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Register a new userMultiple X-ray bursts from long discharges in air
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Added by
Alexander van Deursen
Publication date
2008
fields
Physics
and research's language is
English
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A lightning surge generator generates a high voltage surge with 1.2 microsec. rise time. The generator fed a spark gap of two pointed electrodes at 0.7 to 1.2 m distances. Gap breakdown occurred between 0.1 and 3 microsec. after the maximum generator voltage of approximately 850 kV. Various scintillator detectors with different response time recorded bursts of hard radiation in nearly all surges. The bursts were detected over the time span between approximately half of the maximum surge voltage and full gap breakdown. The consistent timing of the bursts with the high-voltage surge excluded background radiation as source for the high intensity pulses. In spite of the symmetry of the gap, negative surges produced more intense radiation than positive. This has been attributed to additional positive discharges from the measurement cabinet which occurred for negative surges. Some hard radiation signals were equivalent to several MeV. Pile-up occurs of lesser energy X-ray quanta, but still with a large fraction of these with an energy of the order of 100 keV. The bursts occurred within the 4 nanosec. time resolution of the fastest detector. The relation between the energy of the X-ray quanta and the signal from the scintillation detector is quite complicated, as shown by the measurements.
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X-ray and gamma-ray emissions observed in lightning and long sparks are usually connected with the bremsstrahlung of high-energy runaway electrons. Here, an alternative physical mechanism for producing X-ray and gamma-ray emissions caused by the polarization current and associated electromagnetic field moving with relativistic velocity along a curved discharge channel has been proposed. It is pointed out that lightning and spark discharges should also produce a coherent radio-frequency radiation. The influence of the conductivity and the radius of the lightning channel on the propagation velocity of electromagnetic waves, taking into account the absorption, have been investigated. The existence of fast electromagnetic surface waves propagating along the lightning discharge channel at a speed close to the speed of light in vacuum is shown. The possibility of the production of microwave, X-ray and gamma-ray emissions by a polarization current pulse moving along a curved path via synchrotron radiation mechanism during the lightning leader steps formation and the very beginning of the return stroke stage is pointed out. The existence of long tails in the power spectrum is shown, which explains observations of photon energies in the range of 10-100 MeV in the TGF, as well as measured power spectrum of laboratory spark discharge.
The results of the experiments on recording hard gamma radiation and measurements of its angular distribution at the initial stage of a laboratory high-voltage atmospheric discharge are presented. The experiments were performed on an ERG installation at a voltage of $sim 1$ MV, an atmospheric discharge current of up to 12 kA, and a gap of 0.55 m. The duration of the voltage pulse was about 1~$mu$s with a pulse rise time of 150-200 ns. The radiation was recorded by an assembly of 10 identical scintillation detectors installed each 10$^circ$ around the circumference of a quarter of a circle with a curvature of 1 m. In order to separate the radiation with energies from 20 keV to 1.5 MeV, Al and Pb filters of different thicknesses were used. The obtained results show that, as a rule, a multi-beam radiation pattern and several bursts of radiation (each with a directional pattern) are recorded in each shot. In a considerable number of shots, hard radiation with photon energies comparable to or exceeding the maximum electron energy corresponding to the applied voltage is recorded. In these cases, a needle-like radiation pattern is observed, including at large angles to the axis of the discharge. This may indicate the acceleration of electrons in different plasma channels.
We propose to construct a compact and portable x-pinch driver with x-ray radiation performance, comparable to standard x-pinch drivers. Such a new x-pinch driver was recently designed, fabricated and tested at the Idaho Accelerator Center. The generator is based on two slow LTD bricks combined into one solid unit, and can be described by a simple RLC circuit with four fast 140-nF, 100-kV capacitors that store up to 2.8 kJ. The test data reveals that when charged to 80 kV, the driver supplies 185-kA peak-current into a short Ni-wire load with 220-ns, 10-90%, rise time. The total internal inductance of our driver was measured to be about 60 nH. The revised driver model shows that when fully charged to 100 kV, the driver will supply 180-kA peak-current with 150-ns rise-time into the x-pinch load. The corresponding current rise rate is about 1.2 kA/ns. To prove the driver x-pinch efficiency and to estimate the x-ray radiation performance, we could, for example, image an exploding wires, placed in a separate HV pulser, with our x-pinch x-ray radiation source. The study of exploding wires helps to understand the behavior of a warm dense matter, and our x-pinch driver can be part of the diagnostics needed for this study which is currently under progress at the IAC. Our driver contains no oil inside, is very compact and portable, and can be easily relocated to practically anywhere, which makes it an ideal backlighting diagnostic tool in many areas of plasma physics, biology, and industry where a bright, fast, and small x-pinch radiation source is required.
The interaction of streamers in nitrogen-oxygen mixtures such as air is studied. First, an efficient method for fully three-dimensional streamer simulations in multiprocessor machines is introduced. With its help, we find two competing mechanisms how two adjacent streamers can interact: through electrostatic repulsion and through attraction due to nonlocal photo-ionization. The non-intuitive effects of pressure and of the nitrogen-oxygen ratio are discussed. As photo-ionization is experimentally difficult to access, we finally suggest to measure it indirectly through streamer interactions.
We have performed a systematic study of the Bremsstrahlung emission from the electrons in the plasma of a commercial 14.5 GHz Electron-Cyclotron Resonance Ion Source. The electronic spectral temperature and the product of ionic and electronic densities of the plasma are measured by analyzing the Bremsstrahlung spectra recorded for several rare gases (Ar, Kr, Xe) as a function of the injected power. Within our uncertainty, we find an average temperature of ? 48 keV above 100W, with a weak dependency on the injected power and gas composition. Charge state distributions of extracted ion beams have been determined as well, providing a way to disentangle the ionic density from the electronic density. Moreover X-ray emission from highly charged argon ions in the plasma has been observed with a high-resolution mosaic crystal spectrometer, demonstrating the feasibility for high-precision measurements of transition energies of highly charged ions, in particular of the magnetic dipole (M1) transition of He-like of argon ions.
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