S14-3
Characteristic timescales in plasma control
制御に影響する特徴的時間	 
Yoshiteru Sakamoto, Shinzaburo Matsuda1, Satoru Sakakibara2
坂本宜照、松田慎三郎1、榊原悟2
Japan Atomic Energy Agency, Rokkasho Fusion Institute2-166 Omotedate, Obuchi, Rokkasho-mura, Kamikita-gun, Aomori, 039-3212, Japan
原子力機構	  六ヶ所核融合研究所	  〒039-3212青森県上北郡六ヶ所村尾駮表舘2-166
1
Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8550, Japan1
東京工業大学	  原子炉工学研究所	  〒152-8550東京都目黒区大岡山2-12-1
2
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu, 509-5292, Japan2
自然科学研究機構	  核融合科学研究所	  〒509-5292岐阜県土岐市下石町322-6

In order to maintain steady operation with a constant fusion power in DEMO, it is necessary to identify not
only the operation point but also the operation range surrounded by operation limit. The operation range
should be determined by considering plasma characteristic timescales, delay times of diagnostics andresponse times of plasma to actuators. In this presentation, the characteristic plasma timescales in magnetic
control, kinetic control and instability control are summarized together with objective of control, purpose,control variable and actuator.

1. IntroductionIn order to maintain steady operation with aconstant fusion power in DEMO, it is necessary toidentify not only the operation point but also theoperation range surrounded by operation limit.
Since the burning plasma is controlled by actuators
based on diagnostics signals, the operation range isdetermined by considering plasma characteristic
timescales, delay times of diagnostics and response
times of plasma to actuators. In this presentation,
the characteristic timescales of both tokamak andhelical plasmas are summarized.
2. Characteristic timescales in tokamak plasma
controlThe tokamak plasma control can be divided into
three categories that are magnetic control, kinetic
control and instability control. The role of magnetic
control is to maintain the plasma position and shapein order to avoid contact between plasma and firstwall. The role of kinetic control is to keep theplasma within the operation range, which consistsof controls of pressure, current profiles and divertor
detachment. The characteristic time scales intokamak plasma control are summarized in Table I
together with objective of control, purpose, control
variable and actuator. Although magneticmeasurements are required for magnetic control,
magnetic sensors should be located behind thebreeding blank due to severe radiation environment
in DEMO. Therefore, magnetic sensors are
surrounded by conductive structures and thenaffected by eddy currents, which leads to delayed
response and degradation of accuracy. In the case of
the delayed response close to or longer than thecharacteristic time scale of magnetic control,
special countermeasures are required for magneticcontrol. Special consideration is also required forthe short timescale of disruption
3. Characteristic timescales in helical plasma
control
The characteristic timescales in helical plasma
control are summarized in Table II. In the case ofhelical plasma, magnetic configuration is basicallydetermined by helical coil system. Therefore,
requirement of magnetic control is mitigatedcompared to tokamak. Controls of magnetic axis
and correction of magnetic shear profile might berequired in high beta operation regime, wherecharacteristic timescale of ~1 sec is controllable.
Since helical plasma can be operated by
self-ignition condition, kinetic control is basically
performed by pellet injection. Its characteristic
timescale of energy confinement time level iscontrollable from the viewpoints of the delay timesof diagnostics and response times of plasma topellets.
References[1] Program Committee of Technical Study on theDiagnostics for Control of the Fusion DEMO
Reactors: NIFS-Memo-68 (2014).

Table I. Characteristic timescales in tokamak plasma control
Category

Plasma positionand shape control(magneticcontrol)

Plasma pressure
and burning
control (kineticcontrol)

Instability control

Object ofcontrol
Clearance
betweenplasma surface
and first wall
Verticalposition

ControlvariableActuator

To avoid contact between
plasma and first wall

~ 0.1 sec

Magnetic
field

PF coilcurrents

To hold vertical position
within control range~ 0.1 sec

Magnetic
field

Plasma current

To keep total plasma current
constant~ 100 sec

Magneticflux

PF coilcurrents
CS coilcurrents,NBCD,
ECCD

Fuel

To keep density below its
limit (nGW),
To avoid NB shine throughA few 10 sec(τp level)

Density

Heating(pressure)profileTo keep fusion power
constant

A few sec
(τE level)

Plasma current
profileTo sustain high confinement
and to ensure MHD stability~ 100 sec(a few 10 sec inlocal)

Heat andparticle fluxes

To maintain divertor
detachment for protection of
first wall and diverter target

A few sec
(τE level)

NTM

To suppress NTM~ 10 sec

RWM

To suppress RWM∼ τW level

Disruption

To mitigate disruption~ ms – 50ms

PurposeCharacteristic
time scale

Neutron,density,temperature
Safety factor(magneticpitch angle)
Neutralpressure,
Radiations
MagneticislandRotation

Pellet,Gas puff
NB,
ECRF,
ICRF
NBCD,
ECCD
Gas puff
ECCD
NBI
Massivegas inj.

Table II. Characteristic timescales in helical plasma control
Category

Object ofcontrol

Plasma positionand shape control(magneticcontrol)

Magnetic axis

Plasma pressureand burning
control (kineticcontrol)

Fuel

Plasma current

Heat andparticle fluxes
Ideal mode

Instability control

ResistivemodeRadiation
collapse

PurposeTo avoid MHD instability, fastion loss, increase in helicalrippleTo suppress degradation ofmagnetic shear near edge
region due to toroidal current
To keep fusion power constant
To maintain divertor
detachment for protection of
first wall and diverter target
To avoid confinement
degradation by core MHDactivity
To avoid confinement
degradation by edge MHD
activity
To avoid plasma
disappearance

Characteristic
time scale

ControlvariableActuator~ 1 sec

Magnetic
field

PF coilcurrents~ 1 sec

Magneticflux

PF coilcurrents~ 0.1 sec(pellet interval)

Density

Pellet~ τE level

Neutralpressure,
Radiations

Gas puff~ 1 sec

Magnetic
field

PF coilcurrents~ 0.1 sec

Density,
Radiations

Pellet