Tongjun Xu

Distribution of acid-sensing ion channel 3 in the rat hypothalamus

Acid-sensing ion channels (ASICs), the members of the epithelial sodium channel/degenerin (ENaC/DEG) superfamily, are proton-gated voltage-insensitive cation channels. Six ASIC subunits have been identified and characterized in the mammalian nervous system so far. Of these subunits, ASIC3 has been shown to be predominantly expressed in the peripheral nervous system of rodents and implicated in mechnosensation, chemosensation and pain perception. Little is known on ASIC3 in the brain. We thus employed reverse transcription-polymerase chain reaction (RT-PCR) and Western blot to examine the expression of ASIC3 in various rat brain regions, including hippocampus, amygdala, caudate putamen, prefrontal cortex, and hypothalamus. Specific attention was paid to the distribution of ASIC3 in the hypothalamus of rats by using immunohistochemistry. ASIC3 immunoreactivity showed a widespread pattern throughout the hypothalamus, with the highest density in paraventricular nucleus, supraoptic nucleus, suprachiasmatic nucleus, arcuate nucleus, dorsomedial nucleus, median preoptic nucleus, ventromedial preoptic nucleus, and dorsal tuberomammillary nucleus. This study may contribute to the understanding of ASIC3 functions in the CNS.

Modulation of acid-sensing ion channels by Cu2+ in cultured hypothalamic neurons of the rat

Acid-sensing ion channels (ASICs) are known to distribute throughout the nervous system and serve important roles in various physiological and pathological processes. However, the properties of ASICs in the hypothalamus, an important region of diencephalon, are little known. We herein used whole-cell patch-clamp recordings to characterize proton-induced cation currents in cultured hypothalamic neurons of the rat, and attributed these transient inward currents to ASICs based on their electrophysiological and pharmacological properties. We further examined the effects of Cu(2+), the third most abundant trace element, on ASICs in hypothalamic neurons. Our results showed that this divalent cation reversibly and concentration-dependently inhibited the amplitude of ASIC currents, and slowed down the desensitization of ASIC channels. Our results also displayed that Cu(2+) modulated ASICs independent of change in membrane potential and extracellular protons, suggesting a noncompetitive mechanism. Furthermore, micromolar concentration of Cu(2+) attenuated the acid-induced membrane depolarization. Taken together, our data demonstrate a modulatory effect of Cu(2+) on ASICs in native hypothalamic neurons and suggest a role of this endogenous metal ion in negatively modulating the increased neuronal membrane excitability caused by activation of ASICs.

Scheme for proton-driven plasma-wakefield acceleration of positively charged particles in a hollow plasma channel

A new scheme for accelerating positively charged particles in a plasma wakefield accelerator is proposed. If the proton drive beam propagates in a hollow plasma channel, and the beam radius is of order of the channel width, the space charge force of the driver causes charge separation at the channel wall, which helps to focus the positively charged witness bunch propagating along the beam axis. In the channel, the acceleration buckets for positively charged particles are much larger than in the blowout regime of the uniform plasma, and stable acceleration over long distances is possible. In addition, phasing of the witness with respect to the wave can be tuned by changing the radius of the channel to ensure the acceleration is optimal. Two dimensional simulations suggest that, for proton drivers likely available in future, positively charged particles can be stably accelerated over 1 km with the average acceleration gradient of 1.3 GeV/m.

Ultrashort megaelectronvolt positron beam generation based on laser-accelerated electrons

Experimental generation of ultrashort MeV positron beams with high intensity and high density using a compact laser-driven setup is reported. A high-density gas jet is employed experimentally to generate MeV electrons with high charge; thus, a charge-neutralized MeV positron beam with high density is obtained during laser-accelerated electrons irradiating high-Z solid targets. It is a novel electron–positron source for the study of laboratory astrophysics. Meanwhile, the MeV positron beam is pulsed with an ultrashort duration of tens of femtoseconds and has a high peak intensity of 7.8 × 1021 s−1, thus allows specific studies of fast kinetics in millimeter-thick materials with a high time resolution and exhibits potential for applications in positron annihilation spectroscopy.

Ultrafast multi-MeV gamma-ray beam produced by laser-accelerated electrons

Ultrafast multi-MeV high-flux gamma-ray beams have been experimentally produced via bremsstrahlung radiation of laser-accelerated energetic electrons through millimeter-thick copper targets. By optimizing the electron bunches to the charge of 10 nC in a clustering argon gas target, the obtained gamma-ray beam significantly increases to 1010 photons per shot. The gamma-ray beam spectrum has been measured using a differential filtering detector and has a broad spectrum up to 15 MeV, which is approximately consistent with the Geant4 simulation. The generated high-flux high-energy gamma-ray beams are promising sources for photonuclear reaction, non-destructive inspection and clinical applications.

