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An ion beam has the unique feature of being able to deposit its main energy inside a human body to kill cancer cells or inside material. However, conventional ion accelerators tend to be huge in size and cost. In this paper, a future intense-laser ion accelerator is discussed to make the laser-based ion accelerator compact and controllable. The issues in the laser ion accelerator include the energy efficiency from the laser to the ions, the ion beam collimation, the ion energy spectrum control, the ion beam bunching, and the ion particle energy control. In the study, each component is designed to control the ion beam quality by particle simulations. The energy efficiency from the laser to ions is improved by using a solid target with a fine sub-wavelength structure or a near-critical-density gas plasma. The ion beam collimation is performed by holes behind the solid target or a multi-layered solid target. The control of the ion energy spectrum and the ion particle energy, and the ion beam bunching are successfully realized by a multi-stage laser–target interaction.
Energetic electron beam generation from a thin foil target by the ponderomotive force of an ultra-intense circularly polarized laser pulse is investigated. Two-dimensional particle-in-cell (PIC) simulations show that laser pulses with intensity of 1022–1023 Wcm−2 generate about 1–10 GeV electron beams, in agreement with the prediction of one-dimensional theory. When the laser intensity is at 1024–1025 Wcm−2, the beam energy obtained from PIC simulations is lower than the values predicted by the theory. The radiation damping effect is considered, which is found to become important for the laser intensity higher than 1025 Wcm−2. The effect of laser focus positions is also discussed.
In order to realize an effective implosion, the beam illumination
non-uniformity and implosion non-uniformity must be suppressed to less
than a few percent. In this paper, a direct-indirect mixture implosion
mode is proposed and discussed in heavy ion beam (HIB) inertial
confinement fusion (HIF) in order to release sufficient fusion energy in a
robust manner. On the other hand, the HIB illumination non-uniformity
depends strongly on a target displacement (dz) in a reactor. In a
direct-driven implosion mode dz of ∼20 μm was tolerance
and in an indirect-implosion mode dz of ∼100 μm was
allowable. In the direct-indirect mixture mode target, a low-density foam
layer is inserted, and radiation is confined in the foam layer. In the
foam layer the radiation transport is expected in the lateral direction
for the HIB illumination non-uniformity smoothing. Two-dimensional
implosion simulations are performed and show that the HIB illumination
non-uniformity is well smoothed. The simulation results present that a
large pellet displacement of ∼300 μm is tolerable in order to
obtain sufficient fusion energy in HIF.
An attosecond electron beam generation is studied by an intense
short-pulse TEM (1,0) + TEM (0,1)-mode laser with a plasma separator in
vacuum. The TEM (1,0) + TEM (0,1)-mode laser has a ring-shaped intensity
peak in the radial direction. Electrons are accelerated and compressed
near the focus point of the TEM (1,0) + TEM (0,1)-mode laser. However,
after the focus point, some electrons move to its deceleration phase of
the laser pulse and are decelerated. As a result, a longitudinal velocity
deference of electrons generated causes a density lowering. In order to
suppress the deceleration and the density lowering, we set an overdense
plasma-foil separator before the electrons move to the deceleration phase
of the laser pulse. Since only the laser is reflected by the plasma
separator, the electrons do not experience the deceleration phase and the
density of the electron bunch is kept high after passing through the
plasma separator. Consequently, a high-density electron beam is generated
and at the same time, the pulse length of the electron bunch becomes
We propose a focusing mechanism of high-energy ions by an electron
cloud produced by a laser interaction with slab plasma. In our
2.5-dimensional (2.5D) particle-in-cell simulations, the laser intensity
is 2 × 1020 W/cm2, the laser wavelength
λ is 1.053 μm, and the laser spot size is 2.5λ. When the high
intensity laser irradiates slab plasma, electrons are accelerated,
oscillate around the plasma and produce the electron cloud locally at the
sides of the plasma. Because the electrons are localized transversely, a
static electric potential is formed to focus ions and at the same time the
ions are accelerated longitudinally. Though the longitudinal ion
acceleration has been studied well, the ion focusing effect is reported
for the first time in this paper. In our calculations, the maximum energy
and intensity of the protons are 8.61 MeV and 1.89 × 1017
W/cm2, and the diameter of the proton bunch accelerated are
focused to 71.2% of its initial size.
Electron ponderomotive acceleration in a vacuum by a short-pulsed
laser of TEM (1, 0) + TEM (0, 1) mode is studied in this paper using a
3-dimensional (3D) particle simulation. It was found that the laser can
trap electrons in transverse and accelerate them with the longitudinal
ponderomotive force at the same time. Through this electron trapping and
acceleration scheme of TEM (1, 0) + TEM (0, 1) mode laser, the electron
bunch is confined well in transverse and compressed remarkably in
longitudinal. Therefore, a high energy, high density, and low emittances
electron bunch is generated. For example, the result shows that for a
laser with intensity of a0 = eE0
/mωc = 10, the laser spot size of
w0 = 15λ, and the laser pulse length of
Lz = 10λ, the maximum energy gain reaches
301 MeV and the average energy 57.7 MeV. The electron bunch transverse
radius is about 350λ and the longitudinal size about 20λ. The
property of this accelerated bunch is improved compared with that
generated by the laser of TEM (0, 0) mode.
