.

Recent Progress in Chemistry of Multiple Helicenes.

The last decade has witnessed multiple helicenes arising as an interesting class of nonplanar polycyclic aromatics of inherent multihelicity. These molecules present esthetic structures and interesting properties not available to helicenes of single helicity. Herein an overview of multiple helicenes with respect to structures, stereochemical dynamics, synthesis, and applications is provided. Recently reported multiple helicenes are surveyed with an emphasis on molecular structures and stereochemistry of multiple carbohelicenes. After this survey, the synthesis of multiple helicenes through the Scholl reaction is discussed and recent applications of multiple helicenes in organic electronics are summarized. On the basis of these discussions, conclusions are reached on the current status of multiple helicenes and an outlook for this field is provided.

Geometric scalings for the electrostatically driven helical plasma state

A new plasma state has been investigated [Akcay et al., Phys. Plasmas 24, 052503 (2017)], with a uniform applied axial magnetic field in a periodic cylinder of length L=2πR, driven by helical electrodes. The drive is single helicity, depending on mθ+kz=mθ−nζ, where ζ=z/R and k=−n/R. For strong (m,n)=(1,1) drive, the state was found to have a strong axial mean current density, with a mean-field safety factor q0(r) just above the pitch of the electrodes m/n=1 in the interior. This state has possible applications to DC electrical transformers and tailoring of the current profile in tokamaks. We study two geometric issues of interest for these applications: (i) scaling of properties with the plasma length or aspect ratio and (ii) behavior for different helicities, specifically (m,n)=(1,n) for n &amp;gt; 1 and (m,n)=(2,1).

Thermal transport dynamics in the quasi-single helicity state

A dynamical model describing oscillations between multiple and single helicity configurations in the quasi-single helicity (QSH) state of the reversed field pinch [P. W. Terry and G. G. Whelan, Plasma Phys. Controlled Fusion 56, 094003 (2014)] is extended to include electron temperature profile dynamics. It is shown that QSH dynamics is linked to the electron temperature profile because the suppression of mode coupling between tearing modes proposed to underlie QSH also suppresses magnetic-fluctuation-induced thermal transport. Above the threshold of dominant-mode shear that marks the transition to QSH, the model produces temperature-gradient steepening in the strong shear region. Oscillations of the dominant and secondary mode amplitudes give rise to oscillations of the temperature gradient. The phasing and amplitude of temperature gradient oscillations relative to those of the dominant mode are in agreement with experiment. This provides further evidence that the model, while heuristic, captures key physical aspects of the QSH state.