ABSTRACT
An electron heating mechanism based on a resonance between the cyclotron motion of electrons and the radio frequency sheath oscillations is reported in weakly magnetized capacitively coupled plasmas at low pressure. If half of the electron cyclotron period coincides with the radio frequency period, then electrons will coherently collide with the expanding sheath and gain substantial energy, which enhances the plasma density. A relation between the magnetic field and the driving frequency is found to characterize this resonance effect and the kinetics of electrons are revealed at resonance conditions for various driving frequencies.
ABSTRACT
Five forests under diverse site conditions but under identical climate in the Maoershan region of Northeast China were sampled for measuring contents of soil carbon (C), nitrogen (N), and phosphorus (P), soil bulk density, and soil thickness by soil profile horizons. The stands included two plantations (i.e., Pinus koraiensis and Larix gmelinii plantations) and three broadleaved forests (i.e., Quercus mongolica stand, Populus davidiana Betula platyphylla mixed stand, and hardwood stand). Our aim was to examine vertical distribution of the content, density, and stoichio metry of soil C, N and P for the five forest types. The results showed that the contents and densities of soil C, N and P differed significantly among the forest types, with the maxima of the soil C and N at both O and A horizons occurring in the hardwood stand. The contents of C and N decreased significantly with increasing soil depth in all the stands. P content decreased significantly only in the broadleaved stands, and P content had no significant difference among different soil layers in the coniferous stands. The soil C/N at the A horizon, N/P at the O horizon, and the C/P at A and B horizons were significantly different among the forest types. The soil C and N linearly correlated significantly across all the forest types without significant differences in the slopes and intercepts, and the soil N and P, or the soil C and P correlated significantly only in the broadleaved stands. This result suggested that the C-N coupling relationship tended to converge across the forest types, and the N-P and C-P relationships varied with forest types.
Subject(s)
Carbon , Nitrogen , Phosphorus , Soil , China , ForestsABSTRACT
Plant temperature is an important parameter for estimating energy balance and vegetation respiration of forest ecosystem. To examine spatial variation in diurnal courses of stem temperatures (Ts) and its influencing factors, we measured the Ts with copper constantan thermocouples at different depths, heights and azimuths within the stems of two broadleaved tree species with contrasting bark and wood properties, Betula platyphylla and Fraxinus mandshurica. The results showed that the monthly mean diurnal courses of the Ts largely followed that of air temperature with a 'sinusoi dal' pattern, but the Ts lagged behind the air temperature by 0 h at the stem surface to 4 h at 6 cm depth. The daily maximal values and ranges of the diurnal course of Ts decreased gradually with increasing measuring depth across the stem and decreasing measuring height along the stem. The circumferential variation in Ts was marginal, with slightly higher daily maximal values in the south and west directions during the daytime of the dormant season. Differences in thermal properties (i.e. , specific heat capacity and thermal conductivity) of both bark and wood tissue between the two species contributed to the inter specific variations in the radial variation in Ts through influencing the heat exchange between the stem surface and ambient air as well as heat diffusion within the stem. The higher reflectance of the bark of B. platyphylla decreased the influence of solar radiation on Ts. The stepwise regression showed that the diurnal courses of Ts could be well predicted by the environmental factors (R2 > 0.85) with an order of influence ranking as air temperature > water vapor pressure > net radiation > wind speed. It is necessary to take the radial, vertical and inter specific varia-tions in Ts into account when estimating biomass heat storage and stem CO2 efflux.
Subject(s)
Betula , Fraxinus , Plant Stems , Temperature , TreesABSTRACT
We present the experimental evidence of the collisionless electron bounce resonance heating (BRH) in low-pressure dual-frequency capacitively coupled plasmas. In capacitively coupled plasmas at low pressures when the discharge frequency and gap satisfy a certain resonant condition, the high energy beamlike electrons can be generated by fast sheath expansion, and heated by the two sheaths coherently, thus the BRH occurs. By using a combined measurement of a floating double probe and optical emission spectroscopy, we demonstrate the effect of BRH on plasma properties, such as plasma density and light emission, especially in dual-frequency discharges.
