Stopping of heavy ions in a hydrogen plasma

Deceleration of fast ion rarefied beam due to Cherenkov interaction with ion-acoustic waves

The deceleration of a low-density beam of ions in plasma with developed ion-acoustic turbulence arising in strong electric field is described. The time and length of beam deceleration along and across the anisotropy axis of the wave number distribution of ion-acoustic waves are found. It is shown to what extent an increase in the strength of the electric field that generates turbulence is accompanied by a decrease in the time and length of braking. As the beam propagates along the anisotropy axis, its velocity decreases to approximately the velocity of ion sound, and the direction of propagation does not change. When the beam is decelerated with an initial velocity across the anisotropy axis, a velocity component appears along the anisotropy axis during deceleration, which results in the beam deflection from the initial direction. In this case, the modulus of the beam velocity at the end of deceleration is close to the ion sound velocity.

Target Density Effects on Charge Transfer of Laser-Accelerated Carbon Ions in Dense Plasma

We report on charge state measurements of laser-accelerated carbon ions in the energy range of several MeV penetrating a dense partially ionized plasma. The plasma was generated by irradiation of a foam target with laser-induced hohlraum radiation in the soft x-ray regime. We use the tricellulose acetate (C_{9}H_{16}O_{8}) foam of 2  mg/cm^{3} density and 1 mm interaction length as target material. This kind of plasma is advantageous for high-precision measurements, due to good uniformity and long lifetime compared to the ion pulse length and the interaction duration. We diagnose the plasma parameters to be T_{e}=17  eV and n_{e}=4×10^{20}  cm^{-3}. We observe the average charge states passing through the plasma to be higher than those predicted by the commonly used semiempirical formula. Through solving the rate equations, we attribute the enhancement to the target density effects, which will increase the ionization rates on one hand and reduce the electron capture rates on the other hand. The underlying physics is actually the balancing of the lifetime of excited states versus the collisional frequency. In previous measurement with partially ionized plasma from gas discharge and z pinch to laser direct irradiation, no target density effects were ever demonstrated. For the first time, we are able to experimentally prove that target density effects start to play a significant role in plasma near the critical density of Nd-glass laser radiation. The finding is important for heavy ion beam driven high-energy-density physics and fast ignitions. The method provides a new approach to precisely address the beam-plasma interaction issues with high-intensity short-pulse lasers in dense plasma regimes.

Benchmark Experiment to Prove the Role of Projectile Excited States Upon the Ion Stopping in Plasmas

We report on a precision energy loss measurement and theoretical investigation of 100  keV/u helium ions in a hydrogen-discharge plasma. Collision processes of helium ions with protons, free electrons, and hydrogen atoms are ideally suited for benchmarking plasma stopping-power models. Energy loss results of our experiments are significantly higher than the predictions of traditional effective charge models. We obtained good agreement with our data by solving rate equations, where in addition to the ground state, also excited electronic configurations were considered for the projectile ions. Hence, we demonstrate that excited projectile states, resulting from collisions, leading to capture-, ionization-, and radiative-decay processes, play an important role in the stopping process in plasma.

Observation of a high degree of stopping for laser-accelerated intense proton beams in dense ionized matter

Intense particle beams generated from the interaction of ultrahigh intensity lasers with sample foils provide options in radiography, high-yield neutron sources, high-energy-density-matter generation, and ion fast ignition. An accurate understanding of beam transportation behavior in dense matter is crucial for all these applications. Here we report the experimental evidence on one order of magnitude enhancement of intense laser-accelerated proton beam stopping in dense ionized matter, in comparison with the current-widely used models describing individual ion stopping in matter. Supported by particle-in-cell (PIC) simulations, we attribute the enhancement to the strong decelerating electric field approaching 1 GV/m that can be created by the beam-driven return current. This collective effect plays the dominant role in the stopping of laser-accelerated intense proton beams in dense ionized matter. This finding is essential for the optimum design of ion driven fast ignition and inertial confinement fusion.

A detailed understanding of particle stopping in matter is essential for nuclear fusion and high energy density science. Here, the authors report one order of magnitude enhancement of intense laser-accelerated proton beam stopping in dense ionized matter in comparison with currently used models describing ion stopping in matter.

Experimental Validation of Low-Z Ion-Stopping Formalisms around the Bragg Peak in High-Energy-Density Plasmas

We report on the first accurate validation of low-Z ion-stopping formalisms in the regime ranging from low-velocity ion stopping-through the Bragg peak-to high-velocity ion stopping in well-characterized high-energy-density plasmas. These measurements were executed at electron temperatures and number densities in the range of 1.4-2.8 keV and 4×10^{23}-8×10^{23}  cm^{-3}, respectively. For these conditions, it is experimentally demonstrated that the Brown-Preston-Singleton formalism provides a better description of the ion stopping than other formalisms around the Bragg peak, except for the ion stopping at v_{i}∼0.3v_{th}, where the Brown-Preston-Singleton formalism significantly underpredicts the observation. It is postulated that the inclusion of nuclear-elastic scattering, and possibly coupled modes of the plasma ions, in the modeling of the ion-ion interaction may explain the discrepancy of ∼20% at this velocity, which would have an impact on our understanding of the alpha energy deposition and heating of the fuel ions, and thus reduce the ignition threshold in an ignition experiment.

