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Research project: On steady state helicity injection.

Duration: November 2008-March 2009

Advisor: Prof. Paul M. Bellan

The magnetic helicity conservation equation in MHD implies that resistive MHD equilibrium having closed flux surface is not possible. It further implies that magnetic helicity can never be injected into the plasma in steady state. Also, magnetic helicity injection in a sustained spheromak depends on a time dependent change of open flux surfaces to closed flux surfaces.

Files: Chapter 2 of Thesis

Research project: Alfvenic jets associated with spheromak formation.

Duration: November 2006-March 2009

Advisor: Prof. Paul M. Bellan

Spheromak formation in most pulsed spheromak experiments consists of a series of dynamic steps whereby highly localized plasma near the electrodes evolves towards a Taylor state equilibrium. The dynamical evolution stage is often modeled as a series of equilibrium states. However, the experiments at the Caltech Spheromak facility have revealed that non-equilibrium Alfvenic flows are driven during these preliminary stages by unbalanced J × B forces. The flow velocity was measured using time of flight measurements using a novel He-Ne density interferometer. The flow velocities depend on the gas species inertia and lead to a collimated plasma jet with β_thermal ∼1(P. M. Bellan, Phys. Plasmas 10 Pt2, 1999 (2003)).

With high neutral pressure in the fast gas valves, a layer of neutral gas with density ∼10²³/m³ and thickness ∼1 cm is observed in front of the plasma jet. The neutral gas density in the layer was estimated using the Gladstone-Dale relation.

Publication: Phys. Rev. Lett. 103, 105003 (2009); DOI:10.1103/PhysRevLett.103.105003

Video: Thesis defense video. This is a large file. I would suggest downloading it and playing it on VLC media player.

Files: Chapter 5 of Thesis

Research project: The effect of ignitrons on the discharge circuit of the Caltech spheromak experiment.

Duration: August 2006-May 2008.

Advisor: Prof. Paul M. Bellan

Ignitrons are often used in many pulsed pulsed power experiments. In the Caltech spheromak experiment, low inductance ignitrons are used to switch high voltage capacitor bank (∼118 μF,∼1 kJ) across low impedance plasma (∼4 mΩ,~50 nH) in ∼30 μs . However, the impedance of the ignitron significantly exceeds the plasma impedance in the Caltech spheromak experiment. This leads to an inefficient coupling of energy from the capacitor bank to the plasma (~20%). The fraction of the capacitor bank energy coupled to the magnetic field of the spheromak is even lower (~5%). These trends in energy efficiency have also been experienced in other pulsed plasma experiments. Also, because of the relative impedances of the plasma being smaller than the ignitron, the discharge circuit behaves like a current source.

I conducted various experiments to measure/estimate the circuit parameters (impedance of ignitron, plasma and cables) of the discharge circuit of the Caltech Spheromak Experiment.

Files: Chapter 3 of Thesis

Research project: Design of an X-ray imaging system to study reconnecting flux tubes

Duration: June 2006-January 2009

Advisor: Prof. Paul M. Bellan

Merging and reconnection of flux tubes have been studied at the Caltech solar coronal loop simulation experiment. It was observed that merging of adjacent flux tubes in counter-helicity configuration resulted in significantly more X-ray radiation than with the co-helicity situation (Hansen, Tripathi and Bellan 2004). An X-ray imaging system will help localize the source of the radiation.

An existing high speed X-ray camera was upgraded for the experiment. The camera consists of a pin hole with an optional foil filter, a Micro Channel Plate (MCP) intensifier, and a phosphor imaging surface. The MCP intensifier can be triggered for short duration (~10 ns), thus providing very high time resolution compared to the time scale of the experiment (~10 μs). The MCP intensifier plate is sensitive to photons in the UV to hard X-ray band. The image from the phosphor screen is captured by a CCD camera.

Video: Thesis defense video. This is a large file. I would suggest downloading it and playing it on VLC media player.

