Global forest resources assessment 2000 : main report

Parthenium hysterophorus L.(キク科)の胚発生学の研究〔英文〕

High power electron beam generation studies were carried out in a planar diode configuration to investigate the effect of the accelerating gap, diode voltage, and anode-cathode materials on the electrode plasma expansion. The diode voltage has been varied from 145–428 kV, whereas the current density has been varied from 208–2215 A/cm2 with 100 ns pulse duration. It was found that the diode voltage and current follow the bipolar space-charge limited flow model. The anode and cathode plasma expansion velocities were calculated using the perveance data. The plasma expands at 11 cm/μs for 34 mm anode-cathode gap and the plasma velocity decreases for smaller gaps. It was found that the plasma expansion velocity increases significantly due to the cathode edge contribution and the edge contribution is particularly important during the beginning and the end of the accelerating pulse when the diode voltage and the corresponding electric field are comparatively low. It was also observed that the diode current incre...

Plasma expansion and fast gap closure in a high power electron beam diode

High power microwave (HPM) generation studies were carried out in KALI-5000 pulse power system. The intense relativistic electron beam was utilized to generate HPMs using a coaxial virtual cathode oscillator. The typical electron beam parameters were 350 kV, 25 kA, and 100 ns, with a few hundreds of ampere per centimeter square current density. Microwaves were generated with graphite and polymer velvet cathode at various diode voltage, current, and accelerating gaps. A horn antenna setup with diode detector and attenuators was used to measure the microwave power. It was observed that the microwave power increases with the diode voltage and current and reduces with the accelerating gap. It was found that both the peak power and width of the microwave pulse is larger for the velvet cathode compared to the graphite cathode. In a coaxial vircator, velvet cathode is superior to the graphite cathode due to its shorter turn on time and better electron beam uniformity.

High power microwave generation from coaxial virtual cathode oscillator using graphite and velvet cathodes

This paper presents the design and development of a repetitive Marx generator rated at 1 kJ, 300 kV, 12 kA, and 10 Hz and which is suitable to drive the load directly. Bipolar charging of a Marx generator scheme has been adopted to get the following: 1) faster rise time and 2) relatively low charging voltage. The faster rise time is due to the reduced number of spark gaps down to six for 12 in-series capacitors (0.15 μF and 50 kV) of a six-stage bipolar-charging Marx generator. Interstage inductive charging further enhances the efficiency of the overall system as compared to resistive charging. The generator is characterized by an aqueous resistive load for both polarities at the output voltage. Output voltages of the same magnitude have been achieved for both polarities. The matched impedance is found to be 25 Ω for which a suitable reflex triode is designed. Intense electron beams and high-power microwaves have been generated using this Marx generator and reflex triode system. Experimental results demonstrating the Marx generator and the reflex triode operation are presented and discussed.

Development and Characterization of Repetitive 1-kJ Marx-Generator-Driven Reflex Triode System for High-Power Microwave Generation

Intense gigawatt relativistic electron beam has been generated in a high power cylindrical diode in the presence of prepulse. A bipolar prepulse voltage, recorded at the diode, varies both in amplitude and time duration with the Marx generator voltage. It was found that only at the accelerating gap ⩽1.65cm there is some shot to shot variation in the diode voltage and current for the same Marx generator voltage. The anode and cathode plasma expansion velocities were calculated using the perveance data. The plasma expands at 5cm∕μs for 1.85cm radial anode-cathode gap and the plasma velocity decreases for smaller gap. It was found that the effect of the prepulse is less pronounced in the cylindrical diode as compared to planar diode that allows one operation of the cylindrical diode with the gap ⩽1.85cm.

Intense relativistic electron beam generation and prepulse effect in high power cylindrical diode

Intense relativistic electron beam diode has been operated without a prepulse switch. Our previously reported results demonstrated that the large pulse power systems in the presence of prepulse can deliver gigawatt power pulses into a matched load if the anode cathode gap is set larger than that estimated by the Child Langmuir relation. This article reports some more experimental results on the current and voltage characteristics, the shot to shot reproducibility of the diode in the presence of prepulse. Intense electron beam diode behavior was studied for various anode cathode gaps and voltages in presence of prepulse. It has been observed that for small gap, prepulse generated plasma completely fills the anode cathode gap and the diode behaves as plasma filled diode. At a large gap the beam parameters obtained are 420 keV, 22 kA, 100 ns. From the experimentally obtained values of perveance an upper limit was set up for the Marx voltage (or anode cathode voltage) and lower limit for the anode cathode gap in order to avoid the gap closure problem and the diode can be operated with a better shot to shot reproducibility.