Key issues of heavy ion beam (HIB) inertial confinement fusion
(ICF) include an efficient stable beam transport, beam focusing,
uniform fuel pellet implosion, and so on. To realize a HIB fine
focus on a fuel pellet, space-charge neutralization of incident
focusing HIB is required at the HIB final transport just after
a final focusing element in an HIB accelerator. In this article,
an insulator annular tube guide is proposed at the final transport
part, through which a HIB is transported. The physical mechanism
of HIB charge neutralization based on an insulator annular guide
is as follows: A local electric field created by HIB induces
local discharges, and plasma is produced on the insulator inner
surface. Then electrons are extracted from the plasma by the
HIB net space charge. The electrons emitted neutralize the HIB
space charge well.
Numerical analyses show that a 40% mixing inhomogeneity of the deuterium (D) and tritium (T) concentrations in a DT pellet still gives a sufficient fusion energy output in DT inertial confinement fusion (ICF), as long as the D and T total amounts are equal. This new result means that fusion energy output is rather insensitive to the inhomogeneous fuel mixing in the DT ICF pellet.
A numerical and analytical estimation and a one-dimensionalhydrodynamic simulation show that a 30%reduction of the tritium content in a D-T pellet still gives sufficient energy output in aD-T inertial confinement fusion reactor.In other words, the tritiumb content can be reduced significantly without a significant reduction in the D-T fusion energy output.This new result also meansthat the tritium inventory can be reduced significantly before and during the reactor operation in the D-T inertial confinement fusion.This result comes from the contribution of a D-D reaction to the D-T reaction.
A new and simple type of self-magnetically insulated, vacuum ion diode named “Plasma Focus Diode” has been successfully developed with a large solid angle of irradiation and low divergence angle. The diode has a pair of coaxial cylindrical electrodes similar to a Mather-type plasma focus device. Ion-current density of 1·9 kA/cm2 has been obtained on the anode surface under the experimental conditions of diode voltage ∼1·4 MV, diode current ∼180 kA, and pulse width ∼75 ns. The generated ion beam has been two-dimensionally focused (line focusing) with a focusing radius of ∼0·18 mm, giving a maximum ion current density and beam power density at the axis of ∼0·14 MA/cm2 and ∼0·18 TW/cm2, respectively. The motion of electrons in the gap has been numerically simulated by use of a newly developed particle-in-cell computer simulation code, and good agreement has been obtained between the simulation and the experiment.
The interaction between particles and an electromagnetic (EM) wave is investigated numerically in the system of particle Vp × B acceleration by the EM wave. Numerical simulations show that the particle acceleration mechanism works well in the case of the appropriate number density of the imposed particles. When the interaction between particles and the wave is too strong, a part of the trapped and accelerated particles is detrapped. A condition is also presented for the efficient particle acceleration and trapping by the EM wave.
Recent experimental and theoretical research on intense pulsed particle beams involving the ETIGO Project at the Technological University of Nagaoka is reviewed. Experimentally, we review the following: (1) studies on local divergence angle in parameter space, (2) development of self-magnetically insulated “Plasma-Focus Diode,” (3) beam focusing by z-discharged plasma channel, (3) measurement of ablation process of beam-target interaction, (4) time-resolvable measurement of energy and species, (5) inductive postacceleration of highly neutralized ion beam, (6) construction of new pulse-power machine, and pulse-compression experiment by plasma erosion opening switch. A brief summary is also given on the development of 2.5-dimensional particle-in-cell computer simulation code.
Rotating motion of a propagating LIB is analyzed in order to suppress the mixed mode of the Kelvin-Helmholtz instability, the tearing instability and the sausage instability by the action of a self-induced magnetic field in the axial direction. The beams are assumed to be charge-neutralized but not current-neutralized. The steady-state solutions of a propagating LIB with rotation are first obtained numerically. Through the dispersion relation with respect to the ikonal type of perturbations, which are added to the steady-state solutions, the growth rates of instabilities appearing in an LIB are obtained. It is concluded that if the mean rotating velocity of an LIB is comparable to the propagation velocity, in other words, if the induced magnetic field intensity in the axial direction is comparable to the magnetic field intensity in the azimuthal direction, the instability disappears in the propagating ion beam.
Intense light-ion beam (LIB) propagation through a plasma channel is numerically investigated by using a two-dimensional simulation code. Analyses are given for the LIB propagation which couples with the motion of the channel plasma. Although the electron back current neutralizes the LIB current under typical beam and channel plasma conditions, the Lorentz force of the electron back current expands the LIB radially. The expansion of the LIB depends strongly on the density of the channel plasma. A strong current of the order of 1 MA is shown to propagate stably in an argon plasma channel.
Numerical simulations and analyses are given for the implosion of a hollow shell target driven by proton beams. The target consists of three layers of Pb, Al and DT. The Pb and Al layers play roles of a tamper and a pusher, respectively. The main part of the beam energy is deposited in the Al layer. But the process of deposition depends strongly on the distribution of incident angles and particle energies. As the Al layer is heated by proton beams, the layer expands and pushes the DT layer toward the target centre. This type of implosion motion is examined by using a similarity solution for the slab model. To obtain an optimum velocity for the DT implosion, the optimum target size and optimum layer thicknesses are determined. The Rayleigh–Taylor instability, accompanied by the implosion motion is investigated, and the implosion is found to be stable with respect to the chosen target structure. The effects of inhomogeneities on implosion are shown to be severe. The initial fluctuation of the temperature or the density in the Al layer must be less than 3% and the maximum amplitude of the ripples on the initial boundary surface should be less than 3 μm with a view to achieving a high target gain.
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