ALBERT

Notes: In an ongoing effort to investigate plasma plume features yielding high quality diamond-like carbon films, we have applied plasma plume diagnosis and film characterization to examine plume character distinction from KrF laser ablation of both amorphous carbon (a-C) and single crystal graphite (SCG) targets. The advancing plasma plume produced by these structurally different targets are observed to possess quantitatively similar total heavy particle inventory, ionized fraction, and electron thermal content, yet quite different ion kinetic energy, plume profile, C2 formation mechanism, and concentration of complex molecules. Plume electron temperatures are observed to reside in the range 1–3 eV, with those in SCG plumes ∼10%–30% greater than a-C at all spatial positions downstream of the target. For both target cases, we find Te drop off with position away from the target with radiation as the most likely loss mechanism for these noninteracting plumes propagating in vacuum. Electron density is found to be ∼10%–12% lower near the target in SCG than a-C plumes consistent with mass loss inventory measurements, whereas ion fractions are estimated in the range ∼10%–15% for both target cases. All recorded data support the conclusion that the SCG target plasma plume is populated with heavier, more complex molecules than those in a-C which have been shown to be predominantly comprised of C and C+ under vacuum conditions with the addition of C2 at high fill pressure. A significantly smaller profile peaking factor for SCG plumes supports this conclusion. Less energetic and slightly lower temperature SCG plume conditions are consistent with reduced peaking and more massive plume species. Plasma plumes from SCG targets exhibit laser energy (El) dependent peaking, again consistent with more complex molecules increasingly disassociated with El increase. The El dependence further suggests the potential for control of particle size distribution and plume profile peaking, though not independently. Consistent with this scenario is the observation of harder films produced from SCG targets at lower El. Micro-Raman results indicate strongly heterogeneous films deposited by SCG target ablation even under vacuum conditions further supporting the case for more complex structures with greater hardness. Energy balance estimates indicate that ion kinetic energy dominates the balance and that SCG ablation liberates about twice the number of 12C atoms from the target per unit El. As well, high pressure background fill indicates lesser plume energy attenuation for SCG plumes, again suggesting the presence of higher mass particles. © 2000 American Institute of Physics.

Notes: A large, and often neglected, component of grazing intake is behavioural. Grazing animals may employ two strategies to increase their intake rates during grazing: increasing bite mass and decreasing handling time. These strategies are discussed in the context of fasted and non-fasted sheep grazing white clover (Trifolium repens L. cv. Kent wild white) and perennial ryegrass (Lolium perenne L. cv. Parcour). It is concluded that within a forage species sheep have little flexibility for increasing their intake rates by altering their handling times but have some latitude for altering bite masses. Between forage species, differences in intake rates may be more strongly affected by the ‘species-specific’ requirement for mastication than by differences in bite masses per se. Comparisons are also made with grazing strategies of cattle.

Notes: The spatial patterns of white clover and sward surface height (SSH) that developed In established perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) pastures undercutting lent every 4 weeks to 5 cm) and gracing (continuously grazed with sheep to 5 cm) were measured. While clover cover was recorded in 1000 contiguous 5 × 5-cm quadrats down 50-m permanent transects from early spring to late autumn. Measurements of SSH were made at 10-cm intervals down the same transect. Spatial pattern was analysed using two-term local quadrat variance and patch-gap analysis. At least two scales of spatial pattern existed for white clover when defoliation treatments began. White clover was not distributed at random but found in patches (mean size = 1.1 m) where it was finely intermixed with grass. Patches, separated by gaps (regions of no clover) (mean size = 2.3 m), were in turn aggregated into ‘patches of patches’, separated by larger gaps (mean size = 4.1 m). Under grazing the pattern of patches and gaps did not alter. Under cutting, patch size increased and gap size decreased, explaining in part the greater mass and cover of white clover that arose in cut than grazed swards during the experiment. No new patches of white clover due to seedling establishment or clonal growth were observed in either cut or grazed swards. The intensity of pattern increased in both cut and grazed swards, but the increase was greater m cut swards. The initial single scale of spatial pattern of SSH of tall patches (mean size = 1.2 m) separated by short patches (mean size = 2.7 m) did not change under grazing. SSH became uniform under cutting. It is suggested that the response of plants to selective (spatially heterogeneous) grazing is a crucial factor in the development and maintenance of spatial pattern in grasslands. The importance of spatial pattern to our understanding and interpretation of plant-plant and plant-animal interactions and to the composition of temperate grasslands is considered.

