Focused Power Density Research Articles

An innovative optical element for X-ray beam aperture, monitoring, and diagnostic at the CARNAÚBA beamline at SIRIUS/LNLS

The CARNAÚBA beamline is the tender-to-hard X-ray nanoprobe under commissioning for the new source Sirius at the Brazilian Synchrotron Light Laboratory (LNLS). The all-achromatic optics relies on horizontal deflection mirrors, a horizontal secondary source aperture (SSA), and Kirkpatrick–Baez (KB) mirrors to reach beam size down to ~30x30 nm2 at the sample position. To handle the highly focused power density of the pink beam and reach the desired stability for the horizontal secondary source, essential for the desired focusing and coherent properties at the sample position, we present here an SSA device based on a cryogenically cooled Si crystal in a channel-cut geometry. The crystal slabs define the channel aperture, which is controlled by a fine rotation. Simultaneously to the aperture function, the device plays the key role of beam position monitor and beam diagnostic. The scattered signal from the slabs is used to monitor the horizontal beam position during experiments and serves as a feedback input for the pitch control system of the upstream focusing mirror to stabilize the beam intensity and wavefront. The Bragg magnifier geometry of the device spreads the beam over the slabs, allowing the diffracted image of the beam to be collected by an area detector. The beam spread on the slabs also reduces the power density reaching the device, keeping it stable and under controlled temperatures.

Tight focusing of laser light using a polarization converter and an adaptive lens

Tight focusing of laser light using a polarization converter and an adaptive lens is demonstrated. The polarization converter is used to convert the linearly polarized light into a radially polarized light. The adaptive lens is used to focus the radially polarized light on a spot. At the focused spot, a large component of the electric field is along the longitudinal direction, and the focused power density is enhanced. The size of the focused spot can be changed by changing the focal length of the lens. When the focal length of the lens becomes shorter, the focused spot becomes tighter. The mechanisms of the two devices are introduced, and the theory of tight focusing is experimentally supported. Tight focusing of radially polarized laser light with a controllable spot size has potential applications in super-resolution imaging, material processing, and microparticle trapping.

Lithium beam generation and focusing with a radial diode on PBFAII

The performance of a 15-cm-radius applied-magnetic-field ion diode was investigated on the PBFA II accelerator at a power of 23 TW. The power coupling between the accelerator and diode was measured and compared with numerical simulations that show the effects of the electron flow in the MITL. The power coupled to the cathode of the diode was 18 MW. Measurements of the lithium beam generated from an electric-field-emission LiF anode showed a lithium beam power of 9 TW. The lithium beam was ballistically focused in a gas cell filled with 2 torr argon. The resultant focused power density was ∼1.8 TW/cm2 equivalent on a cylindrical target at the centerline of the diode. The focused power was limited by the 20- to 30-mR divergence of the beam caused by the LiF source used and by virtual cathode instabilities in the anode–cathode gap. The ion mode instability in the virtual cathode was studied extensively by measurement of waves in the ion emission pattern from the anode and of the E-P0 correlation between variations in the beam energy and transverse momentum. The instability Played a dominant role in the limitation of the focused lithium power.

Generation of a focused proton beam with a self-magnetically BΘ-insulated ion diode

A self-magnetically BΘ -insulated ion diode was investigated and optimized at the pulsed power line KALIF (2 MV, 50 ns, 2 Ω) to produce a focused and pulsed high-intensity proton beam for generation of high-power density in matter. This diode is characterized by an azimuthal insulating magnetic field that is generated by the diode current. The beam is neutralized by electrons from the cathode vanes, so that neither a gas filled drift space nor foils are needed. The beam therefore propagates in vacuum and repeated shots are possible without changing parts. The protons had energies up to 1.8 MeV and the proton content in the ion beam was about 50%. The maximum total diode current was about 800 kA and its efficiency was as high as 70%. The beam microdivergence was determined to be 1.1°. The radial dependence of the proton current density was measured and is proportional to r−1.74. This result was confirmed by 2D quasistatic particle-in-cell simulations. The focus was optimized by adjusting the anode shape and the gap geometry and by decreasing the beam divergence. The FWHM diameter of the focus at a distance of 300 mm was about 10 mm and the resulting focused power density was 0.13 TW/cm2. The effect of the gap distance on focusing and impedance was investigated and it was found necessary to adjust the gap to better than 0.1 mm over the total 30-cm-diam anode to obtain the best performance. The electron loss and the corresponding erosion at the posts that hold the anode was reduced. The lifetime is now limited by the plastic anode insert to 10–15 pulses. Based on these results a ‘‘small BΘ diode’’ is suggested for which a power density increase of a factor of 3 is estimated.

