E. Nardi

Energy deposition by fast protons in pellet fusion targets

The energy loss of energetic protons in heavy metal targets is calculated at temperatures and densities of interest for pellet fusion. The contributions of bound and free electrons to the stopping power are considered. It is found that in the temperature range of interest the increase in the number of free electrons in the target plasma causes range shortening with increasing temperatures and specific volumes. In order to study this effect on the ablation process, a one‐dimensional calculation of the hydrodynamic flow with energy deposition was carried out.

Electron beam therapy with transverse magnetic fields

Detailed Monte Carlo electron transport simulations were carried out for the purpose of investigating the possibility of improving electron dose distribution for therapeutic applications, by using transverse magnetic fields. The case studied here is that of a 15 MeV electron beam of 6 cm diameter. The electrons pass through 4 cm of field-free tissue and a transverse magnetic field is applied for depth greater than 4 cm. A field of 3 T was found to improve the skin sparing factor by a factor of 2, when compared to field-free irradiation. A field of 2 T could also have a significant effect although less pronounced than 3 T while, for the case at hand, a magnetic field of only 1 T is not effective. The results here include detailed energy deposition contours in three dimensions.

Charge state of Zn projectile ions in partially ionized plasma: Simulations

This study deals with the simulation of the experimental study of Roth et al. ~2000! on the interaction of energetic Zn projectiles in partially ionized laser produced carbon targets, and with similar type experiments. Particular attention is paid to the specific contributions of the K and Lshell target electrons to electron recombination in the energetic Zn ionic projectile. The classical Bohr–Lindhard model was used for describing recombination, while quantum mechanical modelswerealsointroducedforscalingtheLtoKcross-sectionratios.Itwasfoundthatevenforahydrogen-likecarbon target, the effect of the missing five bound electrons brings about an increase of only 0.6 charge units in the equilibrium charge state as compared to the cold target value of 23. A collisional radiative calculation was employed for analyzing the type of plasma produced in the experimental study. It was found that for the plasma conditions characteristic of this experiment, some fully ionized target plasma atoms should be present. However in order to explain the experimentally observed large increase in the projectile charge state a very dominant component of the fully ionized plasma must comprise the target plasma. A procedure for calculating the dynamic evolvement of the projectile charge state within partially ionized plasma is also presented and applied to the type of plasma encountered in the experiment of Roth et al. ~2000!.The low temperature and density tail on the back of the target brings about a decrease in the exiting charge state, while the value of the average charge state within the target is dependent on the absolute value of the cross-sections.

Dynamic screening and charge state of fast ions in plasma and solids

This paper addresses the effect of target plasma electrons on the charge state of energetic ions, penetrating a target composed of bound as well as plasma electrons. Dynamic screening of the projectile Coulomb potential by the plasma electrons brings about a depression in the ionization energy of the ionic projectiles, as has been verified experimentally. This in turn makes the ionization cross-sections larger, while making the recombination cross-section smaller, thereby causing an increase in the ion charge state compared to the case of a gas target. The effect of the plasma environment, where the valence electrons are treated as plasma, is illustrated here for a 2 MeV carbon beam penetrating amorphous carbon targets of varying densities.

Kilovolt X-ray scattering from a Plasma

We describe a diagnostic suitable for the investigation of strong coupling effects in a plasma. The diagnostic is based on x-ray scattering of kilovolt x rays from a plasma and the determination of total x-ray scattering cross sections. The first experimental results of the total scattering cross sections from a strongly coupled plasma measured with kilovolt x rays are presented. The scattering plasma is formed by radiatively heating an Al foil with soft x rays created by Au laser-conversion foil; the plasma is probed with Ti XXII 1s2–1s2p line x rays emitted from a second laser produced plasma. A detailed description of the scattering technique is presented and the potential of x-ray scattering as a diagnostic of strongly coupled plasmas is explored.

