HTS – An Enabling Technology for Spherical Tokamak Fusion Reactors
Paul Noonan
Tokamak Energy
UK Magnetics Society 22 September 2015
Outline
1.
2.
3.
4.
5.
6.
7.
8.
•
•
•
•
•
•
9.
Energy
Fusion
Tokamaks and progress
Some alternatives
Why spherical tokamaks?
Why HTS?
Strategy
Some engineering challenges
Conductor
Protection
Cables
Joints
Radiation
The need for collaboration
Conclusion
Energy
The world needs abundant energy
• Low cost
• Environmentally benign
• Politically acceptable
Business as usual
Death
800
SO2Concentration
1
0.8
600
0.6
400
0.4
200
0.2
0
SO2-Concentration
[ppm]
Number of deaths
1000
0
1
3
5
7
9
11 13 15 17
December 1952
London smog data 1952:
Assuming a linear law
1 early death ~ 100 tons SO2
A modern coal fired power plant
(Nottinghamshire) is projected to lead to
more than 200 deaths per year (AEAT
1998).
IPCC, Climate Change 2014 Synthesis Report
Fusion
How might we provide this energy?
Energy can be gained both by fusing smaller atoms and by breaking up larger ones
Deuterium
Helium
+ energy (17.6 MeV)
Tritium
Neutron
It has been recognised for decades that fusion is a potential solution, but it
has proved very difficult in practice
Early days
The 1946 Fusion Reactor patent of
Thompson & Blackman [1] (Imperial
College 1946) was indeed Compact
Fusion
- But all sorts of instabilities appeared..
20 msec
A conducting vessel helped…
ZETA 1957/8
A small ‘pinch’ device:
R / a = 1.30m / 0.3m,
Ip = 0.5MA
classical confinement was assumed :
→
τ = 65s
→T = 500keV
Hence D-D fusion would be achievable
Early announcement of success proved
wrong; τ ~ 1ms
→ T~ 0.17keV
[1] Thomson, G.P., Blackman M. British patent 817,681. “Improvements in Gas Discharge Apparatus for Producing Thermonuclear Reactions” in Haines M
G, Plasma Phys. Control. Fusion 38 (1996) 643
Adding a strong toroidal field
Cartoon by Boris
Kadomtsev
The T-3 tokamak was claimed to be
much hotter than the pinches studied in
the Western world. A team of Culham
scientists spent a year at the Kurchatov,
proving this was indeed the case, using a
Thomson Scattering diagnostic.
The rest of the World began building tokamaks!
Developments and improvements of the
Tokamak have controlled all the major plasma
instabilities. But energy confinement still poor;
scaling approximately as
τ ~ R2 x BT1.5
To obtain a Gigawatt size power plant, leads to
the ITER project
R = 6.2m, Vol ~ 850m3, BT (at R) = 5.3T
Progress to date
Triple product, n x T x τ, vs time
Progress has been impressive,
but at the expense of building
ever larger machines
Timing is crucial
Beyond ITER we will need DEMO, and only then can we
contemplate commercial fusion reactors.
Can we move more quickly?
Some recent ‘Compact Fusion’ concepts
Lockheed Martin ‘skunkworks’
General Fusion
Helion
All deviate significantly from the well
characterized tokamak route
Tri-alpha
All have very low nTτ at present
HIT SI3 (Jarboe)
Why Tokamak Energy?
Tokamaks still have a very significant lead in nTτ …
… but machines have been getting larger, more expensive and time consuming to build
Design iterations take a long time => learning is slow
However there have been several important developments since 1985:
• The Spherical Tokamak (ST) has high pressure efficiency, β, and hence high pressure, nT
• There is evidence that a high field ST has improved τ
• Re-evaluation of the ITER confinement database yields surprisingly promising τ. Good for ITER,
great for STs
Reagan, Gorbachev, 1985
• High Temperature Superconductors (HTS) can carry very high current density at high fields and
intermediate cryogenic temperatures
Energy gain Qfus = Pfus/Pin can be high in small modular reactors with low fusion power (~100MWe)
Less expensive, shorter development cycle time => faster learning => earlier commercial deployment
Spherical tokamaks
RECORD β ON START
(achieved through NB Heating)
50
40
β
N
=6
1996
1997
1998
A tokamak of aspect ratio A=R/a where 1 < A < 2 is commonly
known as a ‘Spherical Tokamak’
The plasma equilibria appear spherical
(T
ro
yo βN =
nl
im 3.5
it)
30
DIII-D, #80108
βT , %
20 conventional
tokamak
10
β = plasma pressure/magnetic field pressure
0
M. Gryaznevich et al., ‘Achievement of record beta in the START Spherical Tokamak’, PRL 80
(18), 1998, p. 3972
A.Sykes et al, ‘H-mode operation in the START Spherical Tokamak’, PRL 84, 2000, p. 495
0
2
4
6
8
10
normalised plasma current, Ip/aBT
What’s the difference?
High safety factor q
high κ (‘natural’
elongation)
Further advantages
Disruptions less severe:
NSTX (turquoise) and MAST (black)
have both lower peaking fraction AND
lower halo current fraction than
conventional tokamaks
High Bootstrap current:
High b, and high shear, and high elongation
enable high bootstrap fractions to be more
easily attained at low aspect ratio (left) than
conventional (right)
(modelling from JUST)
A = 1.8
A=3.25
The catch!
For example B = 3.2T at 1.4m requires 22.4MA
22.4MA
We have high β and enhanced stability,
but Bt has been low due to the slender centre column …
=
∗ 2
… and Pfus ~ β2 Bt4 Vol
3.2T at 1.4m
So ineffective as a reactor
Use low temperature superconductors?
plasma
shield
centre column
Space is required for a neutron radiation shield …
18T
For the previous example field on a 0.5m diameter centre column is ~18T
… and Pfus ~ β2 Bt4 Vol , so we would like more
But there is only enough room for shielding to reduce the neutron heat load to several 10’s kW
Challenging at LTS temperatures!
HTS
Thermodynamics tells us:
=
− Cooling in the range 25K ~ 35K =>
• much less recycled power
• significantly lower cryo-plant capital cost
8000
7000
6000
5000
4000
HTS technology has progressed to the point where
the available current density is starting to meet our
requirements
3000
2000
1000
0
0
2
4
6
8
10
12
14
16
18
20
University of Houston presentation at the Low Temperature Superconductor Workshop, Napa,
CA, Feb. 16 – 18, 2015
Up to 9T Ic