Inertial Fusion Targets Research Articles

Plasma Dynamics in a Magnetically Insulated Target for Inertial Fusion

Interest in the use of magnetized targets to enhance energy multiplication in inertial confinement fusion has recently been revived after being ignored for some time. The addition of an externally applied magnetic field to a fuel volume to reduce thermal conduction losses represents one approach. The other approach is the subject examined, namely, self-generated fields created inside a target by beams that enter the pellet through a hole. The field, current, and pressure profiles in a two-region spherical plasma that might be representative of the magnetically insulated inertial confinement fushion concept are calculated in a self-consistent manner. The existence of a quasi-equilibrium soon after the formation of the plasma in the target is assumed, the appropriate magnetohydrodynamic equations in a multiregion plasma configuration are solved, and the parameters for such an equilibrium are established. An energy integral is employed to study the stability of these configurations against azimuthally symmetric perturbations, and the results are applied to some experimental as well as reactor-like systems. For certain configurations and input energies, such systems can be stable for the length of the burn. 12 refs., 7 figs.

Cryogenic targets and related technologies at ILE Osaka University

Described are two approaches to make cryogenic inertial fusion targets that have a uniform solid or liquid fuel layer inside a spherical hollow fuel capsule. One is to utilize low-density, low-atomic-number plastic foam to sustain liquid fuel (foam method) and the other is to initiate a glow discharge in the void of a solid fuel layer to symmetrize the solid layer (plasma layering method). In the foam method, we have successfully made a foam shell with a gas barrier to prevent the fuel from evaporation by means of density-matched emulsion method followed by an interfacial polycondensation method. Foam shells having diameters of 600–1550 μm and wall thicknesses of 10–50 μm were coated with a normal density plastic layer having thickness of 4–10 μm. These foam shells were provided for implosion experiments with the Gekko XII glass laser system. In the plasma layering process, we examined the feasibility of the method by performing experiments using krypton gas and liquid nitrogen. A nonuniform solid krypton layer was redistributed into a uniform one after a 30 min discharge induced by an external 2.45 GHz microwave field. Experimental results on the heat generation in shells indicated that this technique can redistribute 10 K solid fuel into a uniform layer using intermittent discharges.

Ignition problems in heavy-ion fusion targets

A brief review is presented of the work performed at the VNIIEF Center (Arzamas 16, Russia) on physical problems relevant to the design of targets for heavy-ion inertial fusion. Target design topics dealt with include indirect-drive schemes, symmetry issues, development of concepts for achieving ignition in the presence of a certain degree of asymmetry. In addition, material properties (such as opacities) are studied both theoretically and experimentally. Recent work on turbulent mixing is also discussed.

Simulations of compression and thermonuclear burn of a radiation-driven inertial-fusion target

One-dimensional numerical simulations of compression and burn of a reactor-size inertial fusion target imploded by a homogeneous thermal radiation field are presented. This target consists of a multi-layered fusion capsule that is surrounded by a solid gold casing whose radius is twice the outer capsule radius. The target absorbs about 5.4 MJ input radiation energy, about 55% of which is absorbed by the capsule while the rest is lost into the casing. The implosion yields an output thermonuclear energy of 770 MJ so that the capsule gain is of the order of 250 and the target gain is about 140. Larger values of the casing to capsule radii ratio, probably required for achieving a symmetric implosion, would lead to smaller gains, but would not change substantially the qualitative physics aspects discussed in this paper.

Studies of thermonuclear reaction initiation conditions in heavy-ion inertial-fusion targets

The present paper considers some aspects of thermonuclear burning initiation in heavy-ion inertial-fusion targets. It is shown that to compensate the negative effects of asymmetries it is necessary to attain the required DT fuel density and temperature with a certain margin. This margin is conveniently expressed in terms of the ignition criterion factorW of Vakchrameev-Mokchov-Popov. When the spherical symmetry is not achieved the compressed DT fuel parameters are substantially reduced. For instance, for a certain hotraum target, irradiated at 35.25° angle,W is halved, while, if it is irradiated from the poles,W is reduced by a factor of 1000.