Spectra from plasma created by very high intensity picosecond laser

The Powerful Picosecond Laser (PP-Laser) System with an output power level of 20–30 GW and focused power density of 10 16 W/cm 2 has been operating as a part of the X-ray laser program at Princeton University for more than 2 yr (presently the PP-Laser System is producing power density which is larger by almost two orders of magnitude). Such a laser system creates an electric field which is greater than the Coulomb field in atoms. This may lead to a number of new physical processes like multiphoton excitation and ionization in a very strong field. Several spectra from the plasma created by the PP-Laser interacting with different solid targets are presented. Spectra were obtained in the range of 6–300 Å using the high resolution NRL-G SFC 3m XUV spectrograph. Very large broadenings of the lines from highly ionized ions were observed. Also presented are spectra recorded in a longer spectral range (up to visible), and some of the spectral line anomalies are discussed.

Light ion beams as the driver for inertial confinement fusion

Because of the high costs associated with producing the ≳1 MJ, 10 ns pulse required to initiate a pellet, the efficiency and relatively low cost of pulse power driven ion beams makes this approach particularly attractive both for proof-of-principle and for reactors in the future. Although pulse power technology for producing multimegavolt and multimegampere electric pulses is well-established, the coupling of this energy into ion beams focused onto targets requires further work. During the last six years, focused ion beam intensity has been improved more than five orders of magnitude to a value of ∠1012 W/cm2. An additional factor of 40 in focused power density will be required; this will be provided by the PBFA II accelerator now being designed and scheduled for completion in 1985. This paper will briefly describe how pulse power systems deliver energy to ion sources, how beams are generated and focused, and a few examples of target irradiation experiments. Finally, a concept for delivering beams to pellets in the center of a reactor wall will be described.

Production of High-Q Ions by Laser Bombardment Method

The expanding plasma produced when an intense pulse of laser radiation is focused in vacuum onto a solid target has been used as a source of highly stripped ions for collision cross-section measurements. Usable fluxes of carbon nuclei at energies of a few hundred eV/charge have been obtained by irradiation of graphite with pulses of CO2 laser radiation at a focused power density of 3 × 1010 W/cm2. Bombardment of aluminum and iron targets at comparable power levels have yielded ions of maximum charges of 9 and 16 respectively. A time-of-flight apparatus has been constructed to utilize the laser source for measurement of electron capture cross sections for highly stripped ions in gases at energies in the few hundred eV/charge range. Apertures collimate an ion beam from the plasma blowoff, and an electrostatic analyzer selects ions from the expanding plasma which have the same energy per charge. The beam is directed through a gas target cell, charge analyzed once more by deceleration, and detected by a channel plate electron multiplier used in a current amplification mode. Electron capture cross sections have been measured for C+q ions, q = 3, 4, 5 in H2 at energies ranging from 150-1160 eV/charge. Preliminary data have been obtained for carbon ions in atomic hydrogen, and similar measurements are planned for O+q and Fe+q ions.

Spectral classifications in the Fe XIX to Fe XXIII isoelectronic sequences

Emission lines are classified belonging to 2s22pn-2s2pn+1 transition arrays in the ions Ti XVII, V XVII to XX, Mn XVIII, MN XIX, Fe XIX and Fe XX. The source of these ions is the plasma produced at metallic surfaces irradiated with a neodymium laser beam with a focused power density in the range of 1012 to 1013 W cm-2 at the target surface. The data reported can be applied to the interpretation of solar flare spectra.

A parametric study of the performance of a TEA CO2laser

An investigation into the operating characteristics of a double discharge TEA CO2 laser is presented, the object of which is to optimize the available focused power density. The existence of an operating regime of enhanced peak power but degraded energy is noted and qualitatively explained.