Chamber Background Pressure Research Articles

Background pressure effects on MeV protons accelerated via relativistically intense laser-plasma interactions

We present how chamber background pressure affects energetic proton acceleration from an ultra-intense laser incident on a thin liquid target. A high-repetition-rate (100 Hz), 3.5 mJ laser with peak intensity of 8 times 10^{18},text {Wcm}^{-2} impinged on a 450 nm sheet of flowing liquid ethylene glycol. For these parameters, we experimentally demonstrate a threshold in laser-to-proton conversion efficiency at background pressures < 8,text {Torr}, wherein the overall energy in ions >1,text {MeV} increases by an order of magnitude. Proton acceleration becomes increasingly efficient at lower background pressures and laser-to-proton conversion efficiency approaches a constant as the vacuum pressure decreases. We present two-dimensional particle-in-cell simulations and a charge neutralization model to support our experimental findings. Our experiment demonstrates that high vacuum is not required for energetic ion acceleration, which relaxes target debris requirements and facilitates applications of high-repetition rate laser-based proton accelerators.

Numerical studies on plasma plume flows from a cluster of electric propulsion devices

Several hybrid fluid–particle simulations are performed to investigate the plasma plume flows from a cluster of Hall thrusters. These simulations adopt a detailed fluid electron model to describe electron properties. A general finite element solver is developed to solve for these electron properties, and the cathode geometry, facility effects, and vacuum chamber background pressure are included in the simulations; these are compared with available experimental data, and indicate that the cathode had effects on some flow property distributions. They also clearly display charge exchange collision effects and the clustering effects. There exist complex three-dimensional near field structures in the ion number density with Hall thrusters in operation. With four thrusters in operation, a severe impingement is expected right behind the cluster center. Compared with axisymmetric simulations, these three-dimensional simulations have several improvements. When compared with the Boltzmann relation, the detailed fluid electron model displays superiority in predicting the electron temperature as well.

Dry etching of new phase-change material Al1.3Sb3Te in CF4/Ar plasma

The dry etching characteristic of Al1.3Sb3Te film was investigated by using a CF4/Ar gas mixture. The experimental control parameters were gas flow rate into the chamber, CF4/Ar ratio, the O2 addition, the chamber background pressure, and the incident RF power applied to the lower electrode. The total flow rate was 50 sccm and the behavior of etch rate of Al1.3Sb3Te thin films was investigated as a function of the CF4/Ar ratio, the O2 addition, the chamber background pressure, and the incident RF power. Then the parameters were optimized. The fast etch rate was up to 70.8 nm/min and a smooth surface was achieved using optimized etching parameters of CF4 concentration of 4%, power of 300 W and pressure of 80 mTorr.

Reactive ion etching of GaSb and GaAlSb using SiCl4

Reactive ion etching of GaSb and GaAlSb using pure SiCl4 was investigated. Etching rate and etching profiles were characterized as functions of working pressure, chamber background pressure, flow rate, and power density. The etching rate and profile of metal organic chemical vapor deposited-grown GaSb and GaAlSb thin films are strongly dependent on background pressure and applied power. These interesting characteristics can be applied to control selective or nonselective etchings for device fabrication. Etching profiles exhibited a high degree of anisotropy and smooth surface morphologies.

Development of a long life, 2 kHz repetition rate X-ray preionizer

A simple and rugged X-ray preionizer system has been developed that is capable of sustained operation at the 2 kHz level with a demonstrated lifetime of2.5× 108 shots and a potential lifetime of >109 shots. At the heart of the system is an advanced form of corona plasma cathode. Cathode wear is minimised by utilising adsorbates from the dielectric surface and not cathode material to form the surface discharge from which the electron beam is extracted. The preionizer chamber background pressure (which is directly proportional to the sticking rate of adsorbates) is found to be of crucial importance in maintaining good cathode emission at high repetition rates. The device emits a rectangular X-ray beam, 5 cm×40 cm, with an X-ray risetime of 20 ns, a FWHM of 60 ns, and a peak energy of 95 keV. Ionization levels of >107 cm−3 in 1 atm of Ne could be generated after the X-rays had passed through 2 mm of aluminium and 1 mm of steel. Good beam uniformity and reproducibility at 2 kHz were shown to be maintained with the aid of a segmented electrode charge collection chamber. The system should be especially suitable for preionizing lasers containing a strongly electron attaching gas mixture.

A simple, controllable source for dosing molecular halogens in UHV

An electrochemical source for the reproducible generation and dosing of halogens in UHV systems is described. This approach minimizes corrosion damage to vacuum hardware and lends itself to the study of both submonolayer and multilayer halogen adsorption. The corrosive action of halogens on ultrahigh vacuum components has necessitated the development of molecular halogen beam sources, which allow in situ generation and dosing of fluorine, chlorine, bromine, and iodine, with a negligible increase in vacuum chamber background pressure. This communication describes the chlorine and bromine sources developed and routinely used in our laboratory. Iodine dosing may be accomplished using a source of similar design. Fluorine, however, because of its highly reactive nature, requires a somewhat different design of source, such as that recently described in detail by Bechtold.1