J.S. Brzosko

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.

A macroscopic study of the neutron, gamma- and X-ray emissivity in the Frascati plasma focus

Experiments were carried out in the neutron-optimized mode of the plasma focus operation with small electrode diameters and condenser bank energies of 250, 390, and 490 kJ. The time sequence of the emitted radiation (neutron and gamma) was realized by the time-of-flight (TOF) method using an NE-102A plastic scintillator and silicon detectors viewing X rays from the plasma exclusively. The detectors were operated in the wide-energy window mode. Special attention was given to the interpretation of neutron TOF traces and their comparison with the absorption analysis and previously measured spectra by nuclear plates. Average values are discussed simultaneously with the results of representative single shots. It was found that two (something more) neutron bursts are typical and, in each case, are accompanied by simultaneous hard X rays. The maxima of emissivities coincide with the dI/dt maximum. A theoretical analysis of the results reveals the existence of long time-confined streams of fast electrons and deuterons (effective-energy -- 80keV) with evidence of their slowing down.

Triton burnup in plasma focus plasmas

Pure deuterium plasma discharge from plasma focus breeds 1.01 MeV tritons via the D(d,p)T fusion branch, which has the same cross section as the D(d,n)3He (En=2.45 MeV) fusion branch. Tritons are trapped in and collide with the background deuterium plasma, producing 14.1 MeV neutrons via the D(t,n)4He reaction. The paper presents published in preliminary form as well as unpublished experimental data and theoretical studies of the neutron yield ratio R=Yn(14.1 MeV)/Yn(2.45 MeV). The experimental data were obtained from 1 MJ Frascati plasma focus operated at W=490 kJ with pure deuterium plasma (in the early 1980s). Neutrons were monitored using the nuclear activation method and nuclear emulsions. The present theoretical analysis of the experimental data is based on an exact adaptation of the binary encounter theory developed by Gryzinski. It is found that the experimentally defined value 1⋅10−3<R<3⋅10−3 can be explained theoretically only if one considers that tritium burnup occurs in the plasma domains of ...

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.

Neutronic Characteristics of the Novosibirsk GDT-NS Fusion Material Irradiation Facility

Abstract A summarized update of neutronic studies on the Novosibirsk Gas Dynamic Trap (GDT) fusion material irradiation facility (FMIF) is presented. The GDT-FMIF neutron source project is based on a mirror-type machine designed to produce 1018 D-T neutrons/s over 10 yr (3 × 1026 neutrons). The proposed massive shielding, susceptible to further shield reductions and optimization, ensures proper shielding against radiation and/or heat overdeposition in accordance with project tolerances. The present shield configuration allows 3.3 m3 of irradiation space around the plasma column: 0.06 m3 receives 0.3 × 1014 thermonuclear uncollided 14-MeV nDT-neutrons/cm2·s (0.5 MW/m2), and 0.7 × 1014 with collision degraded energies (~0.7 MW/m2 total), over 7 of the 8 m of intense flux axial length, the largest nontokamak availability. This allows the irradiation of large (up to 4.5 m long) life-size components (such as welds). The delivered neutron flux relative-gradients are small (< 6.3%/cm). Simulations use the 3DAMC-VINIA Monte Carlo code in its expanded version (drizzle-shower technique, two-step cascade, etc.), ENDF/B6 and EPDL nuclear data files, and a precise model of critical parts of the GDT. Results demonstrate that the GDT-FMIF is a very suitable irradiation test facility as per International Energy Agency specifications for an FMIF. With its 37.5-cm free depth of test space, GDT is the only dedicated facility suitable for a life-size blanket-tritium-breeding/extraction benchmark at a significant neutron flux level (2 MW/m2).

Precise method of the local tritium breeding measurements oriented to future advanced benchmark experiments

The Advanced Benchmark Experiment (ABE) is a new step in benchmark experiments of fusion reactor technology aimed at examining the effects of non-homogeneities due to the discretization of the reactor blanket into breeder and coolant (confined within stainless tube), as well as to openings in the blanket for vacuum pumping, plasma heating, etc. The organization of the openings and any discretization significantly alter local nuclear parameters, particularly the local tritium breeding ratio (LTBR). Prior to designing an ABE, the practical limits of the quality of the experiment should be established and compared with the expected possibilities of numerical calculations. A study of the state of the art in LTBR measurements is presented. The neutron fluence is measured by the charged associated particle method with the use of ..delta..E- and E- silicon detectors. The tritium activity induced through nuclear transmutations of lithium isotopes is measured by a very advanced coincidence-anticoincidence system on direct mixtures of LiNO/sub 3/ water solution and ATOM-LIGHT scintillator (the considered indicator mass is 0.1 g of LiNO/sub 3/).

Time resolved neutron and hard X-ray emission in the dense plasma focus, revisited

Summary form only given. The time dependence of the MeV ion abundance can provide critical information on acceleration mechanisms in the dense plasma of focused discharges. Time resolved measurements of neutrons and hard X-rays are used to gauge the time evolution of ion population. The plasma focus (7 kJ; 17 kV) chamber was filled with O/sub 2/ (10%) and D/sub 2/ (90%) mixture. Time resolved measurements were done at different distances from the pinch (L) and at different angles (/spl theta/) from the electrode axis by NE102 (L=0.15 m; /spl theta/=0/spl deg/) and NE111 (L=3.8 m; /spl theta/=0/spl deg/ & 90/spl deg/) plastic scintillators. Time integrated absolute measurements of total yield of D(d,n)/sup 3/He and /sup 16/O(d,n)/sup 17/F nuclear reactions were used to establish population of low (E<0.5 MeV) and high (E/sub i/>2 MeV) energy ions. Measured neutron and hard X-ray signals (E/sub x//spl ges/100 keV) were unfolded to recover the real time resolved emission signals P/sub n/ & P/sub x/, respectively.

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.