Fusion Neutron Yield Research Articles

Optimized laser production of thermonuclear neutrons from plasma of submicron-sized clusters

The concept of maximizing the D-D fusion neutron yield from the laser-heated large volume of cluster medium by matching the focal spot size and cluster plasma structural scales to the laser pulse intensity was confirmed. For this purpose, the three-dimensional particle-in-cell GEANT4 simulations have been performed by zoning of the large interaction domain. While considering a small domain of the entire interaction volume, which is partitioned into successive zones along laser propagation direction, a special algorithm was proposed allowing to reconstruct the integral spectrum of deuterons and D-D neutron yield. We demonstrate that it makes possible to specify high-performance laser–cluster neutron source following this concept. For example, for the submicron heavy water droplets heated by femtosecond laser pulse of the intensity 3×1019 W/cm2 a D-D neutron yield may reach 107 neutrons per 1 J of deposited laser energy if the intensity contrast ratio prevents premature cluster destruction. Such yield is considerably higher than achieved to date for microstructured targets.

Mitigation strategies for time-resolved x-ray diagnostic data degradation due to high neutron yield of ICF experiments at the NIF.

Inertial confinement fusion experiments taking place at the National Ignition Facility are generating ever increasing amounts of fusion energy, with the deuterium tritium fusion neutron yield growing a hundredfold over the past ten years. Strategies must be developed to mitigate this harsh environment's deleterious effects on the operation and the performance of the time-resolved x-ray imagers deployed in the National Ignition Facility target bay to record the dynamics of the implosions. We review the evolution of these imagers in recent years and detail some of the past and present efforts undertaken to maintain or improve the quality of the experimental data collected on high neutron yield experiments. These include the use of a dump-and-read electronic backend, the selection of photographic film with a low background sensitivity, and the optical filtering of Cherenkov radiation.

Development of a tritium release system for ceramic tritium breeder pebbles irradiated by high intensity DT fusion neutron Generator at INEST

For ceramic breeder pebbles used in fusion energy, the tritium release data under fusion neutron irradiation is very important and valuable. In this study, a tritium release system for ceramic tritium breeder is introduced, which is matched with the High Intensity DT Fusion Neutron Generator (HINEG-CAS). The HINEG-CAS has been built and operated by the Institute of Nuclear Energy Safety Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences (INEST, HFIPS, CAS), achieving a fusion neutron yield of 1012n/s since 2016. Innovative design and fabrication of sample container enables minimization of the unexpected tritium loss. The dedicated tritium release system can monitor the tritium release behavior over time for both tritiated water and tritium gas, using the bubblers-based tritium trapping system with automatic replacement technology, coupled with highly efficient catalytic oxidants and liquid scintillation counter (LSC). We have performed a tritium release experiment with the Li2TiO3 pebbles as ceramic tritium breeder, and obtained the tritium releasing data at room temperature over time and clarified the tritium chemical form in recovery. It is concluded that this newly established system can serve as a good sharing platform for evaluating prime candidate breeder materials, as well as facilitating detailed design or optimization of tritium involved systems (such as the tritium extraction system).

Neutron yield using a semi-Monte Carlo simulating method for D-T fusion neutron sources

Neutron yield and spatial distribution are crucial parameters of a gaseous target D-T neutron source. The exact calculation of these parameters in large-size geometries with complex fluid dynamics is very important for gaseous target neutron source design and subsequent applications. The gaseous target on operating conditions put more requirements on the Multiphysics simulating method. The fusion neutron yield simulating method has been developed using semi-Monte Carlo based on the finite element analysis for Multiphysics. The method has been verified using the model of a D-T fusion neutron source with a windowless gas target and compared with experimental results and the GEANT4 simulations. Compared with the experimental results, the simulated neutron yield error is 8.31%, and compared with the result of GEANT4, the relative error of total neutron yield is 3.34%. The spatial distributions of neutron yield are in good agreement with the results of GEANT4. In particular, the computing time is reduced from ∼200 core-hours with GEANT4 to ∼50 core-hours with the developed semi-Monte Carlo method, greatly improving the calculation efficiency.