Notes: This study used both experimental evidence and a mathematical model to address some differences in interpretation in the literature on the relationship between sward height and the bite dimensions (bite depth, bite area and bite mass) of sheep grazing contrasting vegetation types. Individual non-fasted sheep were released onto small areas (10 × 10m) of white clover (Trifolium repens) and perennial ryegrass (Lolium perenne) and bite dimensions were measured as they grazed across patches (0·7 × 0·7 m) of predetermined sward surface height (SSH). Sward heights were 4, 7, 10, 13 and 16cm for white clover patches and 4, 6, 5, 9, 11·5 and 14cm for ryegrass patches. Four sheep were assigned to each plant species and each sheep grazed one patch of each height (five patches/sheep). Bite depth, bite area and bite mass increased linearly with SSH in both white clover and ryegrass. At a given SSH, bite depth was similar in white clover and ryegrass, but bite area and bite mass were greater in white clover than in ryegrass. The linear relationships observed between bite mass and SSH contrasted with the asymptotic relationships observed in some other studies, but it is suggested that different relationships may arise because of methodological differences between studies. Furthermore, when linear relationships for bite mass were compared with asymptotic relationships for bite mass in a mechanistic model of animals grazing from ryegrass-white clover pastures it was demonstrated that the nature of the relationship had relatively little effect on the relationship between intake rate and SSH. This was because intake rate depended on the fundamental mechanistic relationship between bite mass and prehension bite rate. This relationship meant that the greater bite masses found when linear relationship were assumed were associated with reduced prehension bite rates and thus the effect on intake rate was relatively small. In addition, the predictions of the model regarding the bases of diet selection by animals grazing ryegrass-white clover mixtures were simplified, and stabilized, when linear relationships were assumed.

Notes: A triple Langmuir probe measurement has been implemented to investigate plasma plume character in low fluence (∼3.0 J/cm2) pulsed laser evaporation (PLE) discharges and has been found to be an extremely valuable tool. Absolute plasma plume density estimates are found to reside in the range 1.0×1013–2.0×1014 cm−3 for vacuum pulses. A simple heavy particle streaming model for vacuum pulses allows estimates of the plume ionization fraction of ∼10%. This is consistent with typical deposition inventory suggesting that high kinetic energy ions may play an important role in diamond-like carbon (DLC) film deposition. Electron temperature inferred from the electrostatic probe is found to consistently reside in the range 0.5–3.0 eV, and appears to be uninfluenced by operating conditions and large variations in Ar and N2 fill gas pressure. Consistent with strong plume ion and neutral particle coupling to the background fill, constancy of Te suggests expulsion of background gas by the energetic plume. The leading edge ion plume speed is measured via temporal displacement of spatially separated probe signals on consecutive PLE pulses. Flow speeds as high as 5.0×104 m/s are observed, corresponding to ∼156 eV in C+. The ion flow speed is found to be a strongly decreasing function of fill pressure from an average high of ∼126 eV in vacuum to ∼0.24 eV at 600 mTorr N2. Raman scattering spectroscopy indicates DLC film quality also degrades with fill pressure suggesting the importance of high ion kinetic energy in producing good quality films, consistent with earlier work demonstrating the importance of energetic particles. Optical emission indicates an increase in C2 molecular light intensity with fill gas pressure implying a reduced, if any, role of these species in DLC production. Ion current signal anomalies are often seen during high pressure pulses. It is suggested that this may indicate the formation of high mass carbon clusters during plume evolution in the presence of background gas. Mass diffusivity estimates, based on density decay, suggest the presence of C2+ under these conditions. Demonstration and control of such cluster formation may provide method(s) for controlling novel advanced materials properties. © 1999 American Institute of Physics.