The Science of ITER

T Luce NAS study visit 01 February 2018

Page	  1	  Inside the Tokamak Building

T Luce NAS study visit 01 February 2018

Page	  2	  The ITER Tokamak	  Vacuum Vessel: ~ 8 000 t.
TF Coils: ~ 18 x 360 t.
Central solenoid: ~ 1 000 t.
Total ~ 23 000 t.
R = 6.2 m, a = 2.0 m
B = 5.3 T, I = 15 MA

T Luce NAS study visit 01 February 2018

Page	  3	  What Is the Mission of ITER?
Quo%ng	  from	  the	  Project	  Speciﬁca%on	  (‘the	  top	  level	  ITER	  Technical	  Document’):	  …demonstrate	  the	  scien%ﬁc	  and	  technological	  feasibility	  of	  fusion	  energy	  for	  peaceful	  purposes	  But	  how?	  

T Luce NAS study visit 01 February 2018

Page	  4	  Physics, Engineering, and Operational
Objectives Were Specified	  • Achieve fusion power of 500 MW with Pfus/Pin (≡ Q) ≥ 10
for 300-500 s (i.e., stationary conditions)• Aim at demonstrating steady-state operation with Q ≥ 5• Capable of advanced operational modes and a wide
operating parameter space• Achieve the minimum cost device that meets all thestated requirements
T Luce NAS study visit 01 February 2018

Page	  5	  Why These Conditions?	  
• Fusion power of ~500 MW is the minimum for a power plant• Q ~ 10 is the minimum for a power plant; also dominant self-heating• Stationary conditions imply duration is not limited by physics, but
hardware investment• Direct comparison of inductive and steady-state scenarios inburning plasmas answers a key design question for the next step
• Wide parameter range requirement avoids a ‘point solution’• Minimum cost because … why pay more?
In the following, I would like to explore the science of these—the ITER
design basis and in turn what science questions will be answered
T Luce NAS study visit 01 February 2018

Page	  6	  Physics, Engineering, and Operational
Objectives Were Specified	  • Achieve fusion power of 500 MW with Pfus/Pin (≡ Q) ≥ 10for 300-500 s (i.e., stationary conditions)• Aim at demonstrating steady-state operation with Q ≥ 5• Capable of advanced operational modes and a wide
operating parameter space• Achieve the minimum cost device that meets all thestated requirements
T Luce NAS study visit 01 February 2018

Page	  7	  500 MW Fusion Power 	  • Fusion power for DT reactions
is given by (in the range
where <σv> ∝ T2):
Pfus (MW) ≈ 7.6 10-12 p2 V<σv>∝ T2• Therefore, smaller devices
need higher pressures toreach the 500 MW goal
– Pressure can be limited by
MHD stability or confinement
T Luce NAS study visit 01 February 2018

Page	  8	  MHD Stability Pressure Limits	  •MHD equilibrium is given by:∇p = j x B = (∇ x B) x B/µ0•

There is a dimensionless parameter2µ0p/B2, which is the ratio of thermal tomagnetic energy (β) and is << 1•

In a tokamak, the following parameters
are also small: r/R and BP/BT ∝	   I/aBT•

With 3 small parameters, ratios yield two
O(1) parameters: ι = 1/q = RBP/rBT andβ/(I/aBT), which describe limits on current
and pressure in ideal MHD, predicted bytheory and validated by experiment
T Luce NAS study visit 01 February 2018

Page	  9	  

Pressure Limits and Fusion Power	  •Adding the stability scaling gives:
Pfus (MW) ≈ 2.9 10-2 (f I B)2 Rκ

G Huijsmans,JET– f is the fraction of the maximum pressure limit•For a given operating scenario, I ∝ B and f isconstant, so fusion power rapidly increases
with B => choose highest practical B•

Tokamak scenarios can trade f and I to givethe same fusion power– ITER baseline has max(I) and f ≈ 0.5 to lower
risk of instability, including tearing modes– Other scenarios can have same f x I at lower I=> want to explore a wide operating space
T Luce NAS study visit 01 February 2018

JETPage	  10	  Physics, Engineering, and Operational
Objectives Were Specified	  • Achieve fusion power of 500 MW with Pfus/Pin (≡ Q) ≥ 10for 300-500 s (i.e., stationary conditions)• Aim at demonstrating steady-state operation with Q ≥ 5• Capable of advanced operational modes and a wide
operating parameter space• Achieve the minimum cost device that meets all thestated requirements
T Luce NAS study visit 01 February 2018

Page	  11	  Gain (Q) Is Determined by Confinement	  • For DT fusion, 20% of the fusion power goes to α particles
and can heat the plasma• At the 500 MW operating point, 100 MW of self-heatingpower is available and Q ≥ 10 implies the auxiliary heatingpower is ≤ 50 MW• Reaching 500 MW fusion power implies two conditions:– The available power can heat the plasma to the requiredpressure– The self-heating is effective
T Luce NAS study visit 01 February 2018

