Low Background Gas Pressure Research Articles

Improvement in ion confinement time with multigrid configuration in an inertial electrostatic confinement fusion device.

Improvement in the functionality of an inertial electrostatic confinement fusion (IECF) device has been investigated through kinetic simulation. Previously, we achieved a neutron generation rate of 10^{6} neutrons per second, but higher rates and better plasma confinement are necessary for broader applications. We compared a traditional single-grid IECF device with a triple-grid variant to evaluate the benefits of using multiple grids for ion confinement. Our computational models, using the 2D-3V xoopic code, suggest that the triple-grid device, with its optimized potentials, could significantly enhance ion confinement. The models show that the triple-grid design directs ion beams more effectively to the center, in contrast with the more scattered ion distribution in the single-grid design. This results in longer ion lifetimes in the triple-grid system due to its modified electrostatic fields. In the standard single-grid IECF device, the primary reasons for ion loss are chaotic ion trajectories and interactions with residual gases. By operating the triple-grid device under very low background gas pressure and with a focused field structure, we expect to achieve improved ion confinement.

Dynamics of planar gas expansion during nanosecond laser evaporation into a low-pressure background gas

A numerical study in a one-dimensional planar formulation of the dynamics of the neutral gas expansion during nanosecond laser evaporation into a low-pressure background gas is carried out using two different kinetic approaches: the direct simulation Monte Carlo method and direct numerical solution of the Bhatnagar–Gross–Krook equation. Results were obtained for a wide range of parameters: the background gas pressure, masses of evaporated and background particles, temperature and pressure of saturated vapor on the evaporation surface, and evaporation duration. They are in good agreement with the analytical continuum solution for unsteady evaporation into the background gas. The dynamics of the expansion is analyzed, and the characteristic times and distances that determine the main stages of the expansion process are established. General regularities are obtained that describe the dynamics of the motion of external and internal shock waves and the contact surface as well as the maximum density of evaporated particles and the characteristic temperatures of evaporated and background particles in the compressed layer. The obtained results are important for understanding and describing the change in the mixing layer during nanosecond laser deposition in a low-pressure background gas.

Growth of Al-doped ZnO nanostructures in low pressure background gas by pulsed laser deposition

In this work, the deposition of nanostructures by room temperature pulsed laser deposition (PLD) in low pressure (2.6 Pa) O2, N2, He, and Ar at different substrate positions are studied. The substrates are placed at the normal on-axis/center position; and off-axis i.e. 4.5 cm above and below the center position. Optical emission spectra and ion velocity are measured by using spectrometer and ion probe. The overall intensity of optical emission is in the order of Ar > O2/N2 > He while the ion velocity is in the reverse order. The inverse relation of optical emission and ion velocity suggest that collisions of the ablated specie possibly result in strong emission and impede the ion velocity at the same time. Amorphous films are obtained for the substrates placed at on-axis/center in all the background gases and at the bottom position for O2. However, crystalline ZnO and Zn nanostructures are deposited at the bottom position in N2, He, and Ar. Larger structures are deposited in Ar and the presence of nanorods are detected by TEM. The results further suggest that in N2, He or Ar, the ablated plasma plume is spatially and thermally confined; governed by the properties of the gas. Nucleation of nanostructures occurred in the plasma plume even at low background pressure and deposited on the substrates that are placed at the bottom position. Between the two inert gases, Ar with higher atomic mass and lower thermal conductivity results in high collisions and thermal confinement that leads to some large structure as compared to those obtained in He. In addition, in the presence of N2, He or Ar, Zn crystallites are also detected, which can act as catalyst for nanostructure formation.

Formation of Al-Doped ZnO Nanostructures in Low Pressure Background Gas by Pulsed Laser Deposition

Formation of Al-Doped ZnO Nanostructures in Low Pressure Background Gas by Pulsed Laser Deposition

Transition from ambipolar to free diffusion in an EUV-induced argon plasma

Extreme Ultraviolet (EUV) optical components used in EUV lithography tools are continuously impacted by an exotic and highly transient type of plasma: EUV-induced plasma. Such an EUV-induced plasma is generated in a repetitive fashion upon sending a pulsed beam of high energy (92 eV) photons through a low-pressure background gas. Although its formation occurs on a time scale of ∼100 ns, it is the plasma's decay dynamics on longer time scales that dictates the fluxes and energy distribution of the produced ions. Therefore, the plasma decay also determines the overall impact on plasma-facing EUV optical components. Enabled by electron density measurements using Microwave Cavity Resonance Spectroscopy at a much higher sensitivity, we clearly show the breakdown of the ambipolar field in an EUV photon-induced plasma below electron densities of ∼2 × 1012 m−3 and the—until now—unidentified transition from ambipolar diffusion-driven decay into a decay regime driven by free diffusion. These results not only further improve the understanding of elementary processes in this type of plasma but also have a significant value for modeling and predicting the stability and lifetime of optical components in EUV lithography.

