Investigation On Pulse Current Produced By Intense X-rays-Metal Interactions

By high-energy-density systems, we refer to those having a pressure above one million atmospheres. The units of this pressure can be designated by 1 Mbar,1011 J/m3.In most high-energy-density systems the matter is in the plasma state. High-energy-density systems commonly exist in the universe especially in the late stage of galaxies, for example pulsars are high energy density systems.Some pulsars can generate directed radio radiation having a power of 1020-24 W. Its radiation intensity is inverse proportion to square of frequencyω, which manifests non-thermal radiation character. The radio emission mechanisms of pulsars have not been solved satisfactorily. These indicate there exist some non-thermal emission mechanisms in high-energy-density systems, and these provide some clues for pursuing generating high power (≥1016 W), high energy (≥1010 J) directed electromagnetic pulse.Early investigations on high-energy-density systems focused on astrophysics because of lack of high energy density facilities.Rapid developments of ultra-intense, ultra-short laser, Z pinch, pulse power technique, magnetic loading technique and nice diagnostic instruments provide probabilities to investigate high energy density physics in laboratory. Z pinch plasmas produce the most intense X-ray burst in laboratory, for example the peak power in X-ray burst produced by plasmas at Sandia National Laboratories Z facility can exceed 200 TW, and the duration is typically-6 ns FWHM.The system will be excited after absorbing energy from external sources.The system will transit from excited state to low energy state with energy releasing simultaneously, such as vaporization, mass ejections, rising temperature, thermal radiation and non-thermal emission. Whether the modes of energy releasing can be controlled at our will? The main purpose of this paper is attempting to understand non-thermal emission mechanism in high energy density systems produced by intense X-rays. The main conclusions are:(1)The transmission line shaped homocentric sphere surface have the lowest inductances;the rising time of load current is approximately 80～90 ns and the peak current reached 1 MA using the new transmission line.(2) The electron currents consist of three parts including photoelectric return currents, Compton currents and thermoelectric currents. At the beginning of irradiation photoelectric currents dominate; Compton currents and thermal currents gradually dominate with increase of irradiation intensity of X-rays. The magnitude of thermal current is approximately 1 A in our experiments, and it is typically an order of magnitude of photoelectron current or Compton current. Our consequent experiments excluded the effects caused by splash of plasmas, Rogovsky coils and the dispersion of the cable.(3)Detected the electromagnetic pulse produced by oblique incident X-rays, and the results were unsatisfactory probably because the intensity of X-rays was not high enough to excite intense directed radio electromagnetic pulses.(4)Proposed a micro model to explain the observations. We calculated photoelectric efficiency according the wave nature of electrons.We got the relation between potential experienced by electrons and the refraction index, then we extended Newton equation to descript wave motion. Compton current density and photoelectron current density were～10-1 A/cm2, and the thermal current density was～105 A/cm2 at the surface of Cu irradiated by 106 W/cm2 X-rays;energy deposition depth was～100 nm and the electron kinetic was～101 eV. Thermal current is proportional to Ia (1/3<α<1/2).Electron kinetic and current density are inverse proportional to energy deposition depth, that is, current density decrease with increase of incident photon energy.(5) Discuss the macro mechanism of thermal current according to thermal conduction equation. We calculated the distribution of temperature on the target; the magnitude of temperature was～103 K, and the magnitude of current density was～105 A/cm2.The position where peak current density appeared moved with velocity of～103 m/s away from irradiated area, that is, the electrons moved outward from irradiated area to the boundary. We discussed the effects of incident conditions on energy releasing.(6) Classify the interaction mechanisms according to correlation. Photoelectric effect and Compton effect belong to individual behavior of electrons, and thermal effect manifest weak correlated collective behavior of electrons. Strong correlated collective behavior-charge density modulation state is predicted to appear on the boundary of vacuum/metal irradiated by ultra-intense X-rays, and a highly directed radio electromagnetic pulse will be generated during the process of de-excitation of this state.(7) Charge density modulation state is reasonable according to symmetry theorem. Considering one dimensional case energy stored in charge density state is proportional to product of square of modulation magnitude and square of modulation wavelength. Electron-phonon interactions and magnetic field were main factors producing charge density modulation.