Magnetic spectrometry of deuteron spectra in NX2 plasma focus

Magnetic spectrometry using polymer nuclear track detectors is used for high-resolution measurement of the energy spectrum of deuterons (≫250 keV) emitted from the pinch-column of the NX2 plasma focus (PF). The spectrum is measured for single PF shots. A 25 µm pinhole images the deuteron source on the spectrometer. This pinhole enables differential vacuum to be performed between the plasma focus and spectrometer chambers. Deuteron tracks are measured with an automated scanning system. The detectors are imaged at ×20 objective magnification, for which the CCD field of view is 528×379 µm 2 . Then the spectrum on the detector is scanned in a path comprising 432 images. The recognized and counted tracks are accumulated in a histogram of track displacement in bins representing 100 µm wide equal energy stripes on the detector. A very distinct neutrals spot is clearly visible for each spectrum, resulting from uncharged deuterons passing through the spectrometer in straight-line paths and producing a densely tracked zone. The deuteron trajectory in the magnetic field is calculated in order to obtain the deuteron energy as a function of the distance from the neutrals spot on the detector. The deuteron spectrum d 2 N/dEdΩ is derived from the track displacement histogram. The average number of deuterons per shot is ≈1.5×10 15 . For an empirical power-law deuteron spectrum of the form dN/dE=kE −n , the best fit to the data over the energy range 250 to 500 keV is obtained for n≈4. The deuteron spectrum is used to estimate the beam-target contribution of neutron yield to be Y n =1.8×10 5 neutron per shot. It shows that the beam-target contribution for deuterons with energies more than 250 keV is small compared to the total neutron yield of NX2 plasma focus which is in the order of 10 8 neutron per shot. Hence the fusion neutron yield is predominantly due to deuterons with energies ≪250 keV. The number of Nitrogen-13 nuclides in the activation of graphite through 12 C(d,n) 13 N is estimated using the deuteron spectra and the thick target yield to be Y 13N =3.4×10 6 nuclide per shot.