# Presentation: 534425

## Critical Physics Issues for Tokamak Power Plants

**D J Campbell****1****, F De Marco****2****, G Giruzzi****3****, G T Hoang****3****, L D Horton****4****, G Janeschitz****5****, J Johner****3****, K Lackner****4****, D C McDonald****6****, D Maisonnier****1****, G Pereverzev****4****, B Saoutic****3****, P Sardain****1****, D Stork****6****,****E Strumberger****4****, M Q Tran****7****, D J Ward****6**
**	****1 **EFDA, CSU Garching, Germany
- 	2 Association Euratom-ENEA Frascati, Italy
- 	3 Association Euratom-CEA, Cadarache, France
- 	4 Association Euratom-Max-Planck-Institut für Plasmaphysik, Garching, Germany
- 	5 Association Euratom-Forschungszentrum Karlsruhe, Germany
- 	6 Euratom-UKAEA Fusion Association, Culham, United Kingdom
- 	7 Association Euratom-Confédération Suisse, Lausanne, Switzerland

*This work, supported by the European Communities, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission*

## EU FusionPower Plant Studies

- D J Ward, FEC-18, paper IAEA-CN-77-FTP2/20

**EU studies of commercial power plants developed 4 concepts:**
- size decreases from (A) to (D) with advances in physics and materials
**Relatively simple scaling developed for Cost of Electricity:**
**Initiation of studies to define DEMO device have stimulated review of key physics issues which influence design of power plants**

- Direct influence of Physics

**EU Fusion****Power Plant Studies**

- Availability

- Thermodynamic efficiency

- Net electrical power

## Context for analysis of physics basis for tokamak power plants
Key physics issues for a tokamak fusion power plant:
operating scenarios
confinement properties
current drive requirements
high density, highly radiating regimes
mhd stability
plasma control
-particles
Conclusions

**Key physics issues for a tokamak fusion power plant:**
- operating scenarios
- confinement properties
- current drive requirements
- high density, highly radiating regimes
- mhd stability
- plasma control
- -particles
**Conclusions**

**Synopsis**

## The extrapolation required beyond the level of performance typical of present devices can be characterized relatively simply:
confinement enhancement factor:                                             (1.3 - 1.6)	
beta-normalized:						   (4 -6)
fractional Greenwald density:				   (0.9 - 1.5)
these parameters characterize proximity to operational limits
In power plants, there are of course additional important parameters which influence behaviour and fusion performance:
current drive: fbs, CD
radiation: frad, Zeff
-particle physics: eg  v/vA, , n/n

- confinement enhancement factor:                                             (1.3 - 1.6)
- beta-normalized:						   (4 -6)
- fractional Greenwald density:				   (0.9 - 1.5)
**these parameters characterize proximity to operational limits**
**In power plants, there are of course additional important parameters which influence behaviour and fusion performance:**
- current drive: fbs, CD
- radiation: frad, Zeff
- -particle physics: eg  v/vA, , n/n

**Reactor Plasma Physics I**

## The CoE expression provides an insight into the key elements influencing the economics of a fusion power plant:
operation at high N and high density is favoured for their direct impact on CoE
however, the CoE dependence masks the underlying physics which determines the reactor operating mode and fusion performance
Analysis to date indicates that CoE should be lower for steady-state tokamak designs 
fully non-inductive steady-state operation must be sustained
	 “advanced scenario” implies complex control with limited actuators 
high confinement (H98 > 1), high-N (N > 4li), high current drive efficiency(CD  Te) essential
high density (fGW > 1): efficient use of 			  highly radiating scenarios (frad > 80% to protect divertor)
mhd stability against sources of confinement degradation and disruption
 Can we meet the physics challenges of sustaining steady-state  operation in the regime relevant to power plants ?

- operation at high N and high density is favoured for their direct impact on CoE
- however, the CoE dependence masks the underlying physics which determines the reactor operating mode and fusion performance
**Analysis to date indicates that CoE should be lower for steady-state tokamak designs ******
- fully non-inductive steady-state operation must be sustained
- 	 “advanced scenario” implies complex control with limited actuators
- high confinement (H98 > 1), high-N (N > 4li), high current drive efficiency(CD  Te) essential
- high density (fGW > 1): efficient use of 			  highly radiating scenarios (frad > 80% to protect divertor)
- mhd stability against sources of confinement degradation and disruption
****** ****Can we meet the physics challenges of sustaining steady-state****  operation in the regime relevant to power plants ?**

**Reactor Plasma Physics II**

## Steady-State Operation

- plasma with reversed central shear +sufficient rotational shear

- internal transport barrier  enhanced confinement

- reduced current operation +large bootstrap current fraction

- reduced external current drive +current well aligned for mhd stability and confinement enhancement

- active mhd control

- Steady-state operation +High fusion power density

**Steady-State Operation**

## “Hybrid” Operation

**Hybrid scenario:**
- H-mode plasma with q0 ~ 1
**Recent results from hybrid operation:**
- H98 > 1 relevant to reactor-like scenarios
- N ~ 3.5 without RWM control (high li)
- n ~ 0.8nGW, achieved to date
- significant bootstrap current component extended pulse length
- q0 ~ 1 current profile control less demanding less sensitivity to Alfvén eigenmodes and    TF ripple losses
- edge plasma requirements still crucial - ELM behaviour, radiation