Neutron yield scaling law in laser-cluster fusion experiments

We present a scaling law () of fusion neutron yields (Y) for laser pulse energy (E) in laser-cluster fusion experiments. We compare the available neutron yield data from previous deuterium cluster fusion experiments with those calculated using the cylindrical fusion plasma model. The calculated neutron yields are shown as functions of the incident laser pulse energy, average number density, and ion temperature. Although the deuterium–deuterium fusion reactivity is known to increase rapidly with ion temperature, the neutron yield shows a modest increase above ∼10 keV for a given laser pulse energy. We find the scaling exponent β approaching 1.0 as the ion temperature increases from 1 keV to 100 keV. We explain the observed temperature dependence of β by examining the temperature dependence of the beam–beam and beam–target fusion neutron yields separately. Our scaling law differs from previously reported scaling laws from individual experiments, but it shows an excellent agreement with the scaling law determined by the maximum neutron yields of individual experiments.

Numerical experiments on the total D–D fusion neutron yield versus deuterium pressure for different energy plasma focus devices using the Lee model code

Numerical experiments using the Lee model code were carried out to compute the total D–D fusion neutron yield versus initial deuterium pressure for nine different plasma focus devices [PF50, AACS, PF400J, PF2.2, UNU-ICTP, PFZ200, PF24, PF78, Poseidon]. The computed data were compared with the published experimental results. In addition, the maximum discharge current, the pinch current and the pinch ion number density, as a function of initial deuterium pressure, were discussed, as well as their effect on the neutron yield. The optimum pressures that correspond to the maximum neutron yield Y n were obtained for all considered devices, with reference to the specific plasma focus properties. The scaling laws for the D–D fusion neutron production, in terms of storage energies E 0 and pinch current I pinch, were derived. The scaling of Y n with I pinch and E 0 over the whole range of investigated energies up to 300 kJ has been obtained as follows: . These laws are important in the planning and design of a plasma focus as a potentially powerful pulse neutron source.

High deuteron and neutron yields from the interaction of a petawatt laser with a cryogenic deuterium jet

A compact high-flux, short-pulse neutron source would have applications from nuclear astrophysics to cancer therapy. Laser-driven neutron sources can achieve fluxes much higher than spallation and reactor neutron sources by reducing the volume and time in which the neutron-producing reactions occur by orders of magnitude. We report progress towards an efficient laser-driven neutron source in experiments with a cryogenic deuterium jet on the Texas Petawatt laser. Neutrons were produced both by laser-accelerated multi-MeV deuterons colliding with Be and mixed metallic catchers and by d (d,n)3He fusion reactions within the jet. We observed deuteron yields of 1013/shot in quasi-Maxwellian distributions carrying ∼8−10% of the input laser energy. We obtained neutron yields greater than 1010/shot and found indications of a deuteron-deuteron fusion neutron source with high peak flux (>1022 cm−2 s−1). The estimated fusion neutron yield in our experiment is one order of magnitude higher than any previous laser-induced dd fusion reaction. Though many technical challenges will have to be overcome to convert this proof-of-principle experiment into a consistent ultra-high flux neutron source, the neutron fluxes achieved here suggest laser-driven neutron sources can support laboratory study of the rapid neutron-capture process, which is otherwise thought to occur only in astrophysical sites such as core-collapse supernova, and binary neutron star mergers.

Pushing the limits of existing plasma focus towards 1016 fusion neutrons with Q = 0.01

