Session PO7: DPP/GEC Joint Session: Low Temperature Plasmas II

The Bohm Criterion in Collisional Plasmas -- A Comparison of Different Approaches

The plasma sheath transition in stationary low temperature plasmas is investigated for arbitrary levels of collisionality. The model under study contains the equations of continuity and motion for a single ion species, Boltzmann's equilibrium for the electrons and Poisson's equation for the field. Within this simple but self-consistent model, the arguments of various authors with respect to the existence or non-existence of a collisional modified Bohm criterion are compared. Godyak (1982 Phys. Lett. A 89 80), Valentini (1996 Phys. Plasmas 3 1459), Chen (1997 Phys. Plasmas 5 804) and Brinkmann (2011, J. Phys. D: Appl. Phys. 44, 042002) argued in favor of such a concept, Riemann (1991 J. Phys. D: Appl. Phys. 24 493) and Franklin (2003 J. Phys. D: Appl. Phys. 36 2821) disputed it.   

A New Inductively Driven Plasma Generator (IPG) Source

As part of the partnership between the Center for Astrophysics, Space Physics and Engineering Research (CASPER) at Baylor University and the Institut fur Raumfahrtsysteme (IRS) at the University of Stuttgart, a new design for a modular, inductively driven plasma generator (IPG) source is being developed and tested within CASPER and the IRS. The current IPG design is built on a well-established heritage of modular inductively driven plasma generators designed and operated at IRS. This latest IPG source enables the electrodeless generation of high-enthalpy plasmas and will provide CASPER researchers the ability to operate with various gases at plasma powers of approximately 20~KW. It will also provide minimized field losses and operation over a wide scope of parameters not possible using existing designs requiring flow-controlled stabilization. Both research and technical applications will be discussed.   

Optimizing Persistent Negative Hydrogen Beams from High-Power RF Plasma Sources

High-power RF ion sources produce intense beams of negative H- ions for high-power accelerators using charge-changing injection schemes. The inductively induced RF plasma produces copious amounts of positive ions, electrons, and excited molecules. Energetic electrons rapidly destroy H- ions with their 0.75 eV electron affinity. A $\sim$250 G filter field reflects the energetic electron while the cold electrons, the ions, and the molecules can drift towards the outlet. There slow electrons colliding with highly excited vibrating molecules form H- ions that can be extracted. However, production yields suggest that most negative ions are formed on a conical Mo converter surface, which surrounds the outlet. This appears to be especially true when the surface is covered with a fractional layer of Cs. The persistence of the extracted H- beam suggests that the Cs layer is persistent, likely due to low levels of impurities and hydrogen being to light to sputter Cs atoms from the metallically clean surface. Experimental evidence, data, and simple models will be presented to support our findings.   

A new approach to confine the high-density Nonneutral plasma

In this paper a new method to confine the high-density pure positron plasma is proposed. The limit of the plasma confinement has been discussed and a modified Malmberg-Penning trap with high aspect ratio is presented. A feasibility study of these traps simulated by two different programs: WARP code and the Charge Particle Optics program (CPO). With different configuration of the initial conditions in the simulation, the results have indicated that with the decreasing trap radius, the density of the plasma can increase accordingly, which agree with the analytical prediction.   

Infrared optical actinometry for determining the densities of N- and H-atoms in N2-H2 microwave discharge

N2+H2 plasmas are commonly used to introduce amino groups on the surface of polymers. Optical emission spectroscopy techniques, combined with the use of surface characterization methods may be applied to better control the reactions in the discharge and optimize surface modifications. We have already demonstrated that optical emission spectroscopy in the infrared region can be used to provide additional useful information as compared to the UV-visible region. In connection with this idea, optical IR actinometry may be used to determine N and H atomic density with clear advantages as compared to classical UV-Visible actinometry. In the present study, N2+H2 microwave plasmas with different H2 proportion have been characterized using infrared actinometry. The results allowed monitoring the trend for H and N relative densities as a function of H2/N2 ratio and plasma pressure in the discharge.   

Measurements of resonance broadening constants of mercury lines in ceramic high intensity discharge lamps

Resonance broadening of provides a convenient way to measure mercury density in the arc core of high intensity discharge (HID) lamps. 491.6nm, 577nm, and 1014nm resonance Hg lines are optically thin in the majority of conventional HID discharges. Temperature-dependent broadening of these lines due to Stark, van der Waals, and Doppler effects is weaker than resonance broadening. Several studies showed that temporal and radial averaging have only limited effect on the line profiles. Therefore, resonance broadening of Hg lines may be used for core density measurements in HID discharges with non-transparent ceramic envelopes. Preliminary experiments showed, however, that broadening constants should be validated. Side-on measurements of the broadening constants for the 577nm and 1014nm Hg lines were performed for ceramic HID discharges in the power range from 140 to 600W. The obtained constants show certain deviation from the calculated values. Possible reasons of deviation are discussed. For diagnostic purposes, 1014nm line was found more convenient due to higher sensitivity to pressure variations and less overlapping with emission lines of other elements in the discharge.   

