Fusion Reactor Technology I

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F usion Reactor Technology I

(459.760, 3 Credits)

Prof. Dr. Yong-Su Na

(32-206, Tel. 880-7204)

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Week 1. Magnetic Confinement

Week 2. Fusion Reactor Energetics (Harms 2, 7.1-7.5)

Week 3. How to Build a Tokamak (Dendy 17 by T. N. Todd) Week 4. Tokamak Operation (I): Startup

Week 5. Tokamak Operation (II):

Basic Tokamak Plasma Parameters (Wood 1.2, 1.3) Week 7-8. Tokamak Operation (III): Tokamak Operation Mode Week 9-10. Tokamak Operation Limits (I):

Plasma Instabilities (Kadomtsev 6, 7, Wood 6) Week 11-12. Tokamak Operation Limits (II):

Plasma Transport (Kadomtsev 8, 9, Wood 3, 4) Week 13. Heating and Current Drive (Kadomtsev 10)

Week 14. Divertor and Plasma-Wall Interaction

2

Inherent drawback of Tokamak!

Faraday‘s law

ds dt B

v d

S

⋅

−

= ∫

4

(5)

Steady-state operation d/dt ~ 0

Plasma Current

CS Coil Current (kA)

(MA)

t (s)

Steady-State Operation

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6

Tokamak Operation Modes

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q-profiles

0 1 2 3 4 5

0 0.5 1

strong

weak Reversed

shear

r/a

z Good confinement

z Poor stability

H-modeELMy

Tokamak Operation Modes

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q-profiles

0 1 2 3 4 5

0 0.5 1

strong

weak Reversed

shear

r/a

z Good confinement

z Poor stability

H-modeELMy

z Only “weak” RS plasmas are stable but they require a

delicate active control 8

Tokamak Operation Modes

(9)

Reversed Shear Mode

• Hollow current profile

•

Higher pressure gradient region in the core with steep edge pedestal

• Reversed q-profile

• With negative magnetic shear

High bootstrap current!

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10

Reversed Shear Mode

Reversed shear mode H-mode

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Reversed Shear Mode

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12

Reversed Shear Mode

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Turbulence Stabilisation

• Formation of internal transport barriers to improve confinement - Reversed magnetic shear

- Differential rotation (input power) Stabilises turbulence

xB

• One reason:

Losses of fast ions at the plasma edge Î sheared radial electric field

Î sheared ExB rotation

Î eddies get tilted and ripped apart cause turbulence suppression!

⊗ B

E

(14)

Turbulence Stabilisation

Z

R

Z

R

Contour lines of electric potential

.

Contour lines of electric potential

.

ITB

Temperature profile

• Formation of internal transport barriers to improve confinement - Reversed magnetic shear

- Differential rotation (input power) Stabilises turbulence

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Turbulence Stabilisation

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16

Reversed Shear Mode

Reversed shear mode H-mode

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Non-monotonic current profile Turbulence suppression

High pressure gradients Large bootstrap current

• Operation at lower plasma current: f

_BS ~ β_p ~ I_p^-2

→ Confinement degradation: τ_E ~ H₉₈(y,2) I_p^0.93

→ To get enough fusion power: H₉₈(y,2) > 1 (advanced)

Reversed Shear Mode

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•

Plasma current diffusion into the core from the edge

Current and pressure profile control !

Sustainment of Non-monotonic Current Profile

18

Current density

q-profile

Plasma radius edge centre

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Non-monotonic current profile Turbulence suppression High pressure gradients Large bootstrap current Non-inductive current drive

Current drive and current profile control

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20

Toroidal direction

Ion gyro-motion

Fast ion trajectory

Poloidal direction

Projection of poloidally trapped ion trajectory

R B

• Bootstrap current

http://tfy.tkk.fi/fusion/research/

ASCOT

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Current drive and current profile control

z But more & faster particles on orbits nearer the core (green cf blue) lead to a net “banana current”

Currents due to neighbouring bananas

largely cancel

orbits tighter where field stronger

dr J

_boot

~ dP

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22

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Non-monotonic current profile Turbulence suppression High pressure gradients Large bootstrap current Non-inductive current drive

Cf. NTM control

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time I_p

III: Performance at stable q(r)

I: Reverse q(r)

II: Create ITB q₉₅~ 5

I: Heat during current rise, external current drive (reverse q).

II: Increase heating power to stabilise turbulence (ITB).

Improve plasma confinement, try to increase pressure (β

_N

)

III: Keep going: ITER non-inductive regime: H

_H

≈1.6; β

_N

≈3.0

(ITER: 9MA, 50% external current drive (73MW), 50% bootstrap fraction)

Technique used since mid 1990´s

Reversed Shear Scenario

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I: Form q(r), II: create ITB, III: But discharge terminates (unstable)

0 10

20

0 1 2

0 5 10

0 2 4 6

3 4 5 6 7 8

0 1 2 3

0 10 20 30

[keV][MA][MJ] [MW][1016 /s][arb.]

E040259

P

_NB

I

_P

P

_EC

T

_io

T

_eo

Time [s]

W

_dia

neutron rate m / n = 4 /1 interchange mode

current hole

I: preheat

II: create ITB

JT-60U ^(R₀=3.3m. a=0.8m. )

(26)

• Formation of an ITB at low n

_e

, with 15 MW NBI power

T

_i

> T

_e

, high rotation shear

• ITBs are relatively short lived, only few τ

_E

• Good, transient performance:

H

₈₉

~3, β

_N

~ 3

• ITB not compatible with H-mode edge barrier and large ELMs

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(R₀=1.7m. ,a=0.6m. )

• Less RS

• H

_H98(y,2)

= 1.37

• β

_N

= 2.62

• q

₉₅

= 5

• „Weak“ ITBs

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MSE, Magnetic Probe Measurements Controller

j (r) from Analysis Methods NBI

• Current density profile control at ASDEX Upgrade

28

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•

Modulation combinations of actuators (NBI, LH, ICRH) to infer the coefficients of the state space model of the slow loop.

•

RT Current and Pressure Profile Control

•

Simultaneous control of distributed magnetic and kinetic paramters

•

Dedicated experiments to identify controller coefficients

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RT Current and Pressure Profile Control

• Real time pressure profile and

q-profile control to keep ITB steady

Mazon et al, PPCF 2002

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q_MSE q_min q_min,ref ΔP_LH

MSE LH

MSE

LH

y Transport reduction

at t = 12.4 s

y Time delay in response of q_min

j_OH or j_BS change

Time (s)

q minP LH(MW)P NB(MW) β_N

ref.

command T e(keV)

r/a~0.2

0.4

0.6

•

JT60-U: Real time q_min control with MSE diagnostics and LHCD

Fusion Reactor Technology I