Mark Denning

Properties of microplasmas excited by microwaves for VUV photon sources

Microplasma sources typically take advantage of pd (pressure × size) scaling by increasing pressure to operate at dimensions as small as tens of microns. In many applications, low pressure operation is desirable, which makes miniaturization difficult. In this paper, the characteristics of low pressure microplasma sources excited by microwave power are discussed based on results from experimental and computational studies. The intended application is production of VUV radiation for chemical analysis, and so emphasis in this study is on the production of resonant excited states of rare gases and radiation transport. The systems of interest operate at a few to 10 Torr in Ar and He/Ar mixtures with cavity dimensions of hundreds of microns to 1 mm. Power deposition is a few watts which produces fractional ionization of about 0.1%. We found that production of VUV radiation from argon microplasmas at 104.8 nm and 106.7 nm saturates as a function of power deposition due to a quasi-equilibrium that is established between the electron temperature (that is not terribly sensitive to power deposition) and the population of the Ar(4s) manifold.

Ion Pump Design for Improved Pumping Speed at Low Pressure

Even if ion pumps are widely and mostly used in ultra-high vacuum (UHV) conditions, virtually every existing ion pump has its maximum pumping speed around 1E-6 mbar (1E-4 Pa). Discharge intensity in the ion pump Penning cell is defined as the current divided by pressure . (I/P)This quantity reflects the rate of cathode bombardment by ions, which underlies all of the various pumping mechanisms that occur in ion pumps (chemisorption on sputtered material, ion burial, etc.), and therefore is an indication of pumping speed. A study has been performed to evaluate the influence of magnetic fields and cell dimensions on the ion pump discharge intensity and consequently on the pumping speed at different pressures. As a result, a combination of parameters has been developed in order to design and build an ion pump with the pumping speed peak shifted towards lower pressures. Experimental results with several different test set-ups are presented and a prototype of a new 200 l/s ion pump with the maximum pumping speed in the 1E-8 mbar (1E-6 Pa) is described. A model of the system has also been developed to provide a framework for understanding the experimental observations.

UV emission and probe diagnostics and computational modeling of a low pressure microwave excited microplasma source

Summary form only given. Low-pressure microwave-excited microplasmas have a wide variety of potential applications, including their use as UV photoionization sources for mass spectrometry. Resonant microwave microstrip architectures can be used to initiate and sustain these microplasmas. When operated in a windowless configuration, the plasma plume exiting from an aperture in the plasma confinement structure contains a complex mixture of particle species (UV photons, groundstate neutrals, metastables, and plasma electrons and ions)1. Understanding the makeup of this plume is critical to optimize parameters for photoionization, or any other application where exposure to the plasma plume takes place. The microplasma source under investigation consists of a resonant microstrip pattern on alumina substrate, with an elongated 1.5 mm x 6.5 mm plasma confinement structure. 2.5 GHz microwave power is delivered at up to 5 W. Argon and helium/krypton mixtures are flowed through the confinement region at flow rates up to 10 sccm producing confinement region pressures near 1 Torr, with the plasma plume exiting into high vacuum through a 300μm-by-600μm aperture. The effect of confinement region and microstrip geometries, net absorbed microwave power, and source region pressures on the properties of the plasma in the source region and plume are investigated. Ultraviolet emission and Langmuir probe diagnostics are used to diagnose the plasma. The spatial distribution of the plasma density in the plume is measured for a range of microwave powers and flow rates. We also present computational modeling results of this microplasma using the Hybrid Plasma Equipment Model (HPEM)2 with comparisons to the experiments. The angular distribution of UV flux is observed, revealing a highly directional radiation pattern owing to an elongated source region. This flux magnitude and directionality is additionally a function of source region pressure, due to variation in the spatial distribution of the plasma and resonant UV photon absorption and quenching.

Plasma dynamics of microwave excited microplasmas in a sub-millimeter cavity

Summary form only given. Capacitively coupled microplasmas in dielectric cavities have a range of applications from VUV lighting sources for surface treatment to radical production. Due to the large surface-to-volume ratio of these devices, the wall mediated dynamics of plasma transport are important to the uniformity and confinement of the plasma. For example, there may be applications where a plume of ionized gas is desired from the microcavity - whereas other applications may require a confined plasma emitting only VUV photons.In this paper, we will discuss results from a computational investigation of the plasma dynamics in microwave excited micro plasma VUV lighting sources. A 2dimensional hydrodynamics model, the Hybrid Plasma Equipment Model, has been used in which radiation and electron energy transport are addressed using Monte Carlo techniques. The microdischarges have widths of:: 1 mm and lengths of :: 1 cm, operate at pressures of 1-20 Torr, with microwave power of 2-10s Watt at 2.5 GHz and a flow rate of several sccm. Gases are either pure rare gases or mixtures of rare gases. We found that the plasma operates in a mode that has both normal-glow and abnormal glow characteristics. Under usual operation in argon, plasmas are produced with a peak electron density of 1013 cm-3. The plasma may not fill the microdischarge cavity at low power. As the power is increased, the plasma expands to fill the cavity. In this regard, the plasma operates as a normal glow. The current density, however, increases with increasing power, and so in this regard, the plasma resembles an abnormal glow. The expansion of the plasma will eventually overfill the cavity, at which time a plasma plume is formed. These plasma dynamics are sensitive to gas mixture. The scaling of plasma confinement and VUV production as a function of aspect ratio, power and gas mixture will be discussed.