24P110-P
Requirements for the Control of DEMO Reactor and its System Research andDevelopment
核融合原型炉に求められる制御要件とシステム開発
Shinzaburo Matsuda
松田慎三郎
Tokyo Institute of Technology
2-12-1-N1-26 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
東工大 〒152-8550 東京都目黒区大岡山2-12-1-N1-26

The mission of the DEMO reactor is to demonstrate electricity production in a power plant level.
The key issue for the control of the power plant is how to maintain a constant fusion power output.
However, the diagnostic measurements would provide poor information due to a heavy DEMO
neutron and gamma radiation environment. For compensation, the control system has to besupported by the intelligent simulation codes that predict plasma behavior. A long range program
for the codes development has to be established as one of the most critical R&D for the DEMO
reactor. It is also required for the fusion researchers to make research plans with a clear recognitionof code validation toward the DEMO.

1. Introduction
There are several critical issues toward DEMO
fusion reactors. These issues were discussed more
extensively in a published text book on fusion
reactors [1]. However, because of the urgent issues
for on-going programs or for ITER issues, it has notbeen discussed for quite a long time on how we cancontrol the fusion power plant in a steady state. It
does not mean that the issue is the tokamak current
drive. We assume that any problem on the current
sustainment facing today can be solved by the timeof DEMO stage. Yet we have to recognize theimportance of the control of power plants forelectricity generation.
2. General Requirements
As decided by the report of the Japan Atomic
Energy Committee, the mission of the fusion
DEMO reactor is to demonstrate electricity
generation in a plant scale [2]. It is also understoodthat the DEMO is the last integrated R&D machine
before commercial reactors [3]. This requirement
imposes an additional mission to the DEMO reactor,
namely, “an appropriate level of economical
perspective leading to commercialization”.
The DEMO plant will have control knobs such ascoil currents, heating/current drive system power,
neutral gas input, etc. How can we keep theconstant fusion output by controlling these knobs?
To do so, the control system has to know theresponse, an explicit form of the function in termsof variants of actuators . The feedback or feed
forward control signal should be continuously sent

to keep( , , , ….) = const. The machine
operating reference point will be chosen on themulti-dimensional curved surface of the function. Thus, by the time of the start of operation of
DEMO reactor, a functional form for the plasma
control has to be known and defined clearly.
This simple consideration will lead to more
pragmatic requirements.
1) The multi-dimensional surface should be as
simple as possible. This means that the number ofactuators should be minimized and control logic
must be simple to avoid conflicts among actuators.
2) The operational limit has to be known inadvance on this multi-dimensional surface toprotect the machine from abnormal event such as
disruption, sudden termination of the plasma. With
the help of such information, the operation point
will be chosen at a reasonable distance apart from

Fig. 1 Control system for DEMO with intelligent
plasma simulator

the operational limit.
3. The Environment for the DEMO Reactor
In order to satisfy the requirement “appropriatelevel of economical perspective” for DEMO,
following consideration has to be incorporated into
the DEMO design.
1) The tritium should be self supplied. Thisrequires a high tritium breeding ratio, in a range of
1.05 overall.
2) The machine should be operated at a higher
plasma pressure than ITER in a steady state mode.
One may note that the DEMO reactor is the first
integrated machine of all the components needed
for the commercial reactors, but it is also the first
integrated machine to be operated in steady stateand truly burning state at high Q value (The ITERwill be operated only at Q~5 in steady state mode.).
Therefore, the neutron environment is much harder,
higher than ITER 5 times by flux, and 100 times inorder by fluence.
Such an environment will allow the diagnostic
system to a restricted space and location. In
particular, the position of the magnetic diagnostic
system such as magnetic probes or loop coils tocontrol the position of the plasma would be of agreat concern.
If information by the diagnostic system is muchpoorer than we could expect in ITER, in morestraightforwardly, if the magnetic measurements
can be done only at an insensitive location to theplasma behavior, how can we operate the power
plant in steady state with confidence? What cancompensate the lack of diagnostic information
against DEMO plasma ?
4. Control System of the DEMO Reactor
The Fig.1 shows a possible simple structure of
the DEMO control system. The central control
system can control all the actuators based on theinformation measured by the diagnostic system. It
may be inevitable for the simulation code system
plays an essential role of the DEMO control system
to compensate information and predict plasmabehavior in real time.
Nowadays a variety of simulation codes havebeen developed and are under development toanalyze and understand toroidal plasma, but thecodes required for the DEMO control have toprovide what is going on inside the plasma on thebasis of insufficient data and also predict precisely
the behavior of the plasma. Such codes should behighly reliable, accurate and fast response which
can cover every behavior of the plasma. In otherwords, every plasma behavior must be fully studied
and expressed by these codes by that time.
So the issue is how to develop such a huge codes
system and how to validate these codes before theactual use in the DEMO operation. Because this is
not the issue of choice nor issue of efficiency, thenation that wants to have a DEMO machine must
develop such codes as essential element of themachine, just as R&D on the superconducting
magnet or tritium breeding blanket. Any existingmachine or future machines such as ITER or
satellite tokamaks should be used for code
validation.
5. Impact on the DEMO Staged Operation
Even with a sufficient development andvalidation of the codes before the completion of theDEMO reactor, one may encounter a contradictory
question whether the developed codes canaccurately express the DEMO plasma without
validation by the DEMO machine itself. TheDEMO construction and operation plan has to take
into account of this issue at an early stage..
6. Proposed Study on the Issues
Those issues discussed above are based on theforeseen difficulty of diagnostics of the DEMO
reactor. Therefore, primary efforts have to befocused to study a radiation environment a little
more precisely. Therefore, the proposal is1)
To study and develop a DEMO radiation
map and find possible access of the essential
diagnostics, assess their technical feasibility andpotentials and identify necessary R&D.
2)
For the development of the codes, try todevelop a long range development program, andpropose sharing and collaboration to theinternational society, since a level of resources
needed would be much bigger than thoseenvisaged until now.
Acknowledgments
The author would like to express his appreciation
to the members of the JSPF special committee on
fusion reactor textbook for their helpful discussion.
References[1] “Text Fusion Reactors” JPFR
87 Supplement
2011..
rd[2] “The 3 Phase Basic Program on Fusion

Research and Development” Atomic energy
Commission, 1992.
[3] “C&R of the 3rd Basic Program of Fusion
Research and Development” Atomic Energy
Commission, Nov. 2005.