Session UO6: Sources and Diagnostics

Abstract Withdrawn

Ion extraction from a plasma under an externally applied electric field involve multi-particle and multi-field interactions, and has wide applications in the fields of materials processing, etching, chemical analysis, etc. In order to develop the high-efficiency ion extraction methods, it is indispensable to establish a feasible model to understand the non-equilibrium transportation processes of the charged particles and the evolutions of the space charge sheath during the extraction process. Most of the previous studies on the ion extraction process are mainly based on the electron-equilibrium fluid model, which assumed that the electrons are in the thermodynamic equilibrium state. However, it may lead to some confusions with neglecting the electron movement during the sheath formation process. In this study, a non-electron-equilibrium model is established to describe the transportation of the charged particles in a parallel-plate ion extraction process. The numerical results show that the formation of the Child-Langmuir sheath is mainly caused by the charge separation. And thus, the sheath shielding effect will be significantly weakened if the charge separation is suppressed during the extraction process of the charged particles.   

Microwave Assisted Helicon Plasmas

The use of two (or more) rf sources at different frequencies is a common technique in the plasma processing industry to control ion energy characteristics separately from plasma generation. A similar approach is presented here with the focus on modifying the electron population in argon and helium plasmas. The plasma is generated by a helicon source at a frequency f0 $=$ 13.56 MHz. Microwaves of frequency f1 $=$ 2.45 GHz are then injected into the helicon source chamber perpendicular to the background magnetic field. The microwaves damp on the electrons via X-mode Electron Cyclotron Heating (ECH) at the upper hybrid resonance, providing additional energy input into the electrons. The effects of this secondary-source heating on electron density, temperature, and energy distribution function are examined and compared to helicon-only single source plasmas as well as numeric models suggesting that the heating is not evenly distributed. Optical Emission Spectroscopy (OES) is used to examine the impact of the energetic tail of the electron distribution on ion and neutral species via collisional excitation. Large enhancements of neutral spectral lines are observed in both Ar and He. While small enhancement of ion lines is seen in Ar, ion lines not normally present in He are observed during microwave injection.   

Hollow laser plasma self-confined microjet generation.

Hollow laser beam produced plasma (LPP) devices are being used for the generation of the self-confined cumulative microjet. Most important place by this LPP device construction is achieving of an annular distribution of the laser beam intensity by spot. An integrated model is being developed to detailed simulation of the plasma generation and evolution inside the laser beam channel. The model describes in two temperature approximation hydrodynamic processes in plasma, laser absorption processes, heat conduction, and radiation energy transport. The total variation diminishing scheme in the Lax-Friedrich formulation for the description of plasma hydrodynamic is used. Laser absorption and radiation transport models on the base of Monte Carlo method are being developed. Heat conduction part on the implicit scheme with sparse matrixes using is realized. The developed models are being integrated into HEIGHTS-LPP computer simulation package. The integrated modeling of the hollow beam laser plasma generation showed the self-confinement and acceleration of the plasma microjet inside the laser channel. It was found dependence of the microjet parameters including radiation emission on the hole and beam radiuses ratio.   

EUV laser produced and induced plasmas for nanolithography.

EUV produced plasma sources are being extensively studied for the development of new technology for computer chips production. Challenging tasks include optimization of EUV source efficiency, producing powerful source in 2 percentage bandwidth around 13.5 nm for high volume manufacture (HVM), and increasing the lifetime of collecting optics. Mass-limited targets, such as small droplet, allow to reduce contamination of chamber environment and mirror surface damage. However, reducing droplet size limits EUV power output. Our analysis showed the requirement for the target parameters and chamber conditions to achieve 500 W EUV output for HVM. The HEIGHTS package was used for the simulations of laser produced plasma evolution starting from laser interaction with solid target, development and expansion of vapor/plasma plume with accurate optical data calculation, especially in narrow EUV region. Detailed 3D modeling of mix environment including evolution and interplay of plasma produced by lasers from Sn target and plasma produced by in-band and out-of-band EUV radiation in ambient gas, used for the collecting optics protection and cleaning, allowed predicting conditions in entire LPP system. Effect of these conditions on EUV photon absorption and collection was analyzed.   

