John Rice on Plasma Rotation

PSFC senior research scientist answers questions about his recently published book

PSFC News

February 22, 2022

Senior Research Scientist John Rice’s career at MIT has spanned 50 years, beginning as an undergraduate and graduate student in Physics (S.B., ’75, Sc.D, ’79), and continuing as a research scientist working on MIT’s “Alcator” series of tokamaks, all designed to use high-field magnets to create a compact device. Much of his focus has been devoted to how plasma moves within the doughnut-shaped vacuum chamber, its transport and rotation. His expertise was awarded the 2010 Nuclear Fusion Journal Prize for his article “Inter-Machine Comparison of Intrinsic Toroidal Rotation in Tokamaks”. An APS Fellow since 2006, he is the former Chair of the US Transport Task Force, has served on the executive committees for Atomic Processes in Plasmas, and High Temperature Plasma Diagnostics, and has been an official US member of the ITPA Transport and Confinement Group since 2001. His book Driven Rotation, Self-Generated Flow, and Momentum Transport consolidates an understanding of the topic gained from years of experience at MIT.

What motivated you to write this book?

The genesis of the book was that in 2015, the journal Plasma Physics and Controlled Fusion asked me to write a review article about rotation in tokamaks. My dog Koko had just died and it was a welcome distraction to focus on something to get my mind off of her. Following publication of the review article, Springer Publishing approached me about writing a monograph on the subject, expanded to include rotation measurement techniques and momentum transport, as well as the most recent results on intrinsic rotation. With the blessing of Magnetic Fusion Energy Division Head Earl Marmar, I undertook the project. Intrinsic rotation has been most thoroughly studied on the PSFC’s fusion experiment Alcator C-Mod, and this book highlights 20 years of research on the subject performed in our own back yard.

What is rotation and why is this topic important to study?

Rotation (and its gradient) is important in tokamaks because it can stabilize deleterious magnetohydrodynamic modes, in addition to suppressing turbulence, which can lead to improved confinement. Traditionally, this rotation was provided by neutral beam injection - for Alcator C-Mod, there was no external momentum input, and the observed rotation occurred spontaneously. Intrinsic rotation is simply naturally occurring rotation in a tokamak plasma, without any source of momentum, otherwise known as self-generated flow. In 1996, Earl Marmar suggested that I install an old x-ray spectrometer that we had used on Alcator C onto a tangentially viewing port on C-Mod, and to our surprise, the plasmas were found to be strongly rotating around the torus at velocities up to 150 km/s (from here to Hartford in one second) without any external momentum input. And even weirder, the rotation can abruptly switch direction without any obvious reason. The largest chapter in the book discusses this in detail.

How will better understanding this topic impact research on MIT’s new fusion venture, SPARC?

The main goal of SPARC is to obtain high performance, reactor grade plasmas. One signature of high energy confinement is strong toroidal (around the doughnut) rotation. Measurements of rotation are important inputs into fundamental modeling of the plasmas’ confinement properties. One challenge for SPARC is that the temperatures will be very high, which will affect how we measure the velocity. In C-Mod, to measure the rotation velocity of high temperature plasmas we needed to inject an impurity, such as argon, to provide a source for measurable emission lines. The hydrogen in the plasma, because it is fully ionized, cannot be measured in this way. In SPARC we will need to inject elements with higher atomic numbers, like krypton or xenon, since argon, like hydrogen, would be fully ionized by the higher temperatures.

Topics: Magnetic fusion energy, Plasma science, Technology & engineering, SPARC, Alcator C-Mod tokamak