SUNIST TUTORIAL

FUSION PRINCIPLE

Fusion – a Fundamental and Future Energy Source

Nuclear fusion is the fundamental process that powering the Sun and stars. As known to all that human being depends upon the Sun, which give us light, heat and the air that we breathe. And the Sun powers the atmosphere to give us the winds and rain. Plants and animals that lived hundreds of millions of years ago and depended on the Sun for life formed the coal and oil that we used to generate electricity for light and power. The sun is a huge reactor where the fusion reactions take place, and we live upon the power provided by fusion in the core of the Sun.

To harness fusion seems very rewarding and promising. Since the industrial revolution, the world population has staggeringly expanded and society has become more and more dependent on energy supplies, along with which global demand for energy continues to grow year by year. Whereas the traditional energy sources such as coal, gasoline and natural gases are limited in the number. These fossil fuels take millions of years to form. Once they have gone, they have gone. People paid more and more attention to the emerging crisis of an energy deficit associated with the global environmental impact from greenhouse gases and limits on petroleum availability. Thus the need to find new sources of energy becomes increasingly important. Nuclear fusion shows significant advantages as a future source of energy. The fuels of fusion are abundant, and the fusion reaction is concentrated and inherently safe. Compared with fossil fuels and fission, fusion reactions produce no greenhouse gases or radioactive waste products.

For more information try http://fusedweb.pppl.gov/CPEP/Chart.html

The Fundamental Principle of Fusion

If light nuclei are forced together, they will fuse with a yield of energy because the mass of the combination will be less than the sum of the masses of the individual nuclei. According to Einstein's famous law "E = m c2", the difference in the summed masses of the fusion fuels and the fusion products is released in the form of energy. The ordinary fusion fuels are deuterium and tritium which are isotopes of hydrogen.

The fusion of deuterium and tritium is the most promising of the hydrogen fusion reactions . The reaction requires a temperature of approximately 40 million Kelvin to overcome the coulomb barrier and ignite it, and yields 17.59 MeV of energy. Hydrogen fusion on the earth could also make use of the following deuterium-deuterium fusion reactions:

Compared with the proton-proton fusion of the stars these reactions seem more promising for potential energy sources. Of all these reactions the deuterium-tritium fusion appears to be the most promising and has been the subject of most experiments. In a deuterium-deuterium reactor, another reaction could also occur, creating a deuterium cycle:

For more information try http://iter.rma.ac.be/en/physics/index.php

Lawson’s Criterion-the Requirement of the Realization of Fusion

To overcome the Coulomb barrier, a sufficiently high temperature should be provided to the particles. And the temperature must be maintained for a relative long time and with a sufficient ion density in order to obtain an appropriate net yield of energy for a fusion reaction. To yield more energy than is required for the heating of the plasma, the condition which concerned with ion density and confinement must be met, and which is called Lawson’s Criterion.

For deuterium-tritium fusion :

For deuterium-deuterium fusion :

To avoid dispersing and keep a high temperature, the fuel in the form of plasma should be contained in a limited volume and kept away from structural material. This is called a confinement. There are several confinement methods to realize fusion reaction. In the stars and galaxies, fusion reactions take place under the condition of huge gravity. While in some laboratories laser-beam-driven fusion is the focus of research. For potential nuclear energy sources for the Earth, the deuterium-tritium fusion reaction contained by some kind of magnetic confinement seems the most likely path. And for magnetic confined fusion tokamaks and stellarators seem promising devices.

What are Tokamak and Sperical Tokamak?

Tokamak is a type of experimental fusion reactor in which a spiral magnetic field inside doughnut shaped tube is used to confine the high temperature plasma produced during fusion. A spherical tokamak or ST (sometimes called spherical torus) has a much tighter ring shape, more like a cored apple (illustrated by the inner shape in the figure). Compared with the conventional device this innovative tokamak may have several advantages, such as lower aspect ratio, and one major advantage is the ability to confine a higher plasma pressure under a given magnetic field strength. Since the amount of fusion power produced is proportional to the square of the plasma pressure, by using spherically shaped plasmas, the development of smaller, more economical fusion reactors seems possible.