Energy distribution functions for ions from pulsed EUV-induced plasmas in low pressure N2-diluted H2 gas

The operation of Extreme Ultraviolet (EUV) lithography scanners inherently goes hand-in-hand with the creation of highly transient pulsed plasmas in the optical path of these tools. These so-called EUV-induced plasmas are created upon photoionization events when a pulsed beam of EUV photons travels through the low pressure background gas. It is fully recognized by the lithography industry that EUV-induced plasmas may significantly impact the quality and life-time of expensive and delicate optical elements in the scanner. Research efforts into EUV-induced plasmas impacting plasma-facing surfaces have so far been limited to pure hydrogen (H2) plasmas. However, this hydrogen background gas may occasionally be diluted with a small fraction of another molecular gas such as nitrogen (N2). The impact on the relevant plasma parameters of such molecular contaminants has remained unknown until now. In this letter, we put forward measurements of energy-resolved fluxes of (positive) hydrogen ions, nitrogen ions, and hydrogen-nitrogen ions created in a pulsed N2-diluted EUV-induced plasma in H2 at approximately 5 Pa (typical EUV lithography scanner conditions). The data have been obtained using an electrostatic quadrupole plasma analyzer and show that although the N2-dilution fraction is small (∼2 × 10−3) compared to the H2 partial pressure, implications for the ion flux out of the plasma and the composition thereof are significant. Since the mass of nitrogen-containing ions is much higher in comparison to that of their hydrogen counterparts, and because of their potential chemical activity, this effect has to be taken into account while studying the surface impact of EUV-induced plasmas.

Cryogenic trapped-ion system for large scale quantum simulation

Trapped-ion systems are among the most promising hardware candidates for large scale quantum computing and quantum simulation. In order to scale up such devices, it is necessary to engineer extreme-high vacuum (XHV) environments to prevent background gas from disrupting the ion crystal. Here we present a new cryogenic ion trapping system designed for long time storage of large ion chains. Our apparatus is based on a segmented-blade ion trap enclosed in a 4 K cryostat, which enables us to routinely trap and hold over 100 171Yb+ ions for hours in a linear configuration, due to low background gas pressure from differential cryo-pumping. We characterize the XHV cryogenic environment measuring pressures below by recording both inelastic and elastic collisions between the ion chain and the molecular background gas. We also demonstrate coherent one and two-qubit operations and nearly equidistant ion spacing for chains of up to 44 ions using anharmonic axial potentials, in order to enable better detection and single ion addressing in large ion arrays. We anticipate that this reliable production and lifetime enhancement of large linear ion chains will enable quantum simulation of models that are intractable with classical computer modeling.

Analysis and characterization of microwave plasma generated with rectangular all-dielectric resonators

We report on a computational modeling study of small scale plasma discharge formation with rectangular dielectric resonators (DR). An array of rectangular dielectric slabs, separated by a gap of millimeter dimensions is used to provide resonant response when illuminated by an incident wave of 1.26 GHz. A coupled electromagnetic (EM) wave–plasma model is used to describe the breakdown, early response and steady state of the argon discharge. We characterize the plasma generation with respect to the input power, background gas pressure and gap size. It is found that the plasma discharge is generated mainly inside the gaps between the DR at positions that correspond to the antinodes of the resonant enhanced electric field pattern. The enhancement of the electric field inside the gaps is due to a combination of leaking and displacement current radiation from the DR. The plasma is sustained in over-critical densities due to the large skin depth with respect to the gap and plasma size. Electron densities are calculated in the order of for a gas pressure of 10 Torr, while they exceed 1020 in atmospheric conditions. Increase of input power leads to more intense ionization and thus faster plasma formation and results to a more symmetric plasma pattern. For low background gas pressure the discharge is diffusive and extends away from the gap region while in high pressure it is constricted inside the gap. An optimal gap size can be found to provide maximum EM energy transfer to the plasma. This fact demonstrates that the gap size dictates to a certain extent the resonant frequency and the Q-factor of the dielectric array and the breakdown fields can not be determined in a straight-forward way but they are functions of the resonators geometry and incident field frequency.

