Acceleration of an electron bunch injected in front of a laser pulse generating an accelerating wake wave

A study is made of a promising method for injecting an electron bunch into an accelerating laser-plasma system. A bunch is injected ahead of the front of a laser pulse generating a wake wave that propagates in a direction collinear with the pulse and has a velocity lower than the pulse group velocity. The influence of the initial nonmonoenergetic character of the bunch on its trapping and acceleration is investigated. By appropriately choosing the laser pulse parameters and the bunch injection energy, it is possible to create such conditions for the trapping of an initially nonmonoenergetic bunch by the wake wave that, over a certain acceleration distance, there will be no energy spread of the bunch due to its initial nonmonoenergetic character, a circumstance that allows compact electron bunches to be accelerated to high energies, with a minimum energy spread.     

Potential of compact Compton sources in the medical field

The exceptional improvement of high power lasers and optical cavity finesses in the last fifteen years allows today the development of X-ray sources based on inverse Compton scattering. These compact sources will provide high intensity beams, with a tunable energy in the range 20–100 keV, that can be used in several application including material sciences, structural biology, cultural heritage research and preservation and medical or biomedical preclinical and clinical research. The access to these devices will be easier. Methods currently used only in synchrotron facilities will be available in dedicated work environment such as hospitals, laboratories or museums. Several machines are in design or construction phase, and aim at producing 10¹²–10¹⁴ ph/s. The ThomX machine is the most advanced project and has the potential to be used as the radiation source for biomedical searches, clinical imaging techniques or radiotherapy programs.     

Feasibility of stabilizing a vacuum-diode X-ray source with a laser-plasma cathode

Results are presented from experimental studies of discharge instabilities and the energy and temporal characteristics of a vacuum-diode X-ray source with a laser plasma cathode over a wide range of energies, intensities, and durations of the plasma-forming laser pulse. It is experimentally shown that the vacuum-discharge dynamics and radiation processes in different discharge stages substantially depend on the parameters of the laser radiation. The shortest recorded pulse duration (10 ns) of Ti K-line radiation (4.5 keV) with a total photon number of 10¹¹ is achieved when the laser plasma cathode is produced by a laser pulse with a duration of 27 ps and an intensity of 10¹³ W/cm². It is found that the contrast of characteristic emission against the bremsstrahlung background is maximum when discharge instabilities are suppressed and the accelerating voltage is three to four times higher than the threshold voltage for line excitation.     

Experimental study of the evaporation and expansion of a solid pellet in a plasma heated by an electron beam

Results are presented from experiments on the injection of solid pellets into a plasma heated by an electron beam in the GOL-3 device. For this purpose, two pellet injectors were installed in the device. The target plasma with a density of ～10¹⁵ cm^?3 was produced in a solenoid with a field of 4.8 T and was heated by a highpower electron beam with an electron energy of ～1 MeV, a duration of ～7 s, and a total energy of 120–150 kJ. Before heating, the pellet was injected into the center of the plasma column transversely to the magnetic field. The injection point was located at a distance of 6.5 or 2 m from the input magnetic mirror. Polyethylene pellets with a mass of 0.1–1 mg and lithium-deuteride pellets with a mass of 0.02–0.5 mg were used. A few microseconds after the electron beam starts to be injected into the plasma, a dense plasma bunch is formed. In the initial stage of expansion, the plasma bunch remains spherically symmetric. The plasma at the periphery of the bunch is then heated and becomes magnetized. Next, the dense plasma expands along the magnetic field with a velocity on the order of 300 km/s. A comparison of the measured parameters with calculations by a hydrodynamic model shows that, in order to provide such a high expansion velocity, the total energy density deposited in the pellet must be ～1 kJ/cm². This value substantially exceeds the energy density yielded by the target plasma; i.e., the energy is concentrated across the magnetic field onto a dense plasma bunch produced from the evaporated particle.     

Nonlinear Thomson scattering of a relativistically strong tightly focused ultrashort laser pulse

The problem of nonlinear Thomson scattering of a relativistically strong linearly polarized ultrashort laser pulse tightly focused into a spot with a diameter of D _F ? λ (where λ is the laser wavelength) is solved. The energy, spectral, and angular distributions of radiation generated due to Thomson scattering from test electrons located in the focal region are found. The characteristics of scattered radiation are studied as functions of the tightness of laser focusing and the initial position of test particles relative to the center of the focal region for a given laser pulse energy. It is demonstrated that the ultratight focusing is not optimal for obtaining the brightest and hardest source of secondary electromagnetic radiation. The hardest and shortest radiation pulse is generated when the beam waist diameter is ?10λ.     