Existing conventional megajoule plasma focus machines with 2–3 MA are producing fusion neutron yields of several times 1011 in deuterium operation, the fusion yields predominantly being the beam-gas target. Increasing the current to 10 MA and using 50%–50% D-T mixture will scale the neutron yield towards 1016 D-T fusion neutrons. In this work, we derive the Lawson criterion for plasma focus devices with a beam-target fusion neutron mechanism, so that we may glimpse what future technological advancements are needed for a break-even Q = 1 plasma focus. We perform numerical experiments with a present-day feasible 0.9 MV, 8.1 MJ, 11 MA machine operating in 100 Torr in 50%–50% D-T mixture. The Lee Code simulation gives a detailed description of the plasma focus dynamics through each phase, and provides plasma and yield parameters which show that out of 1.1 × 1019 fast beam ions produced in the plasma focus pinch, only 1.24 × 1014 ions take part in beam-target fusion reactions within the pinch, producing the same number of D-T neutrons. The remnant beam ions, numbering at least 1019, exit the focus pinch at 1.9 MeV, which is far above the 115 keV ion energy necessary for an optimum beam-target cross-section. We propose to regain the lost fusion rates by using a high-pressure D-T-filled drift-tube to attenuate the energy of the remnant beam ions until they reach the energy for the optimum fusion cross-section. Such a fusion enhancement tube would further harvest beam-target fusion reactions by increasing the interaction path length (1 m) at increased interaction density (6 atm). A gain factor of 300 is conservatively estimated, with a final yield of 3.7 × 1016 D-T neutrons carrying kinetic energy of 83.6 kJ, demonstrating Q = 0.01.

Verification of fast heating of core plasmas produced by counter-illumination of implosion lasers

We conducted fast heating of a core plasma produced by counter-illumination of two beam bundles of implosion lasers for two incident directions of the heating laser. For the axial heating configuration, wherein the heating laser was incident along the coaxial direction of the implosion lasers, the size and density of the counter-imploded core plasma were estimated. The incident timing of the heating laser was determined. Although the heating laser was incident during the stagnation of the core plasma, there were no considerable increases in the intensity of X-rays with energies beyond 1 keV and the yield of the thermonuclear fusion neutrons. Effective core heating did not occur due to the interaction of the heating laser with the low-density ablated plasma far from the core. On the other hand, for the transverse heating configuration, wherein the heating laser was incident from the transverse direction of the implosion laser axis, optimization of the incident timing of the heating laser was not achieved. However, two-dimensional radiation-hydrodynamic simulation revealed that a heating laser can directly interact with a core plasma and effective heating is expected. In our scheme, transverse heating configuration appears to be significant.

Development of a wide-range neutron flux monitoring system in EAST

A wide measurement range for a neutron flux monitoring (NFM) system is developed in the Experimental Advanced Superconducting Tokamak (EAST) for fusion neutron yield measurement, comprising an 3He proportional counter, a 235U fission chamber (FC), and a wide dynamic range signal-processing circuit. Two high-sensitivity 3He detectors operate in counting mode, which is suitable for low neutron yield (Yn) conditions with ohmic and radio-frequency heating of plasma. Two other FCs are adapted with newly developed digitizers, operating in counting and campbelling modes to satisfy the conditions of wide dynamic range measurement required by neutral beam injection (NBI) experiment. The anti-magnetic-field interference of the FC is examined using two permanent magnets, whose efficiency are calibrated with 2.46 MeV neutrons produced by a Van de Graaff accelerator. Furthermore, the NFM system is implemented in-situ calibration with a 252Cf neutron source and a remote handling. As a routine diagnostic, the NFM system is successful obtaining the fusion neutron yield for ion cyclotron resonance heating (ICRH) and NBI heating performance evaluation in EAST plasma experiment.

Estimates of the Dependence of the Fusion Neutron Yield on the Initial Plasma Density and Temperature in Fast Pinches

The neutron generation by hot, dense, and fast Z- and Θ-pinch plasma with the parameters achievable at the present development stage of high-power pulsed technology has been calculated. It has been proposed to use a hot plasma with a high density of 1017 cm−3 as an initial state for subsequent compression by Z- and Θ-pinches to achieve the conditions necessary for fusion reactions. Such plasma can be obtained using high-power pulsed plasma accelerators.