Simulations of multipactor breakdown in low-pressure background gas for angled dielectrics in DC

We report on 2D particle-in-cell (PIC) simulations of electrical breakdown incorporating single-surface multipactor with background-gas ionization. The system for study is a semi-infinite, two-electrode Bergeron geometry with an angled dielectric in steady-state fields, with a background gas at variable low pressure. For pressures below a few tens of Pa (a few hundred mTorr), discharges are dominated by multipactor breakdown initiated by a triple-point source; the breakdown condition is well-characterized by an average secondary-emission coefficient greater than unity at the onset of anodic current, otherwise a dark-current develops due to space-charge saturation. The magnitude of the anode current is driven by the development of a multipactor front comprising an enhanced current front followed by a steady-state tail in which dielectric-surface fields no longer support net secondary-emission, resulting in a non-multiplicative arc. Additional avalanche phenomena occur with increased pressure >133.3 Pa (>1 Torr), where space-charge coupling leads to oscillations in an otherwise DC discharge, largely driven by the development and quenching of the multipactor front.

Effects of collision between two plumes on plume expansion dynamics during pulsed laser ablation in background gas

Si and Ge targets were simultaneously irradiated by individual two pulsed lasers, and two plumes from the targets were collided head-on with expectation to prepare hybrid nanoparticles. We investigate effects of He background gas pressure on plume collision dynamics. Three characteristic behaviors of plume expansion dynamics are observed at low, middle, and high background gas pressure regions. Interaction between the two atomic species during plume expansion was small and the effect of collision was hardly observed at a low background gas pressure, 130 Pa, while spatial evolution of the plume was suppressed at middle pressure, 270 Pa, due to collision of the two plumes. At high pressure, 2700 Pa, plume expansion is suppressed by background gas and the effect of a direct collision of two plumes was small. These results indicate that plume collision dynamics, which governs nanoparticle formation, and the mixture of Si and Ge species can be varied by background gas pressure. The deposit near the center of two targets was nanoparticles that were composed of Si and Ge.

Characterization of kinetic energy distributions of ions in high laser irradiance via orthogonal time-of-flight mass spectrometry

The characteristics of laser ionization in low pressure background gas have been investigated through the measurement of temporal and kinetic energy distributions of Al+, Mn+, Nb+, In+, Ta+, and Bi+ produced from a disk comprised of these metallic elements. A Q-switched Nd:YAG laser was utilized with a laser beam at 532nm of wavelength and 1.0×1010W/cm2 of laser irradiance. The kinetic energy was found to be the same for all ablated species at the given pressure, regardless of the atomic mass. The plume propagation translates from a free expansion at 0.5Pa to a collisional and shockwave-like hydrodynamic expansion at 50Pa. A plume splitting exists at 500–1500Pa where only the fast component can be observed with a grounded nozzle voltage. As the nozzle voltage grows up, the thermalized component with increased kinetic energy is found depending on the nozzle voltage.

Kinetic energy and spatial distribution of ions in high irradiance laser ionization source

Characteristics of laser ionization in vacuum and low pressure background gas (He) have been investigated through the measurement of kinetic energy and spatial distributions of copper and tungsten ions. A Q-switched Nd:YAG laser with 532 nm wavelength was utilized and the laser irradiance was fixed at 9 × 109 W cm−2. A plume splitting was observed in the low pressure regime investigated (from 100 Pa to 2000 Pa). The plume propagation translates from a free expansion in vacuum to shockwave-like expansion at relative low pressure and finally diffusion into background gas at relative high pressure environment. A measurement of ion spatial distribution in the ion source has also been carried out for characterizing ions at different pressures and the behaviors of doubly charged, singly charged, and polyatomic ions to reveal the effect of plume–background gas interaction.

Kinetic simulation of an extreme ultraviolet radiation driven plasma near a multilayer mirror

Future generation lithography tools will use extreme ultraviolet radiation to enable the printing of sub-50 nanometer features on silicon wafers. The extreme ultraviolet radiation, coming from a pulsed discharge, photoionizes the low pressure background gas in the tool. A weakly ionized plasma is formed, which will be in contact with the optical components of the lithography device. In the plasma sheath region ions will be accelerated towards the surfaces of multilayer mirrors. A self-consistent kinetic particle-in-cell model has been applied to describe a radiation driven plasma. The simulations predict the plasma parameters and notably the energy at which ions impact on the plasma boundaries. We have studied the influence of photoelectron emission from the mirror on the sheath dynamics and on the ion impact energy. Furthermore, the ion impact energy distribution has been convoluted with the formula of Yamamura and Tawara [At. Data Nucl. Data Tables 62, 149 (1996)] for the sputter yield to obtain the rate of physical sputtering. The model predicts that the sputter rate is dominated by the presence of doubly ionized argon ions.

Precision measurement of the metastable3P2lifetime of neon

The lifetime of the metastable 3P2 state of neon has been determined to 14.70(13) s (decay rate 0.06801(62) 1/s) by measuring the decay in fluorescence of an ensemble of Ne-20 atoms trapped in a magneto-optical trap (MOT). Due to low background gas pressure p < 5 10^-11 mbar and low relative excitation to the 3D3 state (0.5% excitation probability) operation only small corrections have to be included in the lifetime extrapolation. Together with a careful analysis of residual loss mechanisms in the MOT a lifetime determination to high precision is achieved.

