Pulsed plasma

The molecules that are dissociated and the atoms that are ionized during plasma production can be in any state at the start. Steady-state plasmas are formed most often from gases, although Hquids, such as volatile organics, and soHds are also used. Gases and soHds routinely serve as sources of material in pulsed plasma work. 

Pulsed plasmas containing hydrogen isotopes can produce bursts of alpha particles and neutrons as a consequence of nuclear reactions. The neutrons are useful for radiation-effects testing and for other materials research. A dense plasma focus filled with deuterium at low pressure has produced 10 neutrons in a single pulse (76) (see Deuterium AND TRITIUM). Intense neutron fluxes also are expected from thermonuclear fusion research devices employing either magnetic or inertial confinement. 

At the start of each modulation pulse, a sharp peak in optical emission is seen. Similar SiH emission peaks in pulsed plasmas have been found by Scarsbrook et al. [516] and Howling et al. [321]. The sharp peak was claimed to be caused by a pulse of high-energy electrons. Overzet and Verdeyen [517] measured electron densities at a 2.9-MHz excitation frequency and modulation frequencies up to 20 kHz. The optical emission of a SQWM argon plasma was measured by Booth et al. [518], who also performed particle-in-cell modeling. 

Dorai, R. and Kushner, M.J. (2002) Repetitively pulsed plasma remediation of NOx in soot laden exhaust using dielectric barrier discharges, J. Phys. D Appl. Phys. 35, 2954—68. 

Hoffmann, M. R., Hua, I., Hochemer, R. H., Willberg, D., Lang, P., Kratel, A., chapter entitled Chemistry under Extreme Conditions in Water Induced by Electrohydraulic Cavitation and Pulsed-Plasma Discharges , in Chemistry Under Extreme or Non-Classcial Conditions, pp. 429-478, edited by R. Van Eldik and C. D. Hubbards, New York Wiley Heidelberg Spektrum, 1997. 

Phase changes as small as 2 tr x 10 rad have been detected 269) by using a null method where phase drifts of the apparatus are compensated by a calibrated and controllable dielectric in cavity B. The response time of this laser interferometer is less than 5 psec when operated at 3.39 pm and repetitively pulsed plasmas can therefore be investigated. Gerardo etal. 270) even reached a time resolution of... 

Because of the relatively large dispersion from the electrons compared with the almost constant refractivity of the neutrals and the negligible contribution of the ions, it is possible, with simultaneous measurements at two different wavelength, to determine independent values of the density of electrons and of the nonelectronic components in the plasma 274). Alcock and Ramsden 275) used the light from a giant-pulse ruby laser and its second harmonic generated in an ADP-crystal (ammonium dihydrogen phosphate) to probe a pulsed plasma and its time-dependent density in a Mach-Zehnder interferometer. 

A more complicated but very variable process (iii) was the pulsed plasma-initiated polymerization or copolymerization. Here, the desired monotype functional groups are provided by the monomer, which are polymerized in the pulsed plasma. The art in producing such 50 nm thick monotype functionalized polymer coatings lays in carrying out the plasma process under as mild conditions as possible to avoid fragmentation of monomers and to preserve and remain the functional groups of the respective monomers. 

Plasma polymer layers were deposited in the same reactor as described before. However, in this case, the pulsed plasma mode was applied. The duty cycle of pulsing was adjusted generally to 0.1 and the pulse frequency to 103Hz. The power input was varied between P 100 ()() V. Mass flow controllers for gases and vapours, a heated gas/vapour distribution in the chamber, and control of pressure and monomer flow by vaiying the speed of the turbomolecular pump were used. The gas flow was adjusted to 75-125 seem and the pressure was varied between 10 to 26 Pa depending on the respective polymerization or copolymerization process. The deposition rate was measured by a quartz microbalance. 

Pulsed Plasma Polymerization. Allyl alcohol, allylamine and acrylic acid were polymerized in pulsed plasma to retain a maximum in functionalities in the resulting plasma polymer. The retention of functional groups during the plasma process, introduced by functional-group carrying monomers and followed by deposition to polymer layers, was primarily measured by XPS including the chemical derivatization of these groups as described in Experimental. These layers were also checked for side-products by respective IR spectra. The results are summarized in Table 1. 

Table 1 Absolute and relative yields in functional groups at the surface of deposited pulsed plasma polymer layers measured with XPS after derivatization (cf. Experimental, 100 W)...

It should be remembered that the aim of this work was to produce defined layers with monosort functional groups, which can be used for grafting. Now, in contrast to the irregularly structured continuous-wave plasma polymers, the structure of pulsed plasma polymers was so much improved that partial or complete solubility was observed. Therefore, the further chemical processing in solvents and water led to dissolving the layer. Here, also chemically crosslinking copolymers as butadiene, di-vinylbenzene and trivinylcyclohexane were necessary as schematically shown in Fig. 9. 

Fig. 10 Covalent bonding of fluorophores onto primary amino groups of allylamine pulsed plasma polymer layers (a - fluorescein bonding b - dansylhydrazine bonding c - dansylcadaverine bonding)...

It is important to note that the first three monomers are type I monomers, and their behaviors in plasma polymerization, except under pulsed plasma conditions, are very similar. These conspicuous differences observed as a consequence of the use of pulsed discharge may be attributed to the behavior of the aromatic structure under the conditions of LCVD. 

A number of authors describe significant improvements in the polymer coating quality obtained by pulsed plasma polymerisation cf. CW). If these claims are substantiated, it would seem likely that research effort will focus increasingly on this mode of plasma deposition. The question arises as to whether the pulsed environment is simply an extrapolation of the low power CW environment, or whether it represents an entirely different plasma environment altogether. 

Further ultrahigh pressure structural modifications of zinc-blende BN are considered possible, with a rock salt structure preferred over a /i-Sn structured A metastable E-BN phase is obtained during electron-beam-assisted crystallization of BN. Other metastable phases of BN are reported in the condensation products upon sudden cooling in conditions of a pulsed plasma discharge Disordered structures are reported when mixed plasma/chemical processes are usedd Some compounds of type B N are listed in Table... 

The two-dimensional ordered layered linear chain carbon films were produced using a pulsed plasma ion-assisted technology [10]. The films were obtained at a rate of lOOnm/min on various substrates (metals, ceramics, polymers, etc.) with good adhesion and homogeneity of the films on the areas of 150 x ISOmm. ... 

In particular, two main streams of PI applications have been identified (i) PI innovations for reactors (e.g., microreactors, monolith reactors, spinning disc reactors, reactive separations) and (ii) PI technologies for more efficient energy transfer (e.g., ultrasound, pulse, plasma, microwave). Several PI technologies offer important potential, but require important fundamental/strategic research to reach proof-ofconcept on the laboratory scale. These PI technologies are ... 

A pulsed plasma has been used to prepare pinhole-free films from relatively nontoxic N vinylpyrrolidone.323 The pulsing reduced fragmentation of the monomer and cross-linking. This method should be tried with other monomers. Plasmas are often used for the modification of polymer surfaces.324 These methods are relatively rapid and use no solvent. Decorative coatings of TiN and other inorganic compounds can be applied to metals and other inorganic substrates by sputtering, chemical vapor deposition, plasmas, and such, as described in Chap. 4.325... 

---