Plasma effects in the interaction of intense light ion beams with light targets

It is shown that with present‐day ion beam intensities, plasma effects on the range are important in light targets. It is suggested to use the strong dependence of Kα emission on particle energy in order to observe this effect in layered targets. Neutron diagnostics in the case of deuteron beam CD2 target interaction are also considered.

Plasma diagnostics by means of the scattering of electrons and proton beams

Scattering of energetic electron and proton beams by cold matter is significantly different from the scattering of these particles by plasma, which may be either highly ionized or dense strongly coupled plasma. This is due to the difference in the shielding of the target nuclei between the two cases. Quantitatively, we treat the problem by means of the Bethe Moliere multiple scattering theory and the version of this theory for plasma as derived by Lampe. We propose to use this effect as a plasma diagnostic tool, utilizing monoenergetic, well-collimated electron or proton beams produced either by femtosecond laser plasma interactions or by accelerators. The effect is first illustrated for simplicity, by calculating the widths of the angular distribution of scattered particles interacting with the extreme cases of very hot fully ionized carbon, and iron plasmas, and comparing these results to the corresponding cold material. The more relevant case of electron scattering from partially ionized iron and carbon plasmas covering the entire range from a cold to a completely ionized target is also dealt with here. This paper brings up and highlights the difference between scattering by plasma and by cold material in light of the recent proposals to employ particle beams for various fusion applications.

X-ray scattering from a radiatively heated plasma

Abstract This paper describes experiments designed to measure the X-ray scattering cross section of a dense plasma created by radiatively heating an aluminum sample foil. In contrast to previous experiments, we believe that the heating was sufficient to generate a plasma throughout the entire thickness of the foil. Hydrodynamic simulations predict that the plasma conditions were not very uniform, with temperatures ranging from 1–10 eV through the foil. Contrary to initial expectation, there is an absence of a clear peak in the cross section. This may be due to either lower than expected ionization, or to temporal averaging and the non-uniformity of density and temperature, which may smooth out any sharp features. However, the data does show some indication of changes in the cross section as the plasma evolves. The successful measurement of cross sections as a function of angle and time delay shows that the technique has potential as a probe of this important class of plasmas.

Electron beam therapy with coil-generated magnetic fields.

This paper presents an initial study on the issues involved in the practical implementation of the use of transverse magnetic fields in electron beam therapy. By using such magnetic fields the dose delivered to the tumor region can increase significantly relative to that deposited to the healthy tissue. Initially we calculated the magnetic fields produced by the Helmholtz coil and modified Helmholtz coil configurations. These configurations, which can readily be used to generate high intensity magnetic fields, approximate the idealized magnetic fields studied in our previous publications. It was therefore of interest to perform a detailed study of the fields produced by these configurations. Electron beam dose distributions for 15 MeV electrons were calculated using the ACCEPTM code for a 3T transverse magnetic field produced by the modified Helmholtz configuration. The dose distribution was compared to those obtained with no magnetic field. The results were similar to those obtained in our previous work, where an idealized step function magnetic field was used and a 3T field was shown to be the optimal field strength. A simpler configuration was also studied in which a single external coil was used to generate the field. Electron dose distributions are also presented for a given geometry and given magnetic field strength using this configuration. The results indicate that this method is more difficult to apply to radiotherapy due to its lack of symmetry and its irregularity. For the various configurations dealt with here, a major problem is the need to shield the magnetic field in the beam propagation volume, a topic that must be studied in detail.

The interaction of large fast carbon clusters with plasma

The energy loss and disintegration dynamics of large carbon clusters (C 60 molecules) in hot plasmas is calculated. The calculation self-consistently includes the cluster disintegration, the evolution of the charge of the fragment ions, the screened Coulomb forces among them, and the effects of interference between neighboring ions on the energy loss. A range of projectile energies and plasma parameters is considered. A large enhancement factor in the energy deposition due to the interference is found at the early stage of the interaction.