V. Nardi

High yield of 12C(d,n)13N and 14N(d,n)15O reactions in the plasma focus pinch

Abstract Our experiments have established that 12C(d,n)13N and 14N(d,n)15O reaction yields inside 12C+D and 14N+D plasmas of focused discharges are ≈1% of the simultaneous D-D fusion yield for 10% of 12C and/or 14N atoms doped into D2 gas. The data set a low limit, at n > 1021 cm−3, for the density of high-reactivity plasma domains, which are identifiable with localized “hot spots” of earlier experiments.

Observation of plasma domains with fast ions and enhanced fusion in plasma-focus discharges

Abstract Our experiments have established strong azimuthal anisotropy of fusion reaction products and fast ions ejected from the plasma during focused discharges in deuterium or a mixture of deuterium with 3 He and/or 16 O. The anisotropy is explained in terms of localized plasma domains with enhanced nuclear reactivity (emitting isotropically), displaced from the axial pinch region by up to 5.5 cm. The displaced domains generate and trap over 80% of the observed population of fast ions ( E i >2 MeV), produce the bulk of the 16 O (d, n) 17 F reactions ( E th ≅ 2MeV) in the plasma, and have atomic density n ⩾7×10 21 cm -3 . Most of the D+D and 3 He+D fusion yield, which requires low ion energy E i ⩽0.2 MeV), comes from the region close to the plasma-focus symmetry axis.

Inspection of extended objects with fast neutrons: numerical tests

Abstract This paper presents a pilot study on numerical simulations of images produced when 14.7 MeV neutrons are considered for the inspection of extended objects. The simulations are performed using a 3-D Monte Carlo subroutine complex (3D-MCSC-RWR) that is free of limitations in the representation of the system and operates in continuum space on point-wise nuclear data files. Numerically generated images prove that the image of a void ( O = 2 mm , h = 2 mm ) in an iron slab (10 cm) can be recorded by a real-time recording-system if a 10 12 −10 13 neutrons/pulse source is considered, as for the case of a plasma focus machine.

Advantages and limits of 14-MeV neutron radiography

The paper evaluates the potentials of fast-neutron radiography (FNR) for the inspection of bulky, solid objects. Data for both a fast (En = 14.7 MeV) and a slow (En = 0.1 eV) neutron source are compared. The reproduction of images consists of Monte Carlo simulations of (a) the neutron random walk in a slab (iron, SiC ceramic, and polyethelene (CH2)n plastic) with a void, (b) the process of neutron recording in a detector, and (c) a print-out of images. For a general analysis, 3D-MCSC-RWR software operates without simplification of either the FNR design or the nuclear data files. The results first show the feasibility of the use of 14-MeV neutron radiography, then the superiority of FNR over slow-neutron radiography in-the-field when the thickness of the full slab is over 1 cm and requires a resolution better than 0.1 mm. Examples of some numerically simulated images as well as FNR scaling functions are shown. A review of the available fast-neutron sources reveals that only plasma-focus machines would simultaneously meet all FNR requirements: Yn ≥ 1013 n/pulse, small-source dimensions and mobility.

X-ray emission above 10 keV from focused discharges (calorimetry and time-resolved monitoring)

Summary form only given, as follows. Two issues are addressed: [1] The location of the source of hard X-rays (/spl ges/10 keV) in MA focused discharges. [2] The debated correlation of the deuterium-plasma reaction yield with the absolute intensity in different spectral region of the X-ray emission. Correlation, or the lack of it, can be observed when (a) different spectral regions, and/or (b) different source locations in the discharge vessel, are monitored. The hard component emission up to /spl sime/100 keV is recorded by using differential filters. Energy per pulse (kJ) and emitted power (TW) are monitored with filter-fitted calorimeters and with fast scintillation detectors (NE-102). Emission scaling tests utilized two plasma focus machines with Mathers type electrodes hollow anode (APE-10, W/spl sime/7 kJ, an APF-50, W/spl ges/20 kJ). Different emission levels from a series of discharges with D/sub 2/ filling and with D/sub 2/+Ar fillings (Ar partial pressure from 2% to 16%) contribute to discriminating plasma pinch emission from anode emission. X-ray collimators designed for the same purpose establish the cogency of data analysis and of conclusions.

Spectral characteristics of ion emission in pinched discharges

Summary form only given, as follows. The energy-density increase in the current sheet (CS) of MA focused discharges during the CS radial implosion occurs via partially independent processes: (i) The relative increase, /spl Delta/J/J, of the amperian current density, J, on CS. (ii) The increased stretching, /spl Delta/B/sub /spl par///B/sub /spl par//, of the magnetic-field lines of the CS fine structure. Recent experiments with externally-controlled discharge parameters confirm that (i) dominates over (ii), for equal values of the relative variations of J and of B/sub /spl par//. The ion energy after pinch disintegration is the final outcome of the energy densification process addressed here. A comparison of the ion spectrum emitted from plasma focus discharges and from Z-pinch discharges where field-line stretching is minimized, is carried out from Thomson (parabola) spectrometer data. Data from a variety of gas-mixture fillings of the discharge vessels (D/sub 2/, and D/sub 2/+Ar) contribute to the clarification of the ion acceleration process and of the formation and emission of ion clusters.