Sequentially emission line addressing by microwave driven mercury free low pressure lamps

As the use of mercury vapor lamps for lighting purposes will be banned in the European Union after 2015, finding a replacement for mercury in fluorescent lamps has become a challenge. Several low pressure gas discharge systems containing metal halides have been reported in the last decade. Examples are halides of indium and thallium with argon as auxiliary gas, which generate ultraviolet and visible emission lines. The peak emission intensities are adjustable by variation of plasma parameters, which allows addressing the color temperature of the lamp. In this contribution, we report on the effects of auxiliary gas pressure, cold spot temperatures and power densities for low pressure metal halide lamps filled with indium and thallium with regard to its spectral output. Since the guided surface wave discharge is the only method to increase the lamps power without changing the amplitude of the maintenance electrical field; the lamp discharges are sustained by microwave excited guided surface waves. A surfatron is used as coupling device of microwave energy.   

Surface modification of tin oxide by VUV rays and charge particle treatment: An effective method to improve the efficiency of surface catalytic behavior

Plasma processing is a promising method to modify the chemical and physical properties of the semiconductor oxide surfaces. However, the tuning of surface characteristics is also influenced by plasma-emitted VUV and UV radiations. Different combinations of argon and oxygen partial pressures were applied in the capacitively coupled plasma. The highest surface conductivity was achieved with increasing plasma power, which was attributed to the interstitial defects with increased Tamm states created by the following two processes. First, the charge particle bombardment on the oxide surface, which etches the surface atoms and second, the absorption of VUV and UV radiation in the exposed layers of tin oxide nanoflakes scission the covalent bonds connecting the Sn-O atoms. The catalytic behavior of the tin oxide nanoflakes towards reducing gases have been studied as the function of Ar-O partial pressures, plasma power and voltage, and the ambient sensing temperature. Modified surface characteristics were also supported by SEM, TEM and XPS analysis.   

A comparison study of improvement of binding strength of polypyrrole (PPy) coating on polyester using Argon, Oxygen and Nitrogen plasma treatment

In this work, we have studied the performance of different plasma gases (Ar, N$_{2}$, and O$_{2})$ and the factors responsible for the improvement of binding of PPy with both polyester thin film and fabric. The plasma was generated by a radio frequency (RF) generator. The gas pressure (8$\times $10$^ {-2}$mbar) and the RF power (100 W) were kept the same, while treatment time varied between 60 and 180 sec. Treated samples were subsequently coated with PPy. The oxygen plasma treated samples showed much more pronounced changes in the surface topography compared to nitrogen and argon plasma treated samples. The contact angle decreased from 84\r{ } for the untreated sample to 55\r{ } for Argon, 42\r{ } for nitrogen and 35\r{ } for oxygen plasma-treated samples after 120 sec treatment. Abrasion resistance and conductivity measurements suggest effectiveness of different plasma gases in the following order: O$_{2}>$Ar$>$N$_{2}$. XPS results show a decrease in C-C (284.6 eV) and an increase in C-O (286.4 eV) and O-C=O (288.7 eV) percentages for each plasma gas, while oxygen to carbon ratios for oxygen, argon and nitrogen plasma are 0.56, 0.5 and 0.46 respectively. It is concluded that improvement of binding of PPy is both due to increased surface roughness and incorporation of oxygen containing functional groups.   

Inactivation of \textit{Escherichia coli} ATCC 11775 in fresh produce using atmospheric pressure cold plasma

Food-borne outbreaks are associated with the presence of pathogenic bacteria in food products such as fresh produce. One of the target microorganisms is \textit{Escherichia coli} which exhibits resistance to being inactivated with conventional disinfection methods for vegetables. Atmospheric pressure cold plasma (APCP) was tested to disinfect three vegetables with challenge surfaces, lettuce, carrots and tomatoes. The produce was inoculated with the bacteria to reach an initial microbial concentration of 10$^{7}$ cfu/g. Vegetables were initially exposed to the APCP discharges from a needle array at 5.7 kV RMS in argon, processing times of 0.5, 3 and 5 min. Initial results indicate that microbial decontamination is effective on the lettuce (1.2 log reduction) when compared with other vegetables. To claim disinfection, a 3 log reduction or more is needed, which makes APCP treatment very promising technology for decontamination of produce. We propose that with method refinements full disinfection can be achieved using APCP.   

