Plasma control by modification of helicon wave propagation in low magnetic fields

By making use of nonuniform magnetic fields, it is shown experimentally that control of helicon wave propagation can be achieved in a low pressure (0.08 Pa) expanding plasma. The m=1 helicon waves are formed during a direct capacitive to wave mode transition that occurs in a low diverging magnetic field(B₀<3 mT). In this initial configuration, waves are prevented from reaching the downstream region, but slight modifications to the magnetic field allows the axial distance over which waves can propagate to be controlled. By changing the effective propagation distance in this way, significant modification of the density and plasma potential profiles can be achieved, showing that the rf power deposition can be spatially controlled as well. Critical to the modification of the wave propagation behavior is the magnetic field strength (and geometry) near the exit of the plasma source region, which gives electron cyclotron frequencies close to the wave frequency of 13.56 MHz

Detailed plasma potential measurements in a radio-frequency expanding plasma obtained from various electrostatic probes

On-axis plasma potential measurements have been made with an emissive probe in a low pressure (0.044 Pa) rf expanding plasma containing an ion beam. The beam is detected with a retarding field energy analyzer (RFEA), and is seen to disappear at high pressure (0.39 Pa). The emissive probe measurements are in very good agreement with corresponding measurements made with two separate RFEAs, and the results indicate that the floating potential of the strongly emitting probe gives an accurate measure of the plasma potential under the present conditions

Education for sustainability: a sensory ethnography in biodynamic agriculture

Abstract. Since the inception of compulsory education in the Western world, learning in school has privileged our senses of sight, hearing and touch. The senses of smell and taste have been undeveloped or even neglected in formal education based on the assumption that they are not senses of knowledge (Classen, 1999). In the twenty-first century, environmentally injurious phenomena related to climate change and biodiversity loss have profound impacts on our total environments and our whole bodies—especially beyond what is perceptible by vision, hearing, video and text. This thesis uses sensory ethnographic material collected in a biodynamic farm in northern Italy and in the international Slow Food movement to explore how the senses are engaged in generating and redefining values concerning sustainability and sustainable practice. The sensory ethnographic material is buttressed by a history of the senses in Western thought and culture and explores why dominant ways of understanding the senses in the West are out of step with how humans actually learn. Through this discussion it is argued that theoretically, methodologically and practically dissolving Cartesian ontology is a precondition for sustainability of any kind. This sets up the sensory ethnographic material where I draw from cultural and phenomenological theories of the senses, perception and a theory of place to situate the biodynamic farm and Slow Food movement as place-events of sustainable practice, activism, and education. I then explore how sustainable values are learned through one’s multisensory emplacement within such contexts. The thesis is meant to contribute to discussions about how humans learn in the world and provide an opening from which to explore the possibilities of holistically and explicitly educating the senses in non/formal education. Such considerations are aimed at better preparing learners to actively perceive their world beyond the means of pen, paper, video, debate and discussion. The value of this thesis lies in its interdisciplinarity and the possibilities it raises for reappraising the education of the human sensorium in the Anthropocene

Particle-in-cell simulations of ambipolar and nonambipolar diffusion in magnetized plasmas

Using a two-dimensional particle-in-cell simulation, we investigate cross-field diffusion in low-pressure magnetized plasmas both in the presence and absence of conducting axial boundaries. With no axial boundary, the cross-field diffusion is observed to be ambipolar, as expected. However, when axial boundaries are added, the diffusion becomes distinctly nonambipolar. Electrons are prevented from escaping to the transverse walls and are preferentially removed from the discharge along the magnetic field lines, thus allowing quasi-neutrality to be maintained via a short-circuit effect at the axial boundaries

Particle-in-cell simulations of hollow cathode enhanced capacitively coupled radio frequency discharges

A two-dimensional particle-in-cell simulation has been developed to study density enhancement of capacitively coupled rf discharges with multi-slit electrodes. The observed density increase is shown to result from a hollow cathode effect that takes place within the multi-slit electrode configuration, which forms as a result of secondary electron emission due to ion bombardment. By investigating the ionization and power deposition profiles, it is found that rfsheathheating is too weak to sustain the discharge, and that secondary electron acceleration within the sheath is the primary heating mechanism. Due to a capacitive voltage divider formed by the rfsheaths at each electrode, the area ratio of the powered and ground electrodes is observed to have a strong effect on the resulting discharge, and if the ground electrode area is too small, the voltage drop at the powered electrode is too low to sustain a hollow cathodedischarge.The authors gratefully acknowledge financial support from the Lam Research Corporation

Ion beam formation in a very low magnetic field expanding helicon discharge

An ion beam has been measured emerging from a low pressure (0.04 Pa) helicon plasma reactor over a narrow range of magnetic field values (1 mT<B0<3 mT). The presence of the ion beam occurs simultaneously with a large increase in the plasma density for the same applied magnetic field, produced using a single solenoid half the length of the m=1 rfantenna. The peak central plasma density of 1.5×10¹⁷ m⁻³ is measured to be almost 15 times larger than that occurring before or after the increase, and is associated with a steep axial density gradient which follows the gradient of the magnetic field. During this low magnetic field transition the antenna power transfer efficiency is measured to increase from less than 10% to 50%, suggesting some form of localized bulk electron heating in the source region associated with the helicon wave

Electron–cyclotron damping of helicon waves in low diverging magnetic fields

Particle-in-cell simulations are performed to investigate wave propagation and absorption behavior of low-field (B0<5 mT) helicon waves in the presence of a diverging magnetic field. The 1D electromagnetic simulations, which include experimental external magnetic field profiles, provide strong evidence for electron–cyclotron damping of helicon waves in the spatially decaying nonuniform magnetic field. For a dipole-type magnetic field configuration, the helicon waves are absence in the downstream (lower field) region of the plasma and are observed to be completely absorbed. As the magnetic field is changed slightly however, wave damping decreases, and waves are able to propagate freely downstream, confirming previous experimental measurements of this phenomenon

Enhanced sheath heating in capacitively coupled discharges due to non-sinusoidal voltage waveforms

Through the use of particle-in-cell simulations, we demonstrate that the power deposition in capacitively coupled discharges (in argon) can be increased by replacing sinusoidal waveforms with Gaussian-shaped voltage pulses (with a repetition frequency of 13.56 MHz). By changing the Gaussian pulse width, electron heating can be directly controlled, allowing for an increased plasma density and ion flux for the same gas pressure and geometrical operating conditions. Analysis of the power deposition profiles and electron distribution functions shows that enhanced electron-sheath heating is responsible for the increased power absorption

Direct thrust measurements and modelling of a radio-frequency expanding plasma thruster

It is shown analytically that the thrust from a simple plasma thruster (in the absence of a magnetic field) is given by the maximum upstream electron pressure, even if the plasma diverges downstream. Direct thrust measurements of a thruster are then performed using a pendulum thrust balance and a laser displacement sensor. A maximum thrust of about 2 mN is obtained at 700 W for a thruster length of 17.5 cm and a flow rate of 0.9 mg s1, while a larger thrust of 4 mN is obtained at a similar power for a length of 9.5 cm and a flow rate of 1.65 mg s1. The measured thrusts are in good agreement with the maximum upstream electron pressure found from measurements of the plasma parameters and in fair agreement with a simple global approach used to model the thruster.Dr. K. Takahashi would like to thank the Faculty of Engineering at Iwate University (sabbatical year), the TEPCO Research Foundation in Japan, the MEXT in Japan (Grant-in-Aid for Young Scientists A 22684031), and the SP3 laboratory (Australian Research Council Discovery Grant No. DP 1096653) for financial support