Generation of gamma-ray beam with orbital angular momentum in the QED regime

We propose a scheme to generate gamma-ray photons with an orbital angular momentum (OAM) and high energy simultaneously from laser-plasma interactions by irradiating a circularly polarized Laguerre-Gaussian laser on a thin plasma target. The spin angular momentum and OAM are first transferred to electrons from the driving laser photons, and then the OAM is transferred to the gamma-ray photons from the electrons through quantum radiation. This scheme has been demonstrated using three-dimensional quantum electrodynamics particle-in-cell simulation. The topological charge, chirality and carrier-envelope phase of the short ultra-intense vortex laser can be revealed according to the pattern feature of the energy density of radiated photons.

High quality electron bunch generation with CO2-laser-plasma interaction

CO2 laser-driven electron acceleration in low-density plasma is demonstrated using particle-in-cell simulation. An intense CO2 laser pulse of long wavelength excites a wake bubble that has a large elongated volume for accelerating a large number of electrons before reaching the charge saturation limit. A transversely injected laser pulse is used to induce and control the electron injection. It is found that an electron bunch with total charge up to 10 nC and absolute energy spread less than 16 MeV can be obtained. As a result, the charge per energy interval of the bunch reaches up to 0.6 nC/MeV. Intense CO2-laser based electron acceleration can provide a new direction for generating highly charged electron bunches with low energy spread, which is of much current interest, especially for table-top X-ray generation.

Enhanced high harmonic generation and the phase effect in double-sided relativistic laser-foil interaction

High harmonic generation (HHG) from relativistic laser-foil interaction is investigated analytically and through particle-in-cell simulations. Previous work has shown that when two counter-propagating circularly polarized (CP) laser pulses interact with a thin foil, electrons can be well confined spatially to form a high density layer. The layer electrons oscillate in certain transversal direction and radiate intense high order harmonics. It is demonstrated here that there is a critical foil thickness, only below which can high harmonics be generated efficiently. Furthermore, to enhance the intensity in higher order region, the third linearly polarized (LP) short-wavelength laser pulse with much lower intensity is introduced. Analysis and simulations both show that the enhancement is determined by the relative phase δϕ between the driving CP laser pulses and LP pulse. The enhancement at high order is quite considerable and very sensitive to the relative phase δϕ, thus offering not only a way to efficientl...

Ultra-bright, ultra-broadband hard x-ray driven by laser-produced energetic electron beams

We propose a new method of obtaining a compact ultra-bright, ultra-broadband hard X-ray source. This X-ray source has a high peak brightness in the order of 1022 photons/(s mm2 mrad2 0.1\%BW), an ultrashort duration (10 fs), and a broadband spectrum (flat distribution from 0.1 MeV to 4 MeV), and thus has wide-ranging potential applications, such as in ultrafast Laue diffraction experiments. In our scheme, laser-plasma accelerators (LPAs) provide driven electron beams. A foil target is placed oblique to the beam direction so that the target normal sheath field (TNSF) is used to provide a bending force. Using this TNSF-kick scheme, we can fully utilize the advantages of current LPAs, including their high charge, high energy, and low emittance.

Terawatt-scale optical half-cycle attosecond pulses

Extreme-ultravoilet (XUV) attosecond pulses with durations of a few tens of attosecond have been successfully applied for exploring ultrafast electron dynamics at the atomic scale. But their weak intensities limit the further application in demonstrating nonlinear responses of inner-shell electrons. Optical attosecond pulses will provide sufficient photon flux to initiate strong-field processes. Here we proposed a novel method to generate an ultra-intense isolated optical attosecond pulse through relativistic multi-cycle laser pulse interacting with a designed gas-foil target. The underdense gas target sharpens the multi-cycle laser pulse, producing a dense layer of relativistic electrons with a thickness of a few hundred nanometers. When the dense electron layer passes through an oblique foil, it emits single ultra-intense half-cycle attosecond pulse in the visible and ultraviolet spectral range. The emitted pulse has a peak intensity exceeding 1018 W/cm2 and full-width-half-maximum duration of 200 as. The peak power of this attosecond light source reaches 2 terawatt. The proposed method relaxes the single-cycle requirement on the driving pulse for isolated attosecond pulse generation and significantly boosts the peak power, thus it may open up the route to new experiments tracking the nonlinear response of inner-shell electrons as well as nonlinear attosecond phenomena investigation.