Page	  12	  Gain (Q) Is Determined by Confinement	  •Early DIII-D work

As tokamaks are heated, stored energy
increases more slowly than linear with power

H modeL mode•At a certain power, the plasma transitions
spontaneously to very low transport ofparticles and heat at the edge (H-mode)– PLH (MW) = 0.098 mH-1 B0.80 n0.72 S0.94– In ITER, the plasma should be ~5 keV very
near the plasma boundary => good fusion
reactivity in the entire volume– Sufficient power must be applied to sustain thisedge condition

Plasma Pressure– Stored energy/Input power = Confinement time
– Transport is characterized by the ratio of flux togradient

T Luce NAS study visit 01 February 2018

Page	  13	  How Is Tokamak Confinement Predicted?	  
• Three main methods are used to predict energyconfinement:– Engineering parameter scaling– Dimensionless physics parameter scaling– Direct numerical solution of the fundamental equations• Each of these methods has strengths and weaknesses• To predict gain, it is necessary to get self-consistently
the pressure (related to stored energy and the selfheating) and the additional power needed to reach thatpressure
T Luce NAS study visit 01 February 2018

Page	  14	  Engineering Parameter Scaling	  ITER•

Database of confinement time from many
tokamaks fit by linear regression to a power law
dependence on engineering parameters•Strengths:τE ∝ I R2 P-2/3– 3 orders of magnitude in fit extrapolated to target
less than 1 order outside the data
– Easily used for large design surveys• Weaknesses:– Improper linear regression– Burning plasmas enter a regime ofsignificant intrinsic radiation (esp.
cyclotron radiation)
T Luce NAS study visit 01 February 2018

Page	  15	  Dimensionless Parameter Scaling	  • Use similarity arguments to reduce thenumber of scaling parameters to twoC-Mod– Can keep shape, β, and q fixed– Two other dimensionless parameters:
number of gyroradii across the plasma,number of transits before a collision

JETDIII-D• Strengths:– Variables from basic physics– Validated by identity experiments• Weaknesses:– Uncertainties large => cannot beused alone for prediction, but canbe used to check scalings
T Luce NAS study visit 01 February 2018Page	  16	  Direct Numerical Solutions	  • The fundamental equations governing the plasma turbulence
are believed to be known– Modern computers can now solve these for realistic parameters• Strengths:– First-principles physics• Weaknesses:– Not practical for design
optimization, but cancheck design points– Solutions sensitive toboundary conditions that
do not have the samefirst-principles basis
T Luce NAS study visit 01 February 2018

Page	  17	  Self-Heating of Plasmas is a New Regime	  • The charged fusion product of DT reactions is a 3.4 MeV α particle– First time plasmas are heated by an isotropic particle distribution invelocity space– Most calculations all α particle is thermalized except prompt losses• These α particles are faster than the phase velocity of Alfvén
waves in the background plasma• The wave-particle interaction canlead to instability– Waves excited are eigenmodes ofthe background plasma– Think of Cherenkov emission inan active medium where gain ispossible

Fuel	  mixture:	  Deple%on:	  Drive	  from	  alpha-­‐par%cles	  +	  beams	  Damping	  from	  thermal	  plasma	  T Luce NAS study visit 01 February 2018

Page	  18	  Plasma Response is Also in a New Regime	  • From dimensionless parameter arguments, the toroidal
wavelength of the most unstable modes is proportional to thenumber of gyroradii across the plasma radius– NSTX: n≈1, DIII-D: n≈3-5; ITER: n≈15-25• The background particles in ITER will see a wave field more
typical of turbulence than discrete wave fields• Calculations indicate littleloss of heating efficiency
due to α particle transport– But we have to actually
test it!

T Luce NAS study visit 01 February 2018

Page	  19	  Physics, Engineering, and Operational
Objectives Were Specified	  • Achieve fusion power of 500 MW with Pfus/Pin (≡ Q) ≥ 10for 300-500 s (i.e., stationary conditions)• Aim at demonstrating steady-state operation with Q ≥ 5• Capable of advanced operational modes and a wide
operating parameter space• Achieve the minimum cost device that meets all thestated requirements
T Luce NAS study visit 01 February 2018

Page	  20	  Reaching Stationary Conditions	  • Four significant issues must be dealt with to reachstationary conditions:– Supplying the magnetic energy associated with theoperating current– Handling of the stationary heat and particle exhaust(focus on heat here)– Eliminating the transient heat loads associated withstandard H-mode operation– Stable stationary operating scenarios must be realized• Each of these has fundamental implications for thedesign of a burning plasma device
T Luce NAS study visit 01 February 2018