Effects of preneutralization on heavy ion fusion chamber transport

Beams for heavy ion fusion are likely to require at least partial neutralization in the reactor chamber. Present target designs call for higher beam currents and smaller focal spots than most earlier designs, leading to high space-charge fields. Focusing is complicated by beam stripping in the low-pressure background gas expected in chambers. One method proposed for neutralization is passing an ion beam through a plasma before the beam enters the chamber. In this article, the electromagnetic particle-in-cell code LSP is used to study the effectiveness of this form of preneutralization for a range of plasma and beam parameters. For target chamber pressures below a few milliTorr of flibe gas, preneutralization is found to significantly reduce the beam emittance growth and spot size in the chamber.

Infrared photophysics in an ion trap

A Monte Carlo model has been developed which provides a very detailed picture of conditions during multiphoton infrared fragmentation experiments, as performed in an ion trap. Typically, two types of ion traps are used, an ion cyclotron resonance (ICR) instrument, and a quadrupole ion trap. Experiments fall into three separate categories: Low background gas pressure combined with either high or low intensity laser radiation, and moderate background pressures with low intensity laser radiation. Each set of experimental conditions brings to the simulation a dependency on a particular set of variables, and these can be refined to give a self-consistent picture of the complete photofragmentation process. At the low gas pressures (∼10−7 mbar) found in ICR traps, the simulation of experiments run at low laser intensities shows that radiative decay has an important influence on photofragment yield. In the same type of trap, but at high laser intensities, pulse shape and stimulated emission become important. Finally, at pressures found in a typical quadrupole ion trap (∼10−4 mbar), collisions with the helium background have a significant effect on the outcome of infrared excitation; however, the time scale of an experiment is such that radiative decay can also influence the results. The model has been applied to the infrared photofragmentation of the protonated diethyl ether dimer, [(C2H5)2O]2H+, where it successfully accounts for experimental results recorded under each of the three conditions identified above. Under circumstances where photofragmentation is in competition with either radiative or collisional relaxation, the calculations show that fragmentation requires the absorption of up to 20 photons (assumed to come from a CO2 laser, hν≈0.11 eV), as opposed to the 12 photons necessary to match the critical energy of reaction.

Interaction of an expanding gas cloud with a perforated screen

AbstractThe expansion of a high-pressure gas cloud in a vacuum chamber in the presence of a low-pressure background gas is considered. The interaction of the expanding cloud with the background gas and with a perforated screen placed in the path of the flow is modeled numerically. The influence of the permeability of the screen on the overall picture of the interaction and on the parameters of the resulting flow is studied.

Pulsed organometallic beam epitaxy of complex oxide films

We report the results of a pulsed organometallic beam epitaxy (POMBE) process for growing complex oxide films at low background gas pressure (10−4–10−2 Torr) and low substrate temperature (600–680 °C) using organometallic precursors in an oxygen plasma environment. Our results show that POMBE can extend the capability of organometallic chemical vapor deposition to growing complex oxide films with high precision both in composition and structure without the need for post-deposition oxidation and heat treatments. The growth of phase-pure, highly oriented Y-Ba-Cu-O superconducting oxide films {[Tc (R=0)=90.5 K] and Jc (77 K, 50 K gauss)=1.1×105 A/cm2} is given as an example. Similar to the pulsed laser deposition process, the POMBE method has the potential for in situ processing of multilayer structures (e.g., junctions).

Plasma Enhanced OMCVD Atomic Layered Growth of Y-BA-CU Oxide Films

ABSTRACTWe report the results of a pulsed organo-metallic beam epitaxy (POMBE) process for growing complex oxide films at low background gas pressure (10-4 -10-2 torr) and low substrate temperature (600 to 700 C) using organo-metallic precursors in an oxygen plasma environment. Our results show that POMBE can extend the capability of organo-metallic chemical vapor deposition to growing complex oxide films with high precision both in composition and structure without the need for post-deposition oxidation and heat treatments. The growth of phase-pure, highly oriented Y-Ba-Cu-O superconducting oxide films ([Tc (R=0)=90.5K] and Jc (77K, 50K gauss)=l.l×105 A/cm2) is given as an example. Similar to the pulsed laser deposition process, the POMBE method has the potential for in-situ processing of multilayer structures (e.g. junctions).

Gradient B drift transport of high current electron beams

A 1-MeV, 200-kA electron beam was transported 89 cm in a low pressure background gas via gradient B drift in the 1/r azimuthal magnetic field of a current carrying wire. The electron drift velocity was measured and found to be in good agreement with theory. Measurements of x-ray production in the target indicated high transport efficiency.