In situ analysis of formation of carbon nanostructures in arc discharge by optical spectrometry

Arc discharge supported by the erosion of anode materials is one of the most practical and efficient methods to synthesize various high-quality carbon nanostructures due to its relatively high growth temperature. By introducing a non-uniform magnetic field with the component normal to arc current, graphene flakes and single-walled carbon nanotubes can be synthesized in one step. In contrast to the growth processes without magnetic field, the magnetically-enhanced arc is confined by the Lorentz force, which generates the plasma jet and makes effective delivery of carbon particles and heat flux. However, there are still unresolved questions concerning the location of the region of nanoparticle synthesis and growth steps of carbon nanostructures. In this work we carried out in situ analysis of the optical spectrum which can provide a unique investigation of the different transformation processes of the carbon and metal catalyst vapors generated from the vaporization of the anode in arc. The experiments were taken for various electrode gaps and different conditions of external magnetic field. Moreover, SEM, TEM, EDX and Raman spectroscopy were employed to characterize the properties of carbon nanotubes and graphene.   

Plasma Generated and Sustained in Air by the Double Laser Pulses

Sequences of laser pulses offer an advantageous tool providing access to the control of air-plasma dynamics and optical interactions. A detailed 1D model of plasma dynamics, which self-consistently integrates plasma-kinetic, Navier-Stokes, electron heat conduction, and electron-vibration energy transfer equations is developed to quantify the plasma filaments induced in the atmosphere through filamentation with high-intensity ultrashort laser pulses further sustained by long laser pulses. It is shown that near- and mid-infrared laser pulses can tailor plasma decay in the wake of a filament, efficiently suppressing the attachment of electrons to neutral species and dissociative recombination. Laser pulses with higher intensities can give rise to efficient ionization and heating of the postfilament plasma, eventually inducing a highly conductive arc discharge. However, the plasma-filament- heating-pulse longitudinally uniform interaction length is limited due to the self-defocusing of the heating beam. This effect becomes important when subsequent heating laser pulse is focused to values close to breakdown, and it depends on the laser wavelength.   

Effect of Different Gases on OH Radical Concentration in Ar and He Atmospheric Pressure Microwave Plasma Jet

In non-thermal atmospheric pressure plasmas, OH radical generation can be affected by various parameters such as plasma gases, power, relative humidity, etc. An UV pulsed laser cavity ringdown spectroscopy was employed to measure absolute number density of OH (A-X) (0-0) band at 306-310 nm, generated in an atmospheric pressure plasma jet obtained by a 2.45 GHz microwave plasma source. Addition of N$_{2}$, O$_{2}$ and H$_{2}$O to Ar and He plasma jets, and effect on OH radical generation was studied. Optical emission spectroscopy was simultaneously employed to monitor reactive emission species. Addition of molecular gases such as N$_{2}$, O$_{2}$ or H$_{2}$O results in increase in gas temperature. Addition of O$_{2}$ to plasma gas is more favorable for OH generation. Effect of plasma power and gas flow rate was also studied. With increase in H$_{2}$O content, OH radical concentration decays faster along the jet axis. Electron dissociation and dissociative recombination are dominant reaction path way of OH formation within the plasma column and in the downstream region, respectively.   

Femtosecond Control of Plasma Filaments in Atmosphere with Multiple Ultrashort Laser Pulses

A laser pulse propagating through atmosphere self-focuses due to the nonlinear index of refraction modifications from the instantaneous electronic and delayed rotational Raman responses in air. If the pulse power is sufficient, the focused pulse intensity can surpass the ionization threshold, resulting in a plasma filament. The balance between defocusing due to the plasma and focusing due to the instantaneous and delayed Raman responses results in extended propagation at high intensity. Because the rotational Raman response is periodic in time, owing to quantum mechanical discreteness of the rotational eigenfrequencies of the molecules, subsequent laser pulses delayed at the recurrence period will experience the index modification left behind by the previous pulses. Here we present propagation simulations based on experimental parameters showing that extension of the plasma filament is sensitive to subsequent pulse delays of 10 femtoseconds with respect to the recurrence time of the rotational response.   

Formation of plasma channels and mini-filaments under tight focusing of femtosecond laser pulses in air

The peculiarities of self-focusing and filamentation of high- power femtosecond laser pulses focused in air at various numerical apertures (NA) of focusing lens were experimentally and theoretically studied. The effect of multi-filamentation and formation of mini-filaments under tight focusing was experimentally observed and theoretically confirmed. The influence of the NA on the main characteristics of initiated plasma channels such as their radius R, length and concentration of electrons was studied. It was demonstrated that under the tight focusing of a laser beam (NA higher than 0.05) the decrease of transverse dimension of the plasma channel is discontinued at the level of R equals 2 - 4 micron. These mini-filaments were demonstrated to be an efficient and compact source of the third harmonic generation in UV spectral range.