Page	  21	  Supplying the Magnetic Energy	  •A fundamental design consideration for any tokamak is the inner‘radial build’—the space required for the toroidal field coils,
solenoid, and (for burning plasmas) blankets and shielding– There is no demonstrated method to remove completely the needfor a solenoid for the current rise, even if it is not needed forsustaining the plasma– To achieve stationary conditions, the plasma duration should be atleast 2 current relaxation times for any scenario– The magnetic stored energy required to reach maximum current is
far larger than than dissipated by resistivity in current flattop for 2
current relaxation times•ITER solution: >270 Wb of flux, more than 200 Wb is needed to
get to 15 MA– Lower current pulses can then be run substantially longer (up to3000 s)
T Luce NAS study visit 01 February 2018

Page	  22	  Handling Heat and Particle Flux
ITER transients

The challenge is
not small!
HWR 	   	  Re-entryvehicle

Ariane 5/Vulcain 2	  ~1 	  	  	  	  	  	  	  <10	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  85	  	  	  	  	  	  	  	  	  	  	  	  	  	  2000	  Power load [MW/m2]
Arc welding

Rolls Royce
Trent 900

ITER steady-state
T Luce NAS study visit 01 February 2018

Page	  23	  The First Challenge: How Big is the Problem?
•The thickness of the layer in which heat is
exhausted is small (few mm)– ITER design assumption was ~3-4 mmbased on stability considerations

Recent empirical scaling would extrapolateto 1 mm!
– Scaling is consistent with an analytic model
based on particle drifts

Powerflow

PSOL~120MW#••

State-of-the-art numerical solutions indicate
turbulence in ITER should broaden theexhaust layer to ~5 mm– Existing machines cannot reproduce theseconditions
T Luce NAS study visit 01 February 2018

Page	  24	  The Second Challenge: How to Handle the Problem?
••To reduce the heat flux, deliberate
introduction of impurities, such as noble
gases, is envisionedPSOLλq

Preferably, radiation is in the main chamberto spread the energy over a larger surface– Potential problem with dilution from
impurities entering the core plasma– Potential problem staying in H mode if too
much power radiates from the core plasma•

Many experiments have demonstrated
radiation in the divertor in order to ‘detach’the field lines from the divertor targets– The radiating zones and the area over which
the energy is deposited may be too small

Dissipationzone

T Luce NAS study visit 01 February 2018

Page	  25	  The Third Challenge: How to Compute the Problem?
PSOLλq• Present calculations contain many of theseeffects, but are not yet comprehensive• Present-day tokamaks cannot reproduce
the ITER divertor conditions

T Luce NAS study visit 01 February 2018Page	  26	  Transients in Standard H-Mode: ELMs
• The high confinement at
the plasma edge in H
mode is actually too highASDEX-­‐Upgrade	  E.	  	  Wolfrum	  – Pressure increases until
an instability limit isreached• In present-day tokamaks,
this has beneficial effects– Particle content of theplasma can becontrolled by pumping– Impurity content of thecore is reduced

T Luce NAS study visit 01 February 2018

Page	  27	  Single ELMs Might Cause Melting
Scaling of the energy loss
in large ELMs projects
energy near the surface
melt limit of the ITER
divertor~ surface melt limit* ITER-Regr.
JET-C
JET-ILW
MASTT. Eich et al., [39]T. Eich, Nucl. Mater. Energy (2017)

ITER	  7.5,	  15	  MA	  
AUG-C
AUG-W
AUG-W-inner

T Luce NAS study visit 01 February 2018

Page	  28	  Mitigation of ELMs
• The primary method for ELM
mitigation in ITER is application ofResonant Magnetic Perturbations– This has been successfully applied inseveral tokamaks– ITER can apply up to n=4• In addition, a pellet system to trigger
ELMs at a pace much faster thannatural will be available

T Luce NAS study visit 01 February 2018Page	  29	  What About Disruptions?
• A very important reason to operate stationary
discharges in ITER is to end the concern aboutdisruptions– A truly stationary plasma will not disrupt because ofplasma physics– Disruption occurrence should become a component or
power supply reliability issue and not a physics issue• The key question is whether truly stationary operating
scenarios exist– Note: take care that stationary ≠ steady-state
T Luce NAS study visit 01 February 2018Page	  30	  Do Candidate Stationary Scenarios Exist?
• ITER baseline
scenario operated
stably at zero input
torque in DIII-D• Advanced inductive
scenario operated at
low torque JT-60U to28 s

DIII-D 170479

2.0 βNIN
1.5 H98y2

8 ne (1019/m3)~
B(n=1) (G)
6

1.0

4

0.5

2

0.00

I (MA)
1.5 liDiv. Dα (a.u.)