Trapping of electrons and acceleration of the electron bunch in a wake wave

The process of electron trapping by a wake wave excited by a laser pulse in a plasma channel in the case where the electron bunches are injected into the vicinity of the maximum of the wakefield potential at a velocity lower than the wave phase velocity is considered. The mechanism for the formation of a compact electron bunch in the trapping region when only the electrons of the injected bunch that are trapped in the focusing phase mainly undergo the subsequent acceleration in the wakefield is analyzed. The influence of the spatial dimensions of the injected bunch and its energy spread on the length of the trapped electron bunch and the fraction of trapped electrons is studied analytically and numerically. For electron bunches with different ratios of their spatial dimensions to the characteristic dimensions of the wake wave, the influence of the injection energy on the parameters of the high-energy electron bunch trapped and accelerated in the wake-field is studied.     

Diagnostics of plasma produced by femtosecond laser pulse impact upon a target with an internal nanostructure

X-ray diagnostics of the interaction of femtosecond laser pulses with intensities of 10¹⁶–10¹⁸ W/cm² with CO₂ clusters and frozen nanosize water particles is carried out. The stage of cluster expansion and the formation of a plasma channel, which governs the parameters of the formed X-ray radiation source and accelerated ion flows, is studied. The measurements are based on recording spatially resolved X-ray spectra of H- and He-like oxygen ions. Utilization of Rydberg transitions for spectra diagnostics makes it possible to determine plasma parameters on a time scale of t ∼ 10 ps after the beginning of a femtosecond pulse. The role of the rear edge of the laser pulse in sustaining the plasma temperature at a level of ∼100 eV in the stage of a nonadiabatic cluster expansion is shown. The analysis of the profiles and relative intensities of spectral lines allows one to determine the temperature and density of plasma electrons and distinguish the populations of “thermal” ions and ions that are accelerated up to energies of a few tens of kiloelectronvolts. It is shown that the use of solid clusters made of frozen nanoscale water droplets as targets leads to a substantial increase in the number of fast He-like ions. In this case, however, the efficiency of acceleration of H-like ions does not increase, because the time of their ionization in plasma exceeds the time of cluster expansion.     

Numerical 1D PIC-simulations of ion acceleration during laser-plasma interaction: Optimization of a two-component multilayered target structure

Ion beam acceleration is simulated using a one-dimensional 1D2P PIC code. The dependences of the maximum energy and width of the energy spectrum of the generated ion beams on the duration and intensity of laser radiation, as well as on the target parameters (thickness and number of layers, types and densities of atoms), are investigated. The optimal target configuration at which the energy of the accelerated ions is maximum (5–160 MeV for intensities of 5 × 10^{18 −5} × 10²⁰ W/cm²) is found. The optimal target configuration is shown to depend on the intensity and be independent of the laser pulse duration.     

Runaway electron beams in the gas discharge for UV nitrogen laser excitation

The review of the methods for obtaining the runaway electron beams in the gas discharge is performed. The new method is offered, using which the beam is first formed in a narrow gap (∼1 mm) between the cathode and the grid and then it is accelerated by the field of the plasma column of the anomalous self-sustained discharge in the main gap (10–20 mm long). The electron beams with an energy of about 10 keV and current density of 10³ A/cm² at a molecular nitrogen pressure of up to 100 Torr have been obtained experimentally. The results of research of the UV nitrogen laser with an excitation via runaway electron beam and radiation of energy of ∼1 mJ are given. The UV nitrogen laser generation with the energy of ∼1 mJ has been obtained by the runaway electron beams.     

Crater formation in a target under the action of a high-power laser pulse

The formation of craters in targets of various materials under the action of a high-power neodymium-laser pulse at radiation intensities from 10¹⁰ to 10¹⁴ W/cm² was studied experimentally and theoretically. The interaction between the laser beam and solid targets is investigated to determine the efficiency of the ablation loading of various materials and the transformation of the laser energy into the energy of a shock wave.     