Intensity effects in final focusing for heavy-ion inertial fusion with short bunches

Focusing of heavy-ion beams on an indirectly driven inertial-fusion target can be achieved under vacuum conditions in the reactor chamber with a relatively large number of small emittance beams. Effects of space charge can be described in terms of four distinct mechanisms: an increase of the entrance divergence angle; a current sensitivity of the focal spot; the effect of a non-uniform distribution; and effects from finite bunch lengths. The first two can be calculated by means of an envelope model. The resulting current sensitivity formula is confirmed by a computer simulation taking into account the Coulomb forces between interacting bunched beamlets. We find that the current sensitivity is considerable, in particular for charge statesZ>1, since the spot size blow-up scales likeZ4/A2. The effect depends on the total current in a bundle and not on the number of beamlets in it, and thus determines how many bundles have to be used. Effects from finite bunch lengths are largest for large numbers of beamlets and results in intensity losses at the beam spot at the ends of the bunches.

Induction accelerator development for heavy-ion fusion

For approximately a decade, the Heavy-Ion Fusion Accelerator Research (HIFAR) group at LBL has been exploring the use of induction accelerators with multiple beams as the driver for inertial-fusion targets. Scaled experiments have investigated the transport of space-charge-dominated beams (SBTE), and the current amplification and transverse emittance control in induction linacs (MBE-4) with very encouraging results. In order to study many of the beam manipulations required by a driver and to further developed economically competitive technology, a proposal has been made in partnership with LLNL to build a 10 MeV accelerator and to conduct a series of experiments collectively called the Induction Linac System Experiments (ILSE). The major components critical to the ILSE accelerator are currently under development. We have constructed a full-scale induction module and we have tested a number of amorphous magnetic materials developed by Allied Signal to establish an overall optimal design. The electric and magnetic quadrupoles critical to the transport and focusing of heavy-ion beams are also under development. The hardware is intended to be economically competitive for a driver without sacrificing any of the physics or performance requirements. This paper will concentrate on the recent developments and tests of the major components required by the ILSE accelerator.

Intensity effects in final focusing for heavy-ion inertial fusion

Focusing of heavy-ion beams on an indirectly driven inertial fusion target can be achieved, in principle, under vacuum conditions in the reactor chamber. Due to performance limitations in the storage rings of the driver accelerator, a relatively large number of small emittance beams is required. We suggest that the effect of space charge can be described in terms of three distinct mechanisms: an increase of the entrance divergence angle; a current sensitivity of the focal spot; and the effect of a nonuniform distribution. The first two can be calculated by means of an envelope model. The resulting current sensitivity formula is confirmed by a computer simulation taking into account the correct Coulomb forces between interacting beamlets. We find that for propagation in vacuum the current sensitivity is considerable, in particular for charge states Z≥1, since the spot size blow-up scales like Z4/A2. The effect depends on the total current in a bundle—and not the number of beamlets in it—and thus determines how many bundles have to be used. Possible cures are briefly discussed.

Symmetry of illumination and implosion of hotraum targets for heavy ion inertial fusion

The symmetry of implosion of the hotraum target that has been proposed by J. Meyer-ter-Vehn and the author is analysed in the case when the target is driven by a nonspherical ensemble of heavy ion beams. The ion beams, each with a final focus radius rbf ⩾ R (R is the target radius), are all assumed to irradiate the target at the same angle α0 and symmetrically with respect to the equatorial plane. Separate one dimensional (1-D) simulations for the pole and equator target sectors indicate that the l = 2 mode in the asymmetry of fuel implosion vanishes for α0 ≈ 36°. The l = 4 mode can also be eliminated by adjusting the ion current profile across each beam. It is conjectured that the higher asymmetry modes with l ⩾ 6 will be smoothed out by radiative symmetrization to ≲ 1%. A 1-D average (over the target latitude) energy gain of G = 57 is calculated

Simulations of compression and thermonuclear burn of an indirect drive reactor-size inertial fusion target