Intense gigawatt relativistic electron beam generation in the presence of prepulse

Himalayan forest has been threatened by rapid anthropogenic activities, resulting in the loss of forest diversity and climate change. The present study was carried out on four aspects (northern, southern, western and eastern), at three different altitudinal ranges, namely, 1000–1300 m above sea level (m a.s.l.), 1300–1600 m a.s.l. and 1600–1900 m a.s.l., and at three diverse mountain ranges (Kalaghat, Barog and Nangali) of sub-temperate forest ecosystems of the mid Himalayan ranges, to elucidate their influence on vegetation, tree characteristics and ecosystem carbon density. The results revealed that Pinus roxburghii is the most dominant forest community of the mid Himalaya’s forest, irrespective of altitudinal gradient and slope. The south-facing slopes are occupied by the xerophytic tree species frequently found in the lower Shiwalik P. roxburghii forest, whereas the north-facing ones are dominated by mesophyllic species, such as Cedrus deodara and Quercus leucotrichophora, which commonly grows in the northwestern Himalayan temperate forest ecosystem. The maximum stem density (211.00 Nha−1) was found at 1000–1300 m a.s.l., and on the northern aspect (211.00 Nha−1). The maximum stem volume (236.50 m3 ha−1) was observed on the northern aspect at 1000–1300 m a.s.l., whereas the minimum (32.167 m3 ha−1) in the southern aspect at 1300–1600 m a.s.l. The maximum carbon density (149.90 Mg ha−1) was found on the northern aspect and declined with increasing elevation from 123.20 to 74.78 Mg ha−1. Overall, the study establishes that the southern and western aspects are very low in carbon density, whereas the northern aspect represents higher biodiversity as well as carbon and nutrient stocks. Therefore, aspect and altitude should be given due importance for efficient managing of biodiversity and mitigating climate change.

Influence of Aspect and Elevational Gradient on Vegetation Pattern, Tree Characteristics and Ecosystem Carbon Density in Northwestern Himalayas

Earth's mean air temperature is increasing at an accelerating rate, and forest tree species are sensitive to this changing climate. However, tree species react variably to changing climates across different geographical ranges. Thus, eleven multi-purpose tree species (MPTs), Albizia stipulata, Bauhinia variegata, Celtis australis, Dalbergia sissoo, Gmelina arborea, Grewia optiva, Melia azedarach, Morus alba, Robinia pseudoacacia, Sapindus mukorossi and Toona ciliata, of the mid-hill zone of the northwestern Himalayas were selected to analyze the impact of climatic patterns on their phenological events over two decades (1999–2017). The results indicated that phenological advancement (in days) was more pronounced from 1999 to 2006 than from 2006 to 2017. Specifically, leaf emergence phase advancement ranged from 5 (R. pseudoacacia) to 38 days (M. alba). However, advancement in the flower initiation phase for MPTs was not as prominent. The growth period of M. azedarach was prolonged the most (60 days), followed by those of B. variegata (56 days), D. sissoo (51 days), T. ciliata (48 days), S. mukorossi (46 days), R. pseudoacacia (44 days), G. optiva (41 days), G. arborea (20 days), A. stipulate (15 days), M. alba (10 days) and C. australis (7 days). Increasing growing season duration and rate have implications on future composition, nutrient cycling and carbon sequestration of agro-ecosystems of the mid-hill zone of the north western Himalayas.

Impact of climatic patterns on phenophase and growth of multi-purpose trees of north-western mid-Himalayan ecosystem

This paper presents the experimental results of a repetitive Marx generator being developed at BARC. Effect of lead inductance, sparkgaps' alignment, charging inductor, ground inductor and shielding has been studied. Two types of configurations have been adopted in order to reduce the erected Marx inductance and results are compared. In this Marx generator plus-minus charging scheme was adopted for both test setups. Each setup comprised of 2-stage Marx with 4 series capacitors (each rated for 0.15 microfarad, 40nH, 50kV) while discharging giving 0.375 microfarads capacitance. In one of the scheme Type-I, all spark gaps were aligned in line of sight arrangements and successive discharge path were assembled in a zigzag manner so that the induced magnetic field gets cancelled out and effective inductance is reduced. This scheme was terminated to an aqueous load and critical matched condition was achieved at 12 ohm. This scheme had effective capacitance of 0.0375 microfarads, wave shape in lower voltage i.e. 10–15kV charging level. During experiments effect of load was also seen which gave half the open voltage to matched load [z=√(L/C)] and in critical damping condition voltage was 0.7 times of open circuit voltage. These data will be used for developing a 6-stage 1.2 kJ Marx generator for 20 pps burst output. Presently testing is limited to 2pps due to charging power supply limitations. Thus the results can be summarized as follows: (i) line of sight and ultra violet triggering is not effective for a distance of 200mm, (ii) cancellation of induced magnetic field is not effective at 5kA current peak at 100mm distance, (iii) longer length leads in smaller diameter is preferable over larger diameter with smaller length assembly with zigzag leads for giving shorter pulse, higher current and faster rise time.

D. D. P. Kumar

Papers

Intense relativistic electron beam generation and prepulse effect in high power cylindrical diode

Intense gigawatt relativistic electron beam generation in the presence of prepulse

Influence of Aspect and Elevational Gradient on Vegetation Pattern, Tree Characteristics and Ecosystem Carbon Density in Northwestern Himalayas

Impact of climatic patterns on phenophase and growth of multi-purpose trees of north-western mid-Himalayan ecosystem

Experimental results on design aspects of a compact repetitive Marx generator