Calibration of the gamma-ray measurement procedure in the EAST neutron activation system

A neutron activation system based on pneumatic transfer system has been employed to measure the time-integrated neutron yield for the Experimental Advanced Superconducting Tokamak (EAST). Those DD neutrons were detected by indium with the reaction of In115(n,n') In115m, the energy threshold of the reaction was 0.3 MeV which was capable of eliminating the interference of scatting neutrons. A small amount of DT neutron generated by the deuterium plasma was detected by silicon with the reaction of Si28n,pAl28, the energy threshold was 4 MeV so as to discriminate the DD neutron from DT neutron. The γ spectrum of activated samples was obtained with high purity germanium spectrometer. The efficiency for the measurement of fusion neutrons were calibrated by DD/DT neutron accelerator. The key coefficients using to calculate the neutron flux by characteristic γ peak counts have been obtained in this calibration. Furthermore, investigation of the thickness of samples in order to cover the wide range of neutron yields was performed. The neutron activation system was implemented to the measurement of fusion neutron yield in EAST from the experiment campaign of 2018, and obtained a linear with deviation less than ± 10% comparing with fission chamber.

Uranium fusion implosion capsules see improved performance over gold

Bare depleted uranium hohlraum capsules exhibit double the fusion neutron yield and considerably reduced X-ray flux in inertial confinement fusion experiments compared to commonly used gold.

First demonstration of improved capsule implosions by reducing radiation preheat in uranium vs gold hohlraums

In indirectly-driven Inertial Confinement Fusion (ICF) implosions, supra-thermal M-band (>2 keV) radiation from principally 4–3 resonance line transitions generated during laser irradiation at the peak power of Au hohlraum walls can preheat the fusion capsule and reduce compressional pressure. Higher Z, un-lined depleted uranium (DU) hohlraums were used for the first time in ICF implosions on the National Ignition Facility (NIF) to reduce M-band radiation levels while keeping the total radiation flux similar to Au hohlraums. First implosions in DU demonstrate an increase in in-flight density (+15%) of high density carbon capsules, and hence in stagnated hot spot temperature (+15%), hot spot x-ray (+200%) and fusion neutron yields (+100%) compared to Au hohlraums. We show analytically that these changes are consistent with the observed 40% reduction in M-band x-ray flux in DU, and are in agreement with 2D hydrodynamic simulations. This result had a major impact on ICF research on the NIF where a significant fraction of high neutron yield implosions are currently using un-lined DU hohlraums.

Overrun phenomenon and neutron yield in Coulomb explosion of deuterated alkane clusters driven by intense laser field**Project supported by the National Natural Science Foundation of China (Grant No. 11005080).

By using a simplified Coulomb explosion model, the laser-driven Coulomb explosion processes of three deuterated alkane clusters, i.e., deuterated methane (CD4)N, ethane (C2D6)N and propane (C3D8)N clusters are simulated numerically. The overrun phenomenon that the deuterons overtake the carbon ions inside the expanding clusters, as well as the dependence of the energetic deuterons and fusion neutron yield on cluster size, is discussed in detail. Researches show that the average kinetic energy of deuterons and neutron yield generated in the Coulomb explosion of (C2D6)N cluster are higher than those of (CD4)N cluster with the same size, in qualitative agreement with the reported conclusions from the experiments of (C2H6)N and (CH4)N clusters. It is indicated that (C2D6)N clusters are superior to (CD4)N clusters as a target for the laser-induced nuclear fusion reaction to achieve a higher neutron yield. In addition, by comparing the relevant data of (C3D8)N cluster with those of (C2D6)N cluster with the same size, it is theoretically concluded that (C3D8)N clusters with a larger competitive parameter might be a potential candidate for improving neutron generation. This will provide a theoretical basis for target selection in developing experimental schemes on laser-driven nuclear fusion in the future.

Plasma focus neutron anisotropy measurements and influence of a deuteron beam obstacle

The deuterium-deuterium (DD) fusion neutron yield and anisotropy were measured on a shot-to-shot basis for the NX2 plasma focus (PF) device using two beryllium fast-neutron activation detectors at 0° and 90° to the PF axis. Measurements were performed for deuterium gas pressures in the range 6–16 mbar, and positive correlations between neutron yield and anisotropy were observed at all pressures. Subsequently, at one deuterium gas pressure (13 mbar), the contribution to the fusion yield produced by the forwardly-directed D+ ion beam, emitted from the plasma pinch, was investigated by using a circular Pyrex plate to obstruct the beam and suppress its fusion contribution. Neutron measurements were performed with the obstacle positioned at two distances from the anode tip, and also without the obstacle. It was found that ~ 80% of the neutron yield originates in the plasma pinch column and just above that. In addition, proton pinhole imaging was performed from the 0° and 90° directions to the pinch. The obtained proton images are consistent with the conclusion that DD fusion is concentrated (~ 80%) in the pinch column region.