6PNB (MW)f (q=2) (kHz)4

1.0TNB (Nm)

20.5
0.0

00

1234Time (s)56-20

T Luce NAS study visit 01 February 2018

1234Time (s)56Page	  31	  Disruption Mitigation Is Still Mandatory• ITER must be ready to mitigate disruptions at any operating point– Learning to run a stationary discharge will not happen in a day– Components and power supplies will fail• Mitigation pellets aresubstantially larger thanordinary fuelling pellets(leftmost)13	  mm	  17	  mm	  20	  mm	  T Luce NAS study visit 01 February 201828	  mm	  Page	  32	  Physics, Engineering, and Operational
Objectives Were Specified	  • Achieve fusion power of 500 MW with Pfus/Pin (≡ Q) ≥ 10
for 300-500 s (i.e., stationary conditions)• Aim at demonstrating steady-state operation with Q ≥ 5• Capable of advanced operational modes and a wide
operating parameter space• Achieve the minimum cost device that meets all thestated requirements
T Luce NAS study visit 01 February 2018

Page	  33	  True Steady-State Scenario Will Be Difficult	  • With the day-1 heating and current drive systems, fully noninductive operation predictions range from Q=2-5– Even with upgrades, true steady-state scenario is difficult because ofdivertor heat loads at reduced density• However, ITER can definitively compare scenarios with different q
profiles proposed for steady-state operation• Two key aspects of the comparison:– Dependence of energy confinement on q
– Dependence of stability of Alfvén modes excited by fusion α particles
as a function of q
T Luce NAS study visit 01 February 2018

Page	  34	  Physics, Engineering, and Operational
Objectives Were Specified	  • Achieve fusion power of 500 MW with Pfus/Pin (≡ Q) ≥ 10for 300-500 s (i.e., stationary conditions)• Aim at demonstrating steady-state operation with Q ≥ 5• Capable of advanced operational modes and a wide
operating parameter space• Achieve the minimum cost device that meets all thestated requirements
T Luce NAS study visit 01 February 2018

Page	  35	  What Questions Will ITER Answer?	  
• While we are are confident in the design basis forITER, it is still an experiment– This means operation of ITER as envisioned in thedesign basis will validate (or invalidate) its design basis• In the time between now and ITER plasma operation(and especially DT operation), simulation capability
will continue to advance– This means operation of ITER will validate (or invalidate)
the physics and assumptions in a variety of simulations
T Luce NAS study visit 01 February 2018

Page	  36	  Fusion Questions ITER Will Answer	  By design, ITER will answer several key questions for fusionenergy, such as:• Does self-heating work?
• How large does a plasma need to be to achieve dominant
self-heating?
• Are advanced scenarios actually ‘advanced’?
• Is steady-state operation feasible in a burning plasma?
• Is there a self-consistent solution coupling a burning coreplasma to a working heat and particle exhaust solution?
• Can helium ash be exhausted?
• Are plasmas disruption-free in stationary conditions?
T Luce NAS study visit 01 February 2018

Page	  37	  Physics Questions ITER Will Answer	  ITER will address several interesting plasma physics questions,such as:• Does gyro-kinetic theory really describe plasma transport?
• What limits the inverse cascade of turbulent energy at lowwavelength?
• Is there ‘intrinsic’ rotation?
• Do heat and particle transport depend strongly on gyroradius orcollision frequency?
• What is the mechanism of the L-H transition?
• What is the mechanism for runaway generation in a disruption?
• Are high temperature plasmas more or less prone to tearingmodes?
• Do current drive efficiencies increase with temperature?
• What is the origin of isotope scaling?
T Luce NAS study visit 01 February 2018

Page	  38	  When Will ITER Answer These Questions?	  

ITER will can address some questions in the early operational
phases, but system commissioning for DT is the highest priority task
T Luce NAS study visit 01 February 2018

Page	  39	  ITER Hardware Availability	  T Luce NAS study visit 01 February 2018

Page	  40	  Disruption Physics and Mitigation	  • Disruption mitigation isnecessary from early in
PFPO-1– Validity of scaling projections
for the mitigation will be
immediately known• Operation at full field will bedone in PFPO-1 specificallyto test the scaling of runaway
generation with B
T Luce NAS study visit 01 February 2018

Page	  41	  Other Physics Issues Addressed in PFPO	  • Size scaling of L mode in hydrogen• Current and size scaling of the scrape-off layer
power width in L mode
• L mode divertor detachment• Fueling physics• H-mode transition and confinement scaling with size
• Error field sensitivity and correction• Intrinsic rotation• …
T Luce NAS study visit 01 February 2018

Page	  42	  ITER Will Be The First Look At Burning Plasma

T Luce NAS study visit 01 February 2018

Page	  43