**“****Hybrid” Operation**

**“****Hybrid” operation provides long pulse capability (eg technology testing in ITER) ******** possibility of extension to steady-state?**

## Understanding energy confinement in advanced/ hybrid scenarios is at focus of present studies
quoted H-factors are typically target values
Access conditions for both regimes remain uncertain:
progress required in both experimental and theoretical areas
Reactor plasma will differ from present plasmas:
Te ≈ Ti
low momentum input
broad transport barriers required in advanced scenarios
possible non-linear coupling between -heating profile,current profile, transport properties and mhd stability
	 is limiting factor in advanced scenarios confinement or stability?
Lack of understanding of physics and role of edge pedestal is a key limitation on predictive capability (but not the only one)

- quoted H-factors are typically _target values_
**Access conditions for both regimes remain uncertain:**
- progress required in both experimental and theoretical areas
**Reactor plasma will differ from present plasmas:**
- Te ≈ Ti
- low momentum input
- broad transport barriers required in advanced scenarios
- possible non-linear coupling between -heating profile,current profile, transport properties and mhd stability
- 	 is limiting factor in advanced scenarios _confinement_ or _stability_?
**Lack of understanding of physics and role of edge pedestal is a key limitation on predictive capability (but not the only one)**

**Energy Confinement**

## ITPA SSO TG analysis indicates that, at present, hybrid operation exhibits better plasma performance than advanced scenarios

- A C C Sips et al, Plasma Physics Contr Fusion **47** A19  (2005)

**Advanced and “Hybrid” Operation**

## Majority of power plant studies aim for fbs ~ 80-90%
design values of pressure driven currents in ST studies often >90%
 requires advanced scenario with N > N,no wall	(fbs  p,  p  N2)
Remaining non-inductive current driven by mixture of classical H&CD systems:
CD = ne,20R0ICD/Paux  Te
confirmed extensively in experiments
extrapolation of factor 2-10 in Te to reactors
j(r) control also necessary
Technology a related issue:
plug ~ 60% typically assumed
LHCD/ ICRF need reliable coupling
1.5 - 2 MeV NBI often assumed

- design values of pressure driven currents in ST studies often >90%
-  requires advanced scenario with N > N,no wall	(fbs  p,  p  N2)
**Remaining non-inductive current driven by mixture of classical H&CD systems:**
- CD = ne,20R0ICD/Paux  Te
- confirmed extensively in experiments
- extrapolation of factor 2-10 in Te to reactors
- j(r) control also necessary
**Technology a related issue:**
- plug ~ 60% typically assumed
- LHCD/ ICRF need reliable coupling
- 1.5 - 2 MeV NBI often assumed

- T Oikawa et al, Nucl Fusion **21** 1575  (2001)

**Current Drive**

## Are expectations of CD consistent with j(r) control requirements?

- EFPW13  (2005)

**Estimates of ********CD**** in a DEMO-like device with <T****e****> ~ 20keV indicate range of expected CD efficiencies and deposition radii**

**Current Drive:****Profile Control**

**For all PPCS models, with P****EC****=0.33P****aux****, j****ECCD**** of same order as j****equil****(r) at all radii**
- ECCD has significant control capability in power plants

- S Alberti, EFPW13  (2005)

## Sustaining high density operation in the improved confinement scenarios favoured for power plants is challenging:

- C Angione et al, Phys Rev Lett **90** 205003  (2003)

**Density in relevant scenarios generally low**
**Several density limiting mechanisms:**
- no comprehensive theory
- operation above nGW remains challenging
**Decoupling of SOL recycling and core will be important:**
- pellet injection needs to be exploited
**Implications of recent observations of density peaking at low-********* should be explored:**
- impact on transport of high-Z impurities crucial
- -heating beneficial (ITER important)?

**High Density Operation**

**Density Peaking**

## Divertor targets in a power plant are likely to be constrained to the same heat flux limits as ITER: ~10MWm-2
parallel heat flux is >100MWm-2 in ITER and can reach ~1GWm-2 in reactors
-power to plasma typically factor of > 5 greater than in ITER,but reactor divertor target area factor of ~ 1-2 that of ITER,
Tungsten likely to be the material of choice for high power flux surfaces, based on erosion lifetime and tritium retention characteristics
divertor temperature should be <10eV to limit erosion rate
 Only feasible solution to satisfy these constraints appears to be  radiating mantle/ (semi-) detached divertor
implies impurity seeding to promote radiation, while effective impurity control must be retained to minimize core contamination
better understanding of core/ divertor radiation distribution required

