Thesis submitted

[jamesr]

After 3 years and a lot of procrastination I have finally submitted my PhD thesis on “The Geodesic Acoustic Mode in strongly shaped tight aspect ratio tokamaks”

Some interesting results matching experiment to simulations and theory, and all three agree (almost).

Briefly, plasma turbulence is the primary cause of heat and particle losses in tokamaks and other magnetic confinement devices. In certain circumstances non-linear interactions such as vortex stretching in the turbulence can lead to the formation of zonal flows. These zonal flows are analagous to the jet streams on earth or the bands on Jupiter. The zonal flow then forms a barrier to the transport of heat out of the tokamak (just as the jet streams form a barriers in the flow of heat from the equator to the poles). They also form a sink for the turbulent energy – the interplay between the turbulence and flows then forms a self governing predator-prey kind of relationship.

The geodesic acoustic mode (GAM) is an oscillation in due to the compressibility of the zonal flows and couples via three-wave interactions with the turbulence forming a further sink for the turbulent energy. So studying the frequency response of the GAM can give an insight into the underlying flows and turbulence.

A major instability when running tokamaks in high confinement or H-mode with external heating is the Edge Localised Mode (ELM). This is a partial collapse of the edge pressure gradient (think avalanche) which expels hot plasma to the walls and divertor. This is of particular concern for ITER, where the amount of energy available to an ELM is enough to case serious damage to the walls and divertor. The current method proposed for mitigating ELMs is to introduce magnetic perturbations to the edge of the tokamak, stopping the build-up of large ELMs and instead causing more frequency but smaller disruptions, similar to the way avalanches are controlled by purposefully setting off lots of smaller ones.

My research showed however, that the rotation induced at the edge of the plasma due to these resonant magnetic perturbations causes the frequency of the geodesic acoustic mode to increase and then be damped. Once damped the sink for the turbulence is removed and the level of turbulent heat transport can increase, lowering the desired confinement.

A write-up of my last paper is at: http://iopscience.iop.org/0741-3335/labtalk-article/51084

[jamesr]

Oh and of course I found a place to mention pB11 fusion and dense plasma focus’ in the introduction, citing Eric’s 2011 paper.

[Lerner]

Congratulations! This work shows again how attempt to supress one kind of instability funnels energy into another one.

[benf]

Great to hear of your accomplishment James! The preparation time has been arduous but you’ve made it! I have just been reading about disruption mitigation studies (you’re probably familiar with) and some of the proposed solutions to control instabilities, such as injecting fractured cryogenic pellets into the plasma! The problems of “controlling” instabilities appear daunting.

Your research lends greater clarity to the processes going on in tokamak generated plasma fields, glad you were also able to incorporate Eric’s research as well.

[Warwick]

Congratulations James! Are you intending to do a postdoc next?

Are you still near Cov? (reply as pm)

Warwick

[Brian H]

Lerner wrote: Congratulations! This work shows again how attempt to supress one kind of instability funnels energy into another one.

Indeed; confined fusion whack-a-mole!

[jamesr]

Passed my viva today! So am now Dr Robinson ! yay! (with the expected few minor corrections).

Although my research has been on a particular aspect of tokamak related fusion it has always been the big picture of the goal of achieving fusion by whatever means that has inspired me. I Just want to take this opportunity to thank the contributors to this forum over the years to keep me thinking about the issues not just from the scientific perspective, but also the societal implications of fusion in the wider world.

[Henning]

Congratulations Dr Robinson! 🙂

[vlad]

Congratulations!

[Breakable]

Great job Dr Robinson, now you can research whatever you want 😉
http://1.bp.blogspot.com/-dHEMxzxwenw/TlJ2vNA3bsI/AAAAAAAAAOo/yXJegVapaHE/s1600/phd072011s.gif

[AaronB]

That’s great news! Congratulations!

[JimmyT]

Here’s to you Dr Robinson! Congrats.

[Author]

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