Characterization and Comparison of Aluminum, Silicon, and Carbon Laser Ablation Plumes

Laser ablation of solid targets produces plasma plumes with rapidly evolving temperature and density gradients. These gradients can be measured using laser interferometric techniques that allow for the study of the plasma as the plume expands from the target surface and the temperature and density decrease. A systematic study of the temperature and density of aluminum, silicon, and carbon plasma plumes produced with a 2 TW/cm$^{2}$ laser using spectroscopic, interferometric, fast imaging, and charge diagnostics will be presented. Carbon, aluminum, and silicon plumes are of interest because they are closely grouped on the periodic table but have very different material characteristics. Temporally and spatially resolved data was collected to characterize the evolution of the plasma in the plume. To probe the plasmas produced from these materials, optical spectroscopy was employed to identify and measure the temperature of the coexisting neutral and ionized atomic and molecular species. A Mach-Zehnder interferometer was employed to measure electron density. ICCD imaging and shadowgraphy were used to image the plume dynamics. A comparison of plasma evolution for each element will also be presented and will provide data to benchmark plasma codes.   

Laser Induced Ablation of Metals at Different Ambient Conditions: Experiments and Simulation

Laser erosion of metals under different ambient conditions and laser fluences, has been studied using 1064 nm, 6 ns Nd:YAG laser on 1 mm thick W and Al. Experiments were designed to study the effect of various parameters (material properties, laser fluence, ambient gas, ambient pressure) on metal ablation. Using two different ambient gases (air and argon), the metals were ablated over wide range of incident laser fluence to study the effect of ambient gas on metal ablation. To quantify the ablation process, the crater profile was measured using White Light Profilometer which provided information regarding the amount of mass ablation crater shape, and melt formation. These measurements were used for comparing the ablation processes for various conditions. The study is supported by analytical models and computer simulation which shows strong agreement with the experimental data. The ablation yield has very consistent dependence on incident laser fluence. At low laser fluence, with respect to ablation threshold, this dependence is logarithmic while, at high laser fluence the ablation yield is linear function of incident laser fluence. Ambient pressure was found to be significant in ablation processes. Detailed mechanisms of these effects will be presented.   

The effect of density scale length on hot electron generation in relativistic laser interaction with under dense plasma

The effect of plasma density scale length on hot electron generation have been investigated in relativistic regime for under dense plasma using $1D$ PIC simulation. In our simulation, three different density scale lengths, step density, gentle ramp and steep ramp density for two short and long pulse lengths with temporal pulse duration $\tau_{L} =60\,fs$ and $\tau _{L} =300\,fs$, respectively have been used. It is found that laser pulse length and density scale length have considerable effects on the energetic electron generation. The results of simulation indicate that for the step density scale length, with respect to the short laser pulse, electrons are accelerated to higher energy level than the case with the long pulse and other scale lengths. Furthermore, time evaluation analysis of the energy distribution function shows that with the time increment of the pulse propagation, plasma electrons can reach energies about two times higher than the energy level of the long pulse case.   

Kinetic effects during the interaction between high density microplasma and electromagnetic wave.

The interaction between a high-density microplasma and high-power electromagnetic wave is studied by one-dimensional Particle-in-Cell Monte Carlo collisions model coupled with the Maxwell's equations. We find the value of the amplitude of the wave field above which a fully ionized plasma is generated on the picosecond time scale. This fully ionized plasma is obtained only in the skin layer while the ionization degree of the plasma bulk is \textasciitilde 20{\%}. The simulation results show that such non-homogeneous distribution of plasma and gas density influences significantly the heating of plasma electrons and time evolution of the electron energy probability function.   

Laser Induced Fluorescence (LIF) Measurements of Neutral (ArI) and singly-ionized (ArII) Argon in a LargeScale Helicon Plasma