Ab Initio Studies on the Stopping Power of Warm Dense Matter with Time-Dependent Orbital-Free Density Functional Theory

Electronic transport properties of warm dense matter, such as electrical or thermal conductivities and nonadiabatic stopping power, are of particular interest to geophysics, planetary science, astrophysics, and inertial confinement fusion (ICF). One example is the α-particle stopping power of dense deuterium-tritium (DT) plasmas, which must be precisely known for current small-margin ICF target designs to ignite. We have developed a time-dependent orbital-free density functional theory (TD-OF-DFT) method for ab initio investigations of the charged-particle stopping power of warm dense matter. Our current dependent TD-OF-DFT calculations have reproduced the recently well-characterized stopping power experiment in warm dense beryllium. For α-particle stopping in warm and solid-density DT plasmas, the ab initio TD-OF-DFT simulations show a lower stopping power up to ∼25% in comparison with three stopping-power models often used in the high-energy-density physics community.

Charge-Transfer Processes in Warm Dense Matter: Selective Spectral Filtering for Laser-Accelerated Ion Beams

We investigate, both experimentally and theoretically, how the spectral distribution of laser accelerated carbon ions can be filtered by charge exchange processes in a double foil target setup. Carbon ions at multiple charge states with an initially wide kinetic energy spectrum, from 0.1 to 18 MeV, were detected with a remarkably narrow spectral bandwidth after they had passed through an ultrathin and partially ionized foil. With our theoretical calculations, we demonstrate that this process is a consequence of the evolution of the carbon ion charge states in the second foil. We calculated the resulting spectral distribution separately for each ion species by solving the rate equations for electron loss and capture processes within a collisional radiative model. We determine how the efficiency of charge transfer processes can be manipulated by controlling the ionization degree of the transfer matter.

Determination of Hydrogen Density by Swift Heavy Ions

A novel method to determine the total hydrogen density and, accordingly, a precise plasma temperature in a lowly ionized hydrogen plasma is described. The key to the method is to analyze the energy loss of swift heavy ions interacting with the respective bound and free electrons of the plasma. A slowly developing and lowly ionized hydrogen theta-pinch plasma is prepared. A Boltzmann plot of the hydrogen Balmer series and the Stark broadening of the H_{β} line preliminarily defines the plasma with a free electron density of (1.9±0.1)×10^{16}  cm^{-3} and a free electron temperature of 0.8-1.3 eV. The temperature uncertainty results in a wide hydrogen density, ranging from 2.3×10^{16} to 7.8×10^{18}  cm^{-3}. A 108 MHz pulsed beam of ^{48}Ca^{10+} with a velocity of 3.652  MeV/u is used as a probe to measure the total energy loss of the beam ions. Subtracting the calculated energy loss due to free electrons, the energy loss due to bound electrons is obtained, which linearly depends on the bound electron density. The total hydrogen density is thus determined as (1.9±0.7)×10^{17}  cm^{-3}, and the free electron temperature can be precisely derived as 1.01±0.04  eV. This method should prove useful in many studies, e.g., inertial confinement fusion or warm dense matter.

Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter

The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.

The energy loss of ions in plasma is a challenging issue in inertial confinement fusion and many theoretical models exist on ion-stopping power. Here, the authors use laser-generated plasma probed by accelerator-produced ions in experiments to discriminate various ion stopping models near the Bragg peak.

Nuclear-plus-interference-scattering effect on the energy deposition of multi-MeV protons in a dense Be plasma

The nuclear plus interference scattering (NIS) effect on the stopping power of hot dense beryllium (Be) plasma for multi-MeV protons is theoretically investigated by using the generalized Brown-Preston-Singleton (BPS) model, in which a NIS term is taken into account. The analytical formula of the NIS term is detailedly derived. By using this formula, the density and temperature dependence of the NIS effect is numerically studied, and the results show that the NIS effect becomes more and more important with increasing the plasma temperature or density. Different from the cases of protons traveling through the deuterium-tritium plasmas, for a Be plasma, a prominent oscillation valley structure is observed in the NIS term when the proton's energy is close to E_{p}=7MeV. Furthermore, the penetration distance is remarkably reduced when the NIS term is considered.