In this paper we present one-dimensional numerical simulations of compression and burn of a high-gain indirect drive inertial fusion target suitable for use in a reactor system. The target consists of a capsule that contains 4.45 mg of DT fuel and the capsule is enclosed in a solid gold casing. The inner radius of the casing is twice larger than the outer radius of the capsule so that the inner surface area of the hohlraum wall is four times larger than the capsule surface area. The cavity is filled with a homogeneous radiation field whose intensity increases in time in an appropriate manner reaching a maximum radiation temperature of 300 eV. The capsule is driven to thermonuclear burn by this radiation field. The target (capsule+casing) absorbs an input energy of about 5.6 MJ and the implosion produces an output energy of 720 MJ so that the overall energy gain of this target is of the order of 130.

Incineration of 90Sr and 137Cs by an inertial fusion target

We discuss the inertial confinement fusion system as a transmutator of radioactive waste ( 90Sr and 137Cs). An analytic model of the implosion of the target, which is composed of DT fuel and radioactive waste, is used to evaluate its internal energy and the probability of neutron utilization. From the results of this calculation, we could evaluate the energy that is required to transmute radioactive waste.

Radiation-driven inertial fusion targets for implosion experiments: Theoretical analysis and numerical simulations

This paper presents numerical simulations of compression, ignition, and burn of a radiation-driven inertial fusion capsule suitable for use in a heavy-ion-beam-driven laboratory microfusion facility to achieve breakeven (thermonuclear energy output=input radiation energy). These simulations have been carried out using a one-dimensional, three-temperature, Lagrangian computer code medusa-kat [J. Appl. Phys. 60, 898 (1986)]. The basic capsule design is simple and consists of a thin gold microballoon coated with a beryllium ablator. A high-pressure gaseous deuterium–tritium (DT) fuel is filled in the capsule. The capsule is driven by a shaped radiation pulse having a prepulse corresponding to a radiation temperature of 100 eV and a main pulse with a temperature of 300 eV. A parameter study of the capsule gain, G versus input radiation energy over a range 52–76 kJ, has been carried out. The fuel mass has also been varied over a range 0.3–0.8 μg. It has been found that G∼1 can be achieved with an absorbed radiation energy of 70 kJ provided that the fuel mass lies between 0.5 and 0.7 μg. These simulations have also been repeated over the same parameter range but using an unshaped pulse with a constant radiation temperature of 300 eV. An overall reduction of 10%–30% has been observed in the gain curves in this case.

Development of foam shells for cryogenic laser fusion target

Spherical hollow polymer foam shells used as a sustainer of cryogenic fuel for the inertial fusion target have been developed. Foam shells which have 500–1000 μm diameter with a 30–100 μm uniform wall, 4 μm cell size and 40 mg/cm3 density, have been fabricated using a dual-nozzle droplet generator and a freeze-dry technique. Requirements to obtain a uniform thick wall and fine cell size are discussed.

Uniform liquid-fuel layer produced in a cryogenic inertial fusion target by a time-dependent thermal gradient

Targets for inertial confinement fusion are cooled by cryogenic helium gas jets in an environment of room-temperature thermal radiation in an effort to obtain uniform condensed deuterium–tritium (DT) layers. A time-dependent thermal gradient obtained by ohmic heating of the upper nozzle is found to produce a transient state of uniformity in liquid DT just above its freezing point. A replaceable assembly of nozzles with heaters and thermometers is designed to be implemented in the target chamber of the OMEGA laser. Targets are held between the nozzles by spider webs which are attached to an independently positioned fixture. A flow of He of 6×10−5 g/s is sufficient to obtain the uniform DT layer, producing a background pressure in a test chamber of 0.5 mTorr. Targets are characterized interferometrically in two orthogonal views. Interferograms are compared with computed templates to determine the thickness and uniformity of the condensed layer.