Files: Chapter 6 of Thesis

Research project: Design of a quadrature phase interferometer to measure plasma density.

Duration: September 2003-December 2006.

Advisor: Prof. Paul M. Bellan

I developed a quadrature phase homodyne He-Ne interferometer to measure the plasma density in the Caltech Spheromak Experiment. The interferometer was based on the technique suggested by Buchenauer and Jacobson(1977).

Later, I built a novel heterodyne He-Ne interferometer. The design of the interferometer exploited the periodic nature of the laser phase auto-correlation to have significant path length difference between the scene and the reference beams. The interferometer measured phase shifts ~300^o with phase error ~ 1^o. The interferometer measured plasma densities ~10²²/m³ which corresponded to β_thermal ~ 1, during spheromak formation.

Video: Thesis defense video. This is a large file. I would suggest downloading it and playing it on VLC media player.

Publication: Rev. Sci. Instrum. 77, 083503 (2006); DOI:10.1063/1.2336769

Files: Chapter 4 of Thesis

M.S. thesis: Optimal finite alphabet sources over partial response channels.

Duration: November 2001-June 2003

Advisors: Prof. Scott L. Miller and Prof. Krishna R. Narayanan

The performance of the codes, whose power spectrum matches the channel frequency response was analyzed from an information theoretic perspective. A new coding scheme was proposed, which offers substantial improvement over existing schemes for low rate transmission. This scheme is also asymptotically (for rates tending to 0) optimal coding scheme for transmission over ISI channels.

Files: Thesis. Thesis defense presentation. Detailed preliminary version of thesis defense presentation.

Senior year thesis: Evoked potential estimation in time-frequency domain.

Duration: July 2000-April 2001

Advisors: Prof. V. M. Gadre and Prof. S. R. Devasahyam

The project aimed at improving the SNR and estimation of the latency jitter in various epochs of Evoked Potential measurements. A comparative study of the various signal processing techniques available to do the same was done. These techniques included adaptive filtering, wavelet based thresholding and signal subspace decomposition (using filter banks, Karhunen-Loeve expansion and matched pursuit technique). The signal was filtered using `Db6' wavepackets and the components were adaptively filtered.

Files: Senior year thesis. A research paper submitted in the doctoral category at BIOVISION 2001.

Junior year seminar: Smart antenna systems.

Duration: January 2000-March 2000

Advisor: Prof. R. K. Shevgaonkar

A literature survey was conducted on the various problems faced in the wireless communications, and the use of smart antenna systems and space-time signal processing techniques to counter the problems. CDMA based mobile communication was discussed as an application of Smart Antenna Systems.

Junior year electronic design project: Prosthetic hand control using audio cues.

Duration: January 2000-May 2000

Advisors: Prof. R. Lal and Prof. S. R.Devasahayam

A circuit for control of an artificial limb depending on the duration/type of a whistle was implemented. The circuit was quite robust to changes in the ambiance. The project has been successfully tried out on an impaired person.

Files: Report.

Internship at Western Outdoor Interactive Pvt. Ltd., Mumbai.

Duration: May 2000-June 2000

Coordinator: Mr. Amit Shah

A palm pilot was interfaced to a desktop using the Infra Red port. The desktop had the operating system Windows 3.1. The project was used to assist aircraft maintenance staff.

Yantriki.

"Yantriki" was the annual robotics competition held at IIT Bombay. Currently, robotics competitions at IIT Bombay are held as a part of the annual Technology Festival - Techfest.

In a team of four students, we developed a "carrom miner" for Yantriki 1998. The machine designed to strike carrom coins. It was developed by mostly scrap material, for e.g., gear boxes from old toys, motors from tape players and had its controls in a lunch box.

For Yantriki 1999, I was a part of a team of fours students who built an autonomous android, "A Sumo Wrestler." It was also built from mostly scrap material and was controlled by a simple C code. It was interfaced to the computer through the parallel printer port.