Investigation of Early Plasma Evolution Induced by Ultrashort Laser Pulses

Early plasma is generated owing to high intensity laser irradiation of target and the subsequent target material ionization. Its dynamics plays a significant role in laser-material interaction, especially in the air environment^1-11.Early plasma evolution has been captured through pump-probe shadowgraphy^1-3 and interferometry^1,4-7. However, the studied time frames and applied laser parameter ranges are limited. For example, direct examinations of plasma front locations and electron number densities within a delay time of 100 picosecond (ps) with respect to the laser pulse peak are still very few, especially for the ultrashort pulse of a duration around 100 femtosecond (fs) and a low power density around 10¹⁴ W/cm². Early plasma generated under these conditions has only been captured recently with high temporal and spatial resolutions¹². The detailed setup strategy and procedures of this high precision measurement will be illustrated in this paper. The rationale of the measurement is optical pump-probe shadowgraphy: one ultrashort laser pulse is split to a pump pulse and a probe pulse, while the delay time between them can be adjusted by changing their beam path lengths. The pump pulse ablates the target and generates the early plasma, and the probe pulse propagates through the plasma region and detects the non-uniformity of electron number density. In addition, animations are generated using the calculated results from the simulation model of Ref. ¹² to illustrate the plasma formation and evolution with a very high resolution (0.04 ~ 1 ps).Both the experimental method and the simulation method can be applied to a broad range of time frames and laser parameters. These methods can be used to examine the early plasma generated not only from metals, but also from semiconductors and insulators.     

Study of the glow dynamics in a laser-produced plasma jet expanding across the magnetic field

Results are presented from experimental studies of the glow dynamics of a plasma jet generated during the irradiation of a plane aluminum target by an iodine laser pulse with the wavelength 1.315 μm. The laser pulse energy was 330–480 J, the pulse duration was 0.5 ns, and the focal spot diameter was 3 mm, the laser intensity on the target surface being ∼10¹³ W/cm². The jet expanded across an external magnetic field with the strength ∼1 kOe. The residual air pressure in the vacuum chamber was ∼10⁻⁵ Torr. The spatiotemporal behavior of the jet glow was investigated using a nine-frame camera in two mutually perpendicular directions (along and across the magnetic field). The results of measurements indicate azimuthal asymmetry of the jet expansion.     

Ion recombination and energy balance in lead and gallium laser erosion plasmas

The ion recombination times, the energy balance characteristics, and the time behavior of the radiation intensity of lead and gallium laser plasmas are determined by analyzing the trailing edges of the waveforms of various spectral lines. Reasons for different decay rates of the spectral lines are analyzed. Experiments on laser erosion of lead and gallium targets under the action of repetitive neodymium laser radiation with a pulse duration of 20 ns, wavelength of 1.06 μm, repetition rate of 12 Hz, and peak intensity on the target surface of 10⁸–10⁹ W/cm² were carried out at a residual air pressure of 3–12 Pa.     

Design of an experiment on wakefield acceleration on the VEPP-5 injection complex

Relativistic beams produced by the VEPP-5 injection complex (Budker Institute of Nuclear Physics, Siberian Division, Russian Academy of Sciences) can be used to generate plasma waves with a longitudinal electric field of 1 GV/m. A part of the electron (or positron) driver bunch is accelerated by this field over a distance of up to 1 m. The main advantage of the proposed design over the previous wakefield acceleration experiments is the beam preparation system capable of compressing bunches to a length of σ_z = 0.1 mm in the longitudinal direction and producing an optimal longitudinal profile of the beam density. The main parameters of the planned device are as follows: the electron energy at the entrance to the plasma is 510 MeV, the number of particles in the bunch is 2 × 10¹⁰, the plasma density is up to 10¹⁶ cm^?3, the number of accelerated particles is up to 3 × 10⁹, and their energy spread is less than 10%. The physical project of the experiment is presented, and the results of computer simulations of the beam-plasma interaction are described.     

Simulation of the generation of characteristic X radiation under vacuum heating of electrons by a femtosecond laser pulse

An analytic model of the generation of characteristic X radiation under vacuum heating of electrons at the surface of a massive target by a p-polarized nonrelativistic femtosecond laser pulse is considered. The results of calculations satisfactorily describe the measured data on the output of K _α radiation generated by laser pulses with a wavelength of 1.24 μm and peak intensities of 5 × 10¹⁶–2 × 10¹⁷ W/cm², incident at an angle of 45°.     

Acceleration of electron bunches injected into a wake wave

The process of trapping and acceleration of nonmonoenergetic electron bunches by a wake wave excited by a laser pulse in a plasma channel is investigated. The electrons are injected into the vicinity of the maximum of the wakefield potential with a velocity lower than the wave phase velocity. The study is aimed at utilizing specific features of a wakefield with substantially overlapped focusing and accelerating phases for achieving monoenergetic electron acceleration. Conditions are found under which electrons in a finite-length nonmonoenergetic bunch are accelerated to high energies, while the energy spread between them is minimal. The effect of energy grouping of electrons makes it possible to obtain compact high-energy electron bunches with a small energy spread during laser plasma acceleration.     

