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Center for Fusion Science
Introduction
 At
the beginning of the twenty-first century mankind is faced with the
serious problem of meeting the energy demands of a rapidly
industrializing population around the globe. This, against the
backdrop of fast diminishing fossil fuel resources (which have been
the main source of energy of the last century) and the increasing
realization that the use of fossil fuels has started to adversely
affect our environment, has greatly intensified the quest for
alternative energy sources. In this quest, fusion has the potential
to play a very important role and we are today at the threshold of
realizing net energy production from controlled fusion experiments.
Fusion is, today, one of the most promising of all alternative
energy sources because of the vast reserves of fuel, potentially
lasting several thousands of years and the possibility of a
relatively ‘clean’ form of energy, as required for use in
concentrated urban industrial settings, with minimal long term
environmental implications. The last decade and a half has seen
unprecedented advances in controlled fusion experiments with the
discovery of new regimes of operations in experiments, production of
16MW of fusion power and operations close to and above the so-called
‘break-even’ conditions. A great deal of research has also been
carried out in analysing various socio-economic aspects of fusion
energy.
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The D–T power plant
 In
the D–T reaction, a deuterium and a tritium nucleus produces aHe4
nucleus (namely an alpha particle), with a kinetic energy of 3.5MeV,
and a 14MeV neutron. The alpha particle is positively charged, and
hence interacts with the other charged particles in the plasma; the
neutron does not interact and escapes the plasma region.
The neutrons produced in the D–T reaction carry four fifths of the
released energy beyond the plasma facing components into a stopping
region or blanket, where they are slowed down,
thereby heating the blanket. Coolants circulating within the blanket
and the plasma facing components transfer the heat out of the
reactor area to produce steam and to generate electricity in a
conventional way (see figure 1). So in the case of a fusion reactor,
the heat is not extracted from the thermonuclear burning fuel, but
from the blanket and the first wall. The blanket also serves another
essential purpose in a D–T reactor: producing, or ‘breeding’ the
tritium fuel required by the reactor.
The D–T reaction is self-sustained—i.e. the plasma is ignited—when
the alpha particles, which carry the remaining fifth of the energy
released per reaction, remain confined long enough within the plasma
to transfer their kinetic energy to other confined nuclei through
collisions, and the energy confinement of the plasma is sufficiently
good so that heating by these alpha particles can maintain it at the
required burn temperature. This fraction of the energy eventually
impinges as radiation and energetic particles on the first solid
wall facing the plasma.
In order to achieve ignition in a 50–50 D–T plasma, a temperature
between 10 and 20 keV (that is, between about 100 and 200 million
degrees centigrade) is needed and the condition on the triple
product is [20] nDT(0)Ti(0)τE > 6 × 1021 m−3 keVs. In a reactor, of
course, more power has to be produced by the thermonuclear reactions
(fusion power) than is spent in maintaining the conditions in which
these reactions occur (input power). It is useful to define a
quality factor Q as the ratio of the fusion power to the external
input power: Q thus describes the ‘gain’ in power. In an ignited D–T
reactor, the power necessary to sustain the nuclear reactions comes
entirely from within the plasma itself,
through alpha particle heating. In that case, once ignition is
achieved, no external input power is needed, and Q=∞. It is however
quite possible to operate a reactor without achieving ignition. In
that case, the alpha particles do not provide all the heating for
the D–T mixture, and extra power must be applied at all times to
heat the plasma. This means feeding back some of the reactor power
to heat the plasma: Q has then a finite value, and the reactor is
said to operate in a ‘driven’ regime.
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Progress in fusion science and technology
The past decade and a half has seen a number of major
advances and exciting discoveries on all the different routes to
controlled fusion. The key parameters for measuring the progress are
defined by the fusion triple product nTτE. This product now stands
at 1.5 × 1021 keVsm−3. The progress of the achieved values of the
triple product over the past decades and the projected value of the
next-step device, ITER, is shown in figure 3. Up to 16 MW of fusion
power has been produced for several seconds for the first time, and
several concepts of high heat and particle flux handling have been
tested. Major discoveries include the sudden transition of the
plasma into a state with greatly reduced transport loss of heat.
This has significantly reduced the required size of a prototype
reactor, making it feasible and cost-effective.
Several new technologies have been developed. New heating schemes as
well as heat removal schemes and successful development of extremely
powerful lasers are some of the key technological advances of this
decade.
While new and alternative magnetic confinement schemes are
continuously developing, the tokamak approach has reached vital
milestones in (a) pushing the nTτE product ever closer to that for
ignition, (b) increasing the figure of merit Q equivalent for a D–T
plasma beyond unity and (c) demonstrating near steady-state
operation without disruptions, with improved confinement and
acceptable power and particle exhaust.
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The international collaboration:
ITER
1. Introduction of ITER
The ITER(formerly the short form for the International Thermonuclear
Experimental reactor and meaning ‘the way’ in Latin) project [13a],
which had been initiated at the time of the previous status report,
has been successfully pursued under IAEA auspices. The overall
programmatic objective of ITER is to demonstrate the scientific and
technological feasibility of fusion energy for peaceful purposes in
the shortest possible time and to investigate fusion energy science
phenomena with long term implications. ITER would accomplish this by
demonstrating the controlled ignition and extended burn of
deuterium–tritium plasma, with steady state as an ultimate goal, by
demonstrating technologies essential to a reactor in an integrated
system and by performing integrated testing of some of the
high-heat-flux and nuclear components required to utilize fusion
energy for practical purposes.
ITER binds together two major strands in the development of fusion
as a source of energy: the exploitation of the established potential
of the tokamak configuration to reach reactor conditions and use of
international collaboration as the means to share the burden of
costs and to accelerate progress by pooling resources and expertise.
2. Present status of the ITER programme.
 The Conceptual Design Activity for ITER was already completed at the
beginning of the last decade, and the EDA had started. The original
EDA of ITER was completed by the participating Parties (EU, Japan,
Russian Federation and USA) in July 1998. During the EDA, the
Parties decided to prepare a detailed, complete engineering design
of ITER and to collect all providing the first comprehensive design
of a fusion reactor based on well-established physics and
technology. However, towards the end of the EDA it was recognized by
the Parties that due to financial constraints, it was difficult to
procure a financial commitment towards the construction of ITER. The
parties recognized at the same time that the original goal for ITER
as a ‘one step’ to the DEMO fusion reactor was still valid. Thus it
was decided to attempt a redesign of ITER keeping the original goals
intact as much as possible, but reducing the cost of construction to
about 50% of that envisioned in the EDA. A Special Working Group of
the Parties developed the new technical guidelines [108] for
minimizing costs by reducing the goals, but still retaining the
overall programme objectives of the ITER EDA agreement. These
guidelines were the following.
For plasma performance;
• achieve extended burn in inductively driven plasma with Q ≥ 10 for
a range of operating scenarios with a pulse duration sufficient to
achieve stationary conditions with respect to all characteristic
plasma time scales,
• aim at demonstrating steady-state operation using a noninductive
current drive withQ > 5 and
• the possibility of controlled ignition should not be precluded.
For engineering performance;
• demonstrate the availability and integration of essential fusion
technologies,
• test components for a fusion reactor and
• test tritium breeding module concepts.
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