Sino-UNIted Spherical Tokamak Overview

Research Objectives and Roles of SUNIST

Sunist is short for Sino-UNIted Spherical Tokamak, which is the first spherical tokamak device in China. The objectives of the Sunist are to expand the understanding of low-aspect-ratio plasma, and sustain maintainable target plasma through non-induced startup. The SUNIST Lab was founded in 2004, consists of: Department of Engineering Physics, Tsinghua University (DEP), Institute of Physics, Chinese Academy of Sciences (IOP), and keeping very close collaboration with: Southwestern Institute of Physics (SWIP),Institute of Plasma Physics, Chinese Academy of Sciences (IPPAS).

A Model of the Torus

External view of the Torus

Main parameters

Aspect Ratio: 1.3, 0.3m/0.23m (Major Radius/Minor Radius)
Elongation: 1.6
Toroidal Magnetic Field: 1500G
Flux Swing of Ohmic Field: 60mV.s
Plasma Current: 50kA

Online Tour of the Sunist Facility

The SUNIST machine is a Spherical Tokamak device of approximately 1.4 meters in diameter and 2.2 meters high. At the heart of the machine there is a toroidal vacuum vessel of major radius 0.3 meters minor radius 0.23 meters. The device weighs less than 1000kg and is supported directly on the ground. The maximum plasma current is 50kA.

The main component of the magnetic field, the so-called toroidal field, is provided by 12 D-shaped coils surrounding the vacuum vessel. This field combined with that produced by the current flowing in the plasma, the poloidal field, form the basic magnetic fields for the tokamak magnetic confinement system. The massive forces created when the toroidal coils are energized are resisted by a tightly-fitted mechanical shell.

Additional coils positioned around the outside of the mechanical shell are used to shape and position the plasma.

Assembly of the vacuum vessel

The out radius of the vacuum vessel is 1.2 meters, and the inner radius (the radius of central solenoid) is 0.12 meters. The total bulk of the vacuum vessel is approximately 1 cubic meter. The surface of the vessel is about 2.3 square meters. Vacuum is initially achieved by the turbo molecular pump and maintained by the sputter-ion pump. The background pressure is about .The inner wall of vacuum vessel is processed, firstly by baking using PTC (Positive Temperature Coefficient Resistant) whose Curie point temperature is 160 Celsius, then through glow discharge and siliconization.

External view of the vacuum vessel

Picture of the first time siliconization

Duration: 1 hour Gas: He (8) + SiH4 (2) Pressure: ~ 0.5 Pa IGDC: ~ 0.8A

Designed magnetic surface

Control and Data Acquisition system of SUNIST

The control system of SUNIST is composed of industrial PC, timer with ISA interface, digital I/O and A/D. The working environment is MCGS configuration software. Charge and discharge processes of the three group of magnet electrical sources, surveillance of the running state and interlock protection of the electrical source, together with the diagnostic and startup signals of relevant system are the main objects of this control system.The data acquisition system of SUNIST has 32 A/D conversion channels. The maximum sampling rate is 2MHz.

Diagnosis of SUNIST

The most essential electromagnetic diagnostic tools have been installed on the SUNIST, including Rogowsky coils, flux loops, 2-D Mirnov magnetic probes, Langmuir probes, soft X-ray surface barrier diode array, etc. They can measure plasma current, loop voltage, loop flux, electron density and electron temperature and their fluctuation at edge, MHD instability analysis, equilibrium analysis, and so on.

SUNIST Operational History and the Future Plan

Non-induced startup of plasma current, 2003-2005
Edge plasma fluctuation, 2004-2006
Vertical field contribution to plasma current, 2005-2007
Plasma micro-instabilities, 2005-2007
Hardware upgrade of SUNIST, 2005
Wave and plasma on SUNIST, 2006-2009
Non-induced buildup and sustain of IP, 2006-2009
Hardware upgrade for RF system, 2006-2009

The SUNIST Facilities Site

SUNIST is a spherical tokamak located at Department of Engineering Physics, Tsinghua University, Beijing, P.R.China.