Disassembly time of deuterium-cluster-fusion plasma irradiated by an intense laser pulse.

Energetic deuterium ions from large deuterium clusters (>10nm diameter) irradiated by an intense laser pulse (>10(16)W/cm(2)) produce DD fusion neutrons for a time interval determined by the geometry of the resulting fusion plasma. We present an analytical solution of this time interval, the plasma disassembly time, for deuterium plasmas that are cylindrical in shape. Assuming a symmetrically expanding deuterium plasma, we calculate the expected fusion neutron yield and compare with an independent calculation of the yield using the concept of a finite confinement time at a fixed plasma density. The calculated neutron yields agree quantitatively with the available experimental data. Our one-dimensional simulations indicate that one could expect a tenfold increase in total neutron yield by magnetically confining a 10-keV deuterium fusion plasma for 10ns.

Diagnosing magnetized liner inertial fusion experiments on Za)

Magnetized Liner Inertial Fusion experiments performed at Sandia's Z facility have demonstrated significant thermonuclear fusion neutron yields (∼1012 DD neutrons) from multi-keV deuterium plasmas inertially confined by slow (∼10 cm/μs), stable, cylindrical implosions. Effective magnetic confinement of charged fusion reactants and products is signaled by high secondary DT neutron yields above 1010. Analysis of extensive power, imaging, and spectroscopic x-ray measurements provides a detailed picture of ∼3 keV temperatures, 0.3 g/cm3 densities, gradients, and mix in the fuel and liner over the 1–2 ns stagnation duration.

Measurements of fusion neutron yields by neutron activation technique: Uncertainty due to the uncertainty on activation cross-sections

The neutron activation technique is routinely used in fusion experiments to measure the neutron yields. This paper investigates the uncertainty on these measurements as due to the uncertainties on dosimetry and activation reactions. For this purpose, activation cross-sections were taken from the International Reactor Dosimetry and Fusion File (IRDFF-v1.05) in 640 groups ENDF-6 format for several reactions of interest for both 2.5 and 14MeV neutrons. Activation coefficients (reaction rates) have been calculated using the neutron flux spectra at JET vacuum vessel, both for DD and DT plasmas, calculated by MCNP in the required 640-energy group format. The related uncertainties for the JET neutron spectra are evaluated as well using the covariance data available in the library. These uncertainties are in general small, but not negligible when high accuracy is required in the determination of the fusion neutron yields.

Fusion-neutron-yield, activation measurements at the Z accelerator: design, analysis, and sensitivity.

We present a general methodology to determine the diagnostic sensitivity that is directly applicable to neutron-activation diagnostics fielded on a wide variety of neutron-producing experiments, which include inertial-confinement fusion (ICF), dense plasma focus, and ion beam-driven concepts. This approach includes a combination of several effects: (1) non-isotropic neutron emission; (2) the 1/r(2) decrease in neutron fluence in the activation material; (3) the spatially distributed neutron scattering, attenuation, and energy losses due to the fielding environment and activation material itself; and (4) temporally varying neutron emission. As an example, we describe the copper-activation diagnostic used to measure secondary deuterium-tritium fusion-neutron yields on ICF experiments conducted on the pulsed-power Z Accelerator at Sandia National Laboratories. Using this methodology along with results from absolute calibrations and Monte Carlo simulations, we find that for the diagnostic configuration on Z, the diagnostic sensitivity is 0.037% ± 17% counts/neutron per cm(2) and is ∼ 40% less sensitive than it would be in an ideal geometry due to neutron attenuation, scattering, and energy-loss effects.