Fine structure of ion-cluster disintegration modes in PF discharges

Summary form only given. A characteristic of the disintegration mode of ion clusters and superclusters is the pairing of the D/sub 2//sup +/ ions, which form the bulk of the ion population emission at 67/spl deg/ from the axis of a plasma focus discharge. A Thomson (parabola) spectrometer is used for the determination of ion and ion cluster species and of their energy spectrum. The parabolic pattern of each ion species is recorded on a CR-39 target, with ion track etching in NaOH solution.

Relation between /sup 16/O(d,n) reaction yield and the fine structure of D(d,n) neutron emission in the Dense Plasma Focus

Summary form only given. When DPF is fired with the vessel filled by a mixture of HZ (C,N,O) and LZ (D/sub 2/,/sup 3/He) elements, a strong yield Y(HZ+LZ) of HZ+LZ reactions is observed. These reactions differ strongly in energy of the involved ions: E/sub i//spl ges/1 MeV for HZ+LZ and E/sub i//spl les/0.3 MeV for LZ+LZ. Even though, HZ+LZ and LZ+LZ reactions do not necessary occur in the same space location, the corresponding yields are correlated as Y(HZ+LZ)-Y/sup 2/(LZ+LZ). To understand this phenomenon the time structure of D(d,n)-neutrons and hard X-ray emission have been correlated with the time integrated Y(HZ+LZ). The plasma focus (7 kJ; 17 kV) was fired with the vessel filled by D/sub 2/+O/sub 2/ mixture. The dP/sub n//dt signal was unfolded to the real time resolved neutron emission signal dY/sub n//dt with a novel method accounting precisely for the signal distortion due to effects such as neutron scattering, radiation detection and signal formation/transmission/recording process. An example of measured unfolded signals for a shot with strong radioactivity and hard X-ray spikes, is shown. Implications of this type of data on the plasma domain of the enhanced nuclear reactivity model are discussed.

Inductance modulation effects on the PF reaction yield

Summary form only given. A variation of the interelectrode spacing as a function of the axial coordinate z is used to assess the bearing on the D+D neutron yield per pulse, Y, of the inductance [L(z) dl/dt] and of the resistivity [ldL/dt] term amplitude during the axial propagation of the plasma current sheet in the interelectrode gap of plasma focus discharges. Two funnel-shaped electrodes were assembled coaxially in a PF machine to replace the two Y-optimizing cylindrical electrodes. Two configurations (A), (B) were used, both with the same center electrode (anode) of 30/spl deg/ conic aperture. In configuration (A) the outer electrode (cathode) had the same conic aperture /spl theta/=30/spl deg/ of the anode. In configuration (B), the cathode aperture was /spl theta//sub c/>30/spl deg/. It was found that, systematically, Y(B)>Y(A), under identical operation conditions of capacitor charging voltage and filling pressure. The current-sheet propelling force j_(r)/spl times/B_(r) near the anode surface is essentially the same in (A) and (B). The electrode current variations dl/dt were monitored by a Rogowski coil at the electrode breech. The velocity of the current sheet v(z) measured via magnetic probes in the interelectrode gap determines dL/dt/spl equiv/(dl/dz)v for the assigned dl/dz. Reaction yield Y and emission of ion and ion cluster beams from the PF pinch depend critically on the current distribution between pinch region and breech region.

Fusion-reaction-yield scaling with PF plasma current density

Summary form only given, as follows. The scaling of the neutron yield Y from D+D reactions and, specifically, of its fluctuations /spl Delta/Y from discharge to discharge was experimentally determined with four plasma focus machines, as a function of the peak plasma-current density J. The powering energy levels were W=6 kJ for one PF machine (PF/sub 1/), 20 kJ for two identical PF machines [but one (PF/sub 2/) with, the other (PF/sub 3/) without, field distortion element at the electrode breech to sharpen the plasma current distribution], 35 kJ and 50 kJ for a fourth machine (PF/sub 4/). The comparative tests at 20 kJ and 35 kJ had the same peak electrode current I=1 MA the same charging voltage of the capacitor bank 25 kV, the same ratio inductance/capacitance (L/C), but with C(PF/sub 4/)=2C(PF/sub 2/) and L(PF/sub 4/)/spl equiv/[T(PF/sub 4/)/2/spl pi/]/sup 2//C(PF/sub 4/)=2L(PF/sub 2/). T(PF/sub 4/)/spl sime/1.4 T(PF/sub 4/), is the measured period of the complete PF electric circuit, including the propagating current sheet between coaxial electrodes. The bearing on Y, /spl Delta/Y of the breech overvoltage during the pinch collapse and of the variations of J is experimentally determined. The data indicate that MaxY(PF/sub 4/)/spl sime/mean value Y/sub Av/(PF/sub 2/) of Y, Y/sub Av/(PF/sub 2/)/spl sime/3Y/sub Av/(PF/sub 3/). The neural network analysis connecting Y and the current sheet multiplicity in the same discharge, was carried out as in earlier experiments.