In order to investigate the role of neutral dynamics in helicon discharges in the HelCat (Helicon-Cathode) plasma device at U. New Mexico, a Laser Induced Fluorescence (LIF) system has been developed. The LIF system is based on a \textgreater 250 mW, tunable diode laser with a tuning range between 680 and 700nm. For neutral Argon, the laser pumps the metastable ($^{\mathrm{2}}$P$^{\mathrm{0}}_{\mathrm{3/2}})$4s level to the ($^{\mathrm{2}}$P$^{\mathrm{0}}_{\mathrm{1/2}})$4p level using 696. 7352 nm light. The fluorescence radiation from decay to the ($^{\mathrm{2}}$P$^{\mathrm{0}}_{\mathrm{1/2}})$4s level at 772. 6333 nm is observed. For singly ionized Argon, the laser pumps the 3s$^{\mathrm{2}}$3p$^{\mathrm{4}}(^{\mathrm{3}}$P)3d level to the 3s$^{\mathrm{2}}$3p$^{\mathrm{4}}(^{\mathrm{3}}$P)4p level using 686.3162nm light. The fluorescence radiation from the decay to the 3s$^{\mathrm{2}}$3p$^{\mathrm{4}}(^{\mathrm{3}}$P)4s level is observed. The system design, and velocity measurements in the axial, azimuthal and radial directions for ArI, and in the axial direction for ArII will be presented.   

A Plasma Edge Electron Density Diagnostic Based on a Doppler-free Measurement of Stark Broadening

Passive spectroscopic measurements of Stark broadening have been reliably used to determine electron density for decades. A low-density limit of 1e19 m\textasciicircum -3 exists using these passive techniques due to Doppler and instrument broadening. At Oak Ridge National Laboratory, a novel diagnostic approach for measuring electron density using Stark broadening is currently under development and is capable of extending the low-density limit to 1e16 m\textasciicircum -3. The diagnostic is based on measuring the spectral line profile of a Balmer series transition using Doppler-free saturation spectroscopy, a laser-based absorption technique. The spectrum is then fit to a quantum mechanical model using the Explicit Zeeman Stark Spectral Simulator (EZSSS) code to extract the electron density. The increased sensitivity to the electron density is realized because Doppler-free saturation spectroscopy (DFSS) can greatly reduce the Doppler broadening and essentially eliminate the instrument broadening. DFSS has been successfully employed to measure spectral data in a magnetized (500-800 G), low temperature (5 eV), low density (1e17-1e18 m\textasciicircum -3), He/H2 and He/CH4 plasma in the mTorr pressure range. Experimentally measured pi and sigma H-alpha spectra, fit using the EZSSS code, will be presented. A quantitative model to accurately predict crossover peaks and dips will also be given.   

Magnetic Field Measurements In Magnetized Plasmas Using Zeeman Broadening Diagnostics

The Zeeman effect has been used to measure the magnetic field in high energy density plasmas. This method is limited when plasma conditions are such that the line broadening due to the high plasma density and temperature surpasses the Zeeman splitting. We have measured magnetic fields in magnetized laser plasmas under conditions where the Zeeman splitting was not spectrally resolved. The magnetic field strength was determined from the difference in widths of two doublet components, using an idea proposed by Tessarin et al. (2011). Time-gated spectra with one-dimensional space-resolution were obtained at the Nevada Terawatt Facility for laser plasmas created by 20 J, 1 ns Leopard laser pulses, and expanding in the azimuthal magnetic field produced by the 0.6 MA Zebra pulsed power generator. We explore the response of the Al III 4s $^{\mathrm{2}}$S$_{\mathrm{1/2\thinspace }}$-- 4p $^{\mathrm{2}}$P$_{\mathrm{1/2,3/2}}$doublet components to the external magnetic field spatially along the plasma. Radial magnetic field and electron density profiles were measured within the plasma plume. This work was supported by the DOE/OFES grant DE-SC0008829 and DOE/NNSA contract DE-FC52-06NA27616.   

X-Ray Radiography of Laser-Driven Shocks for Inertial Confinement Fusion

Side-on x-ray radiography of shock waves transiting through the planar plastic ablator and cryogenic fuel layer will be used to study shock timing, shock coalescence, shock breakout, and hydrodynamic mixing at the ablator--fuel interface. The injection of ablator material into the fuel can potentially compromise implosion target performance. The difference in refractive indices of the ablator and the fuel can be exploited to image shocks transiting the interface. An experiment to probe the ablator--fuel interface and a postprocessor to the hydrodynamic code \textit{DRACO} that uses refraction enhanced imaging to view shocks are presented. The advantages of this technique to view shocks are explored and additional applications such as viewing the spatial location of multiple shocks, or the evolution of nonuniformity on shock fronts are discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.