Holographic interferometry evidence that β-decay energy can symmetrize solid deuterium–tritium samples

Thin solid samples of deuterium–tritium (DT) have been seen to sublime and redistribute within spherical inertial fusion targets due to the heat generated by beta decay. We have frozen thin DT samples within glass shells of 890 and 3750 μm diameter, and observed the evolution of the solid towards more symmetric distributions using real time holographic interferometry. Holographic interferometry provides more sensitivity than direct imaging or classical interferometry to observe the thin (10–30 μm) solid samples in these shells.

Analysis of Physical Processes in ICF Target Chambers: Application to the Laboratory Microfusion Facility

When a high-gain inertial fusion target explodes, roughly one third of the energy will be released in the form of x-rays and energetic ions, which can damage the wall of the chamber. One method of ...

Fusion power implosion of laser-type light-ion beam driven reactor-size inertial fusion targets including radiation transport effects

This Letter presents simulation results of compression, ignition and burn of an umtamped (laser-type) inertial confinement fusion target which may be used in a reactor system driven by 33 MeV Li + ions. These simulations have been carried out using a one-dimensional, three-temperature (electron, ion and radiation), Lagrangian ion-beam fusion code MEDUSA-KAT. The importance of thermal radiation in the simulations of such reactor-size targets has been thoroughly discussed. This target is imploded by an appropriately shaped input power pulse with a peak power 250 TW and an input energy of 4.6 MJ. The target yields an output energy of 580 MJ so that the overall energy gain is about 126.

Non-LTE effects in inertial confinement fusion target chambers

In previous studies of transport processes in inertial confinement fusion target chambers, the radiative properties of the background plasma were calculated under the assumption of local thermodynamic equilibrium (LTE). In this paper, the authors present a study of the equation of state and the radiative properties of high temperature, low-to-moderate density (≲ 1021 cm−3) plasmas for the determination of the conditions under which non-LTE effects become important and for an assessment of the importance of non-LTE processes in target chambers during high yield inertial fusion target explosions. For this purpose, two-body (radiative and dielectronic) and three-body (collisional) recombination and de-excitation processes are considered in calculating the steady state ionization and excitation populations. The results of this study indicate that non-LTE processes generally become important at temperatures of ≳1, 10 and 100 eV for plasma densities of 1018, 1019 and 1021 cm−3, respectively. Radiation hydrodynamic simulations utilizing the equation of state and the opacities for a non-LTE argon plasma were performed to study the response of a background gas to an inertial fusion target explosion. These calculations indicate that non-LTE processes are often the dominant atomic processes in the background plasma and that they can strongly affect the radiative and shock properties as energy is transported away from the point of the target explosion.

Rippled shock front solutions for testing hydrodynamic stability simulations

The response of a shock front to arbitrary small perturbations can be calculated analytically. Such rippled shock front solutions are useful for determining the accuracy of hydrodynamic simulation codes such as lasnex [Comments Plasma Phys. Controlled Fusion 2, 51 (1977)], which are used to compute perturbation growth in inertial fusion targets. The lasnex fractional errors are of order κ2L2, where κ is the transverse wavenumber of the perturbation, and L is the largest zone dimension. Numerical errors are about 25% for a calculation using 26 zones per transverse wavelength.

The deposition rate dependence of plasma polymerization on the radius of curvature of the substrate: H K Lintz et al,J Vac Sci Technol,A6, 1988, 1869–1871

In the plasma polymerization deposition of CHx onto microspherical inertial fusion targets we have found that the coating rate varies with substrate diameter. Spherical shells ranging in diameter from 100 to 1000 μ were coated with several microns of plasma polymerized trans‐2‐butene under identical conditions and their coating thicknesses measured interferometrically. We found that the coating rate variation with shell radius could be described by an equation having the form dr/dt=A/(R+r)+B, where dr/dt is the coating rate, R the initial sphere radius, r the coating thickness, and A and B parameters related to the plasma conditions. We also found that there exists a functional relationship between the parameters A and B which is dependent on the rf power density. A model has been developed to show how this observed rate dependence relates to the buildup of an electrical charge on the nonconducting substrate surface. The electric field which arises is dependent on the radius of curvature of the substrate ...