A biresonant plasma source based on a gapped linear microwave vibrator

The operating principle of a novel microwave plasma source—a linear microwave vibrator with a gap—is discussed. The source is placed on a microwave-transparent window of a chamber filled with a plasma-forming gas (argon or methane). The device operation is based on the combination of two resonances—geometric and plasma ones. The results of experimental tests of the source are presented. For a microwave frequency of 2.45 GHz, microwave power of ≤1 kW, and plasma-forming gas pressure in the range 5 × 10⁻²–10⁻¹ Torr, the source is capable of filling the reactor volume with a plasma having an electron density of about 10¹² cm⁻³ and electron temperature of a few electronvolts.     

Laser-generated plasmas at INFN-LNS

Hot plasmas can be generated by fast and intense laser pulses ablating solids placed in vacuum. A Nd:Yag laser operating at the fundamental and second harmonics with 9-ns pulses (maximum energy of 900 mJ) focused on metallic surfaces produces high ablation yields of the order of μg/pulse and dense plasma that expands adiabatically at supersonic velocity along the normal to the target surface. The plasma emits neutral and charged particles. Charge states up to 10+ have been measured in heavy elements ablated with intensities of the order of 10¹⁰ W/cm². The ion temperature of the plasma is evaluated from the ion energy distributions measured with an ion energy analyzer. The electron temperature is measured through Faraday cups placed at the end of long drift tubes by using time-of-flight technique. The neutral temperature is measured with a special mass quadrupole spectrometer placed along the normal to the target surface. The plasma temperature increases with the laser pulse intensity. The ion temperature reaches values of the order of 400 eV, the electron temperature is of the order of 1 keV for hot electrons and 0.1 eV for thermal electrons, and the neutral temperature is of the order of 200 eV. The experimental apparatus, the diagnostic techniques, and the procedures for the plasma temperature characterization will be presented and discussed in detail. Published in Russian in Fizika Plazmy, 2006, Vol. 32, No. 6, pp. 558–564. The text was submitted by the authors in English.     

Optical characteristics and parameters of the plasma of a barrier discharge excited in a mixture of mercury dibromide vapor with nitrogen and helium

Results are presented from experimental and theoretical studies of the optical characteristics and parameters of the plasma of an atmospheric-pressure barrier discharge excited in a HgBr₂: N₂: He mixture, which was used as the working medium of a small-size (with a radiation area of 8 cm²) exciplex gas-discharge radiation source. The mean radiation power of 87 mW was achieved at the radiation wavelength λ_max = 502 nm. The electron energy distribution function, the transport characteristics, the specific energy lost in the processes involving electrons, the electron temperature and density, and the rate constants of elastic and inelastic electron scattering by the components of the working mixture were calculated as functions of the reduced field E/N. The plasma of a discharge excited in a HgBr₂: N₂: He mixture can be used as the working medium of a small-size blue-green radiation source. Such a source can find application in biotechnology, photonics, and medicine and can also be used to manufacture gas-discharge display panels.     

Monte Carlo study on the secondary cancer risk estimations for patients undergoing prostate radiotherapy: A humanoid phantom study

AimThe aim of this study was to estimate the secondary malignancy risk from the radiation in FFB prostate linac-based radiotherapy for different organs of the patient.BackgroundRadiation therapy is one of the main procedures of cancer treatment. However, the application the radiation may impose dose to organs of the patient which can be the cause of some malignancies.Materials and methodsMonte Carlo (MC) simulation was used to calculate radiation doses to patient organs in 18 MV linear accelerator (linac) based radiotherapy. A humanoid MC phantom was used to calculate the equivalent dose s for different organs and probability of secondary cancer, fatal and nonfatal risk, and other risks and parameters related to megavoltage radiation therapy. In out-of-field radiation calculation, it could be seen that neutrons imparted a higher dose to distant organs, and the dose to surrounding organs was mainly due to absorbed scattered photons and electron contamination.ResultsOur results showed that the bladder and skin with 54.89 × 10⁻³ mSv/Gy and 46.09 × 10⁻³ mSv/Gy, respectively, absorbed the highest equivalent dose s from photoneutrons, while a lower dose was absorbed by the lung at 3.42 × 10⁻³ mSv/Gy. The large intestine and bladder absorbed 55.00 × 10⁻³ mSv/Gy and 49.08 × 10⁻³, respectively, which were the highest equivalent dose s due to photons. The brain absorbed the lowest out-of-field dose, at 1.87 × 10⁻³ mSv/Gy.ConclusionsWe concluded that secondary neutron portion was higher than other radiation. Then, we recommended more attention to neutrons in the radiation protection in linac based high energy radiotherapy.