Tokamaks and Spheromaks

Kevin Blondino

5 November 2012

“We say that we will put the sun into a box. The idea is pretty. The problem is, we don’t know how to make the box.”

-- Pierre-Gilles de Gennes

Summary of Fusion

Fusion of light nuclei into heavier ones releases energy.– Most popular candidate for reaction is the D-T

cycle. Others include D-D and D-3He.

Overwhelming majority hypothesize that magnetic confinement is the way to go.

Lawson criterion is to be met for fusion to be a viable energy source.

What is a Tokamak?

The confinement of plasma as a torus using external magnetic fields.– Field lines are required to move around the torus

in a helical shape, generated by a toroidal and poloidal field.

Most popular candidate for thermonuclear fusion.

Russian acronym for “toroidal chamber with magnetic coils.”

The overall goal is to produce a magnetic field that follows around the torus while also wraping around it.

History of the Tokamak

Invented in the 1950’s by Igor Tamm and Andrei Sakharov in the Kurchatov Institute.

Introduced to the public in 1968, with results that demolished competition from every other design.

Still the most developed for fusion: ITER, NSTX, Pegasus Toroidal Experiment, and many more.

What is a Spheromak?

Confinement of plasma through self-induced magnetic field– The current due to the flow of plasma creates a

magnetic field, which in turn, confines it.

Less popular candidate for fusion Considered a compact toroid Name is the arrangement of plasma, not the

device that generates it

History of the Spheromak

Initially developed to study magnetohydrodynamical waves in astrophysical plasma in 1959 by Hannes Alfvén (Alfvén waves).

ZETA machine provided boom in design ideas, including the spheromak.

By the 1980’s, tokamaks surpassed confinement times by orders of magnitude.

SSX and SSPX in 1994

Pros

Tokamak:– Highly scalable– Relatively simple to control and model

Spheromak:– Much less upkeep power required– Generally smaller– No complicated magnets required

Cons

Tokamak:– Large power requirement due to “brute force”

method– Cryogenics required for superconducting magnets

Spheromak:– Hard to scale up– Plasma behavior is complex and hard(er) to

predict and control

General Problems with Fusion

High energy neutrons could be damaging Turbulence! – something not quite understood

– Some hypothesize that there is some underlying quantum mechanical effect that has not been taken into account or not yet discovered that causes it.

– Sputtering – when higher mass particles are mixed into the fuel, lowering its temperature.

“…squeezing a balloon – the air will always attempt to pop out somewhere else.”

KSTAR

Korea Superconducting Tokamak Advanced Research at the National Fusion Research Institute in Daejon, South Korea.

Completed in 2007; first plasma in July 2008 Features fully superconducting magnets Uses hydrogen and deuterium fuels (D-D

cycle), but not deuterium-tritium

ITER

International Thermonuclear Experimental Reactor being built in southern France; will be the largest and most powerful tokamak

EU, India, Japan, China, Russia, South Korea, and the US is funding and running

Designed to produce 500 MW output for 50 MW input

First plasma production scheduled for 2020 DEMO, the successor to ITER, will be the first power

plant scheduled to make fusion energy in 2033.

SSPX

Sustained Spheromak Physics Experiment at Lawrence Livermore National Lab

Completed in 1999 One of the only spheromaks actively

researching fusion

References

http://plasma.physics.swarthmore.edu/ssx/ http://en.wikipedia.org/wiki/Tokomak http://en.wikipedia.org/wiki/Spheromak http://www.iter.org/ https://www.llnl.gov/str/Hill.html