Measurements of Ion Stopping Around the Bragg Peak in High-Energy-Density Plasmas

For the first time, quantitative measurements of ion stopping at energies around the Bragg peak (or peak ion stopping, which occurs at an ion velocity comparable to the average thermal electron velocity), and its dependence on electron temperature (T(e)) and electron number density (n(e)) in the range of 0.5-4.0 keV and 3×10(22) to 3×10(23) cm(-3) have been conducted, respectively. It is experimentally demonstrated that the position and amplitude of the Bragg peak varies strongly with T(e) with n(e). The importance of including quantum diffraction is also demonstrated in the stopping-power modeling of high-energy-density plasmas.

Predictions for the energy loss of light ions in laser-generated plasmas at low and medium velocities

The energy loss of light ions in dense plasmas is investigated with special focus on low to medium projectile energies, i.e., at velocities where the maximum of the stopping power occurs. In this region, exceptionally large theoretical uncertainties remain and no conclusive experimental data are available. We perform simulations of beam-plasma configurations well suited for an experimental test of ion energy loss in highly ionized, laser-generated carbon plasmas. The plasma parameters are extracted from two-dimensional hydrodynamic simulations, and a Monte Carlo calculation of the charge-state distribution of the projectile ion beam determines the dynamics of the ion charge state over the whole plasma profile. We show that the discrepancies in the energy loss predicted by different theoretical models are as high as 20-30%, making these theories well distinguishable in suitable experiments.

Measurement of charged-particle stopping in warm dense plasma

We measured the stopping of energetic protons in an isochorically heated solid-density Be plasma with an electron temperature of ∼32  eV, corresponding to moderately coupled [(e^{2}/a)/(k_{B}T_{e}+E_{F})∼0.3] and moderately degenerate [k_{B}T_{e}/E_{F}∼2] "warm-dense matter" (WDM) conditions. We present the first high-accuracy measurements of charged-particle energy loss through dense plasma, which shows an increased loss relative to cold matter, consistent with a reduced mean ionization potential. The data agree with stopping models based on an ad hoc treatment of free and bound electrons, as well as the average-atom local-density approximation; this work is the first test of these theories in WDM plasma.

Thyristor stack for pulsed inductive plasma generation

A thyristor stack for pulsed inductive plasma generation has been developed and tested. The stack design includes a free wheeling diode assembly for current reversal. Triggering of the device is achieved by a high side biased, self supplied gate driver unit using gating energy derived from a local snubber network. The structure guarantees a hard firing gate pulse for the required high dI/dt application. A single fiber optic command is needed to achieve a simultaneous turn on of the thyristors. The stack assembly is used for switching a series resonant circuit with a ringing frequency of 30 kHz. In the prototype pulsed power system described here an inductive discharge has been generated with a pulse duration of 120 micros and a pulse energy of 50 J. A maximum power transfer efficiency of 84% and a peak power of 480 kW inside the discharge were achieved. System tests were performed with a purely inductive load and an inductively generated plasma acting as a load through transformer action at a voltage level of 4.1 kV, a peak current of 5 kA, and a current switching rate of 1 kA/micros.

Excimer laser pumped by an intense, high-energy heavy-ion beam

High-energy heavy ions are an ideal tool to generate homogeneously excited, extended volumes of nonthermal plasmas. Here, the high-energy loss (dE/dx) and absolute power deposition of heavy ions interacting with matter has been used to pump an ultraviolet laser. A pulsed 70 MeV/u 238U beam with up to 2.5 x 10(9) particles in approximately 100 ns beam bunches was stopped in a 1.2 m long laser cell filled with a 1.6 bar Ar-Kr-F2 mixture (typically 50%:49.9%:0.1%). Laser effect on the 248 nm KrF* excimer transition is clearly demonstrated.

Shock compression of condensed matter using intense beams of energetic heavy ions

In this paper is presented, with the help of sophisticated two-dimensional hydrodynamic simulations, a suitable design with optimized parameters for a heavy-ion beam-matter interaction experiment that will be carried out at the Gesellschaft fur Schwerionenforschung (GSI) Darmstadt by the end of the year 2001 when the upgrade of the existing accelerator facility will be completed. Our simulations show that this upgraded heavy-ion beam is capable of generating strong shocks in solid targets that compress the target material to supersolid densities and generate multi-mbar pressures. This will open up, at the GSI, the possibility of investigation of the equation-of-state properties of matter under such extreme conditions. Numerical simulations can predict the experimental results with reasonable accuracy, which is helpful in designing the diagnostic tools for the experiment.

Laser-assisted stopping power of a hot plasma for a system of correlated ions

The laser-assisted stopping power of a fully ionized plasma for the system of two correlated test charges is investigated. The general expressions for the stopping power are applied to a low-density and a low-temperature plasma in a low-energy beam-plasma experiment [J. Jacoby et al., Phys. Rev. Lett. 74, 1550 (1995)]. The effect of the interaction between the beam test charges, described by a correlation term, is to increase the stopping power of the laser-assisted plasma compared to the case where the charges are infinitely separated. However, the laser field affects the correlation between the test charges and contributes to decrease the plasma stopping power, as compared to the laser-free dicluster case.