Measurements of Energy Spectra of Fast Electrons from PF-1000 in the Upstream and Downstream Directions 1
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R. Kwiatkowski , K. Czaus , E. Skladnik-Sadowska , M.J. Sadowski , K. Malinowski , 1 2 2 2 3 J. Zebrowski , L. Karpinski , M. Paduch , M. Scholz , and P. Kubes 1
The Andrzej Soltan Institute for Nuclear Studies (IPJ), 05-400 Otwock-Swierk, Poland 2 Institute of Plasma Physics and Laser Microfusion (IPPLM), 01-497 Warsaw, Poland 3 Czech Technical University (CVUT), 166-27 Prague, Czech Republic
ABSTRACT The paper describes measurements of energy spectra of electrons emitted in the upstream direction along the symmetry-axis of the PF-1000 facility, operated with the deuterium filling at 21 kV, 290 kJ. The measurements were performed with a magnetic analyzer. The same analyzer was used to measure also electron beams emitted in along the symmetry-axis in the downstream direction. The recorded spectra showed that the electron-beams emitted in the upstream direction have energies in the range from about 40 keV to about 800 keV, while those in the downstream direction have energies in the range from about 60 keV to about 200 keV. These spectra confirm that in the PF plasma column there appear strong local fields accelerating charged particles in different directions.
Scheme of PF-1000 experiment and positions of the magnetic analyzer
Magnetic analyzer of electrons connectors
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support of a film upstream direction magnetic analyzer
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mostly ion beams
magnetic analyzer
mostly e-beams
magnetic analyzer
collimator permanent magnets
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The applied magnetic analyzer was equipped with an input collimator and solid magnets deflecting the investigated electron beams. To record the deflected electrons the use was made of an X-ray film shielded with a thin metal foil, which converted them into Xrays. To eliminate the saturation effect the use was made of two X-ray films placed as a sandwich, when the first film worked as an absorption filter in front of the second one. Both films were wrapped up with a light-tight Al-foil of 30 µm in thickness, placed on a support made of several thin W-wires, and irradiated in the analyzer during a single PF shot.
In the described experiments the PF-1000 facility was equipped with coaxial electrodes of 230 mm and 400 mm in diameter. The discharges were triggered at the initial pressure of 0.75-2.1 hPa D2, and they were powered from a condenser bank charged to 21 kV, 290 kJ. Magnetic analyzer was located at the z-axis behind the main collector plate (in the upstream direction at z = -160 cm) or upon the end-plate of the vacuum chamber (in the downstream direction at z = 200 cm).
Experimental results - downstream direction
Experimental results - upstream direction
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Electron-induced images recorded on the 1 film (top row) and the 2 film (bottom row) for different PF1000 shots: left column - for shot #8361, right column - for shot #8355
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Electron-induced images recorded on the 1 film (top) and the 2nd film (bottom).
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Color-enhanced densitogram from the 1 film and the corresponding energy distribution of electrons.
The quantitative analysis of the exposed films showed that the population of the fast electrons emitted in the downstream direction was considerably lower and their energy spectra extended from about 60 keV to about 200 keV.
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Zero points were produced by fast neutral particles and they corresponded to electrons of infinite energy. Bright vertical lines on the recorded images corresponded to positions of thin W-wires, which helped to scale the images, while the energy scale was known from the analyzer calibration.
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Color enhanced isodensity profiles of the electron images and the energy spectra obtained from the 1 film (left) and the 2nd (right).
It was found that at the investigated experimental conditions the energy spectrum of the electron beams emitted in the upstream direction extended from about 40 keV to about 800 keV. Those results confirmed that accelerating fields in PF plasma are considerably higher that the voltage applied between the electrodes.
The main aim of electron beam measurements within the PF-1000 facility, which was performed during the recent experimental campaigns in 2009, was to record the fast electrons both in the upstream and downstream directions and to compare their energy spectra.
Summary and conclusions Measurements of intense electron beams, which were carried out at different labs, were concentrated on the beams emitted in the upstream direction, i.e. at an angle of 180 to the z-axis. Such electron beams during their interactions with a solid anode generate intense pulses of X-rays, which are recorded in many PF experiments. If a tubular anode is applied, the upstream electron beams can penetrate through the central channel and be recorded behind the main collector plate. It was, however, suspected that fast electrons can be emitted also in other directions.
The most important results of the described studies can be summarized as follows: 1. The measurements with the magnetic analyzer in the upstream direction have confirmed that PF -1000 discharges can emit many fast electrons of relatively high energy (40-800 keV); 2. The measurements along the z-axis have delivered a new experimental evidence that fast electron beams can be emitted also in the downstream direction, although their intensity is lower and the energy spectrum is more narrow (60-200 keV);
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3. The both observations show that the electromagnetic fields responsible for the acceleration of charged particles can reach relatively high values.