- parallel heat flux is >100MWm-2 in ITER and can reach ~1GWm-2 in reactors
- -power to plasma typically factor of > 5 greater than in ITER,but reactor divertor target area factor of ~ 1-2 that of ITER,
**Tungsten likely to be the material of choice for high power flux surfaces, based on ****erosion lifetime**** and ****tritium retention**** characteristics**
- divertor temperature should be <10eV to limit erosion rate
****** ****Only feasible solution to satisfy these constraints appears to be****  radiating mantle/ (semi-) detached divertor**
- implies impurity seeding to promote radiation, while effective impurity control must be retained to minimize core contamination
- better understanding of core/ divertor radiation distribution required

**Power Exhaust/ Impurities I**

## J Dies et al, FEC-21, paper IAEA-CN-149-FT/P5-41

- 80-90% of loss power will need to be radiated in core and divertor - significant fraction in core
- radiation fraction must be maintained with acceptable core impurity concentration and plasma performance (Matthews: Prad  (Zeff-1)
- synchrotron and bremsstrahlung not insignificant - improved modelling treatment essential
- demonstration required for viable reactor scenario:plasmas with radiation dominated by seeded impurities using high-Z wall

- Synchrotron
- Bremsstrahlung

- G F Matthews et al, J Nucl Mater **241-243** 450 (1997)

**Power Exhaust/ Impurities II**

- EU studies

## Transient events are of even greater significance than in ITER
availability and first wall lifetime considerations set severe limitations on frequency and magnitude of pulsed events
Disruptions will essentially have to be eliminated
typical estimates in literature set frequency at 0.1 -1 per year
issues:
thermal quench:	  ~ 1GJ
current quench:	  ~ 1GJ
runaway electrons: >10MA if not suppressed

- availability and first wall lifetime considerations set severe limitations on frequency and magnitude of pulsed events
**Disruptions will essentially have to be eliminated**
- typical estimates in literature set frequency at 0.1 -1 per year
- issues:
- thermal quench:	  ~ 1GJ
- current quench:	  ~ 1GJ
- runaway electrons: >10MA if not suppressed

**Power Exhaust/ Impurities III**

**ELMs too will essentially have to be eliminated**
- ELM-enhanced erosion might already set PFC lifetime limits in ITER
**** ELM  control/ suppression techniques

- A Loarte et al, FEC-18 (2000)

## Reactor requirement for high- (N>3) arises from two major considerations:
high fusion power: high system efficiency (minimize recirculating power fraction)
high bootstrap current fraction: minimize current drive power 
				fbs ~ -0.5h()Nqc
In advanced scenarios, it is assumed that equilibrium can be optimized to operate near ideal mhd limit:
resistive wall modes limiting- RWM control with stabilizing wall and active feedback required
(m, n) requirements? importance of rotation?
neoclassical tearing modes with m/n > 2 might also be an issue
In hybrid regime, it is assumed that adequate  can be sustained (<3.5) without exceeding “no-wall” ideal limit:
neoclassical tearing modes limiting - critical mode m/n = 2/1- control via localized ECCD demonstrated (power requirements?)

- high fusion power: high system efficiency (minimize recirculating power fraction)
- high bootstrap current fraction: minimize current drive power
**	****	****	**	fbs ~ -0.5*h*()Nqc
**In **_**advanced scenarios**_**, it is assumed that equilibrium can be optimized to operate near ideal mhd limit:**
- resistive wall modes limiting- RWM control with stabilizing wall and active feedback required
- (m, n) requirements? importance of rotation?
- neoclassical tearing modes with m/n > 2 might also be an issue
**In **_**hybrid regime**_**, it is assumed that adequate ******** can be sustained (<3.5) without exceeding “no-wall” ideal limit:**
- neoclassical tearing modes limiting - critical mode m/n = 2/1- control via localized ECCD demonstrated (power requirements?)

**MHD Stability at High-******

## MHD Stability at High-

- modes with n>1 are likely to be most unstable as -limit approached
- confirmed in experiments ?

- G Pereverzev et al, FEC-21, paper IAEA-CN-149-FT/P5-23

- 2/1 NTMs can be stabilized in hybrid regime while retaining high- and confinement quality

- C C Petty et al, FEC-20, paper IAEA-CN-116

**MHD Stability at High-******

## Advanced scenario

**Multiple confinement barriers**

**Hybrid scenario**

**Edge confinement barrier**

**Power Plant Scenarios**

## To develop steady-state operation and prepare the Physics Basis for DEMO/ Power Plants, fusion programme must address, in particular:
exploration of relevant scenarios and characterization of their access/ transport properties
demonstration of required degree of current profile control with required level of current drive efficiency 
consistency of scenario with high density/ high radiation regime with acceptable level of fuel dilution - high-Z wall materials!
realization of sustained gain in accessible beta through active control of mhd instabilities
establish satisfactory -particle confinement (ITER)

**To develop steady-state operation and prepare the Physics Basis for DEMO/ Power Plants, fusion programme must address, in particular:**
- exploration of relevant scenarios and characterization of their access/ transport properties
- demonstration of required degree of current profile control with required level of current drive efficiency
- consistency of scenario with high density/ high radiation regime with acceptable level of fuel dilution - high-Z wall materials!
- realization of sustained gain in accessible beta through active control of mhd instabilities
- establish satisfactory -particle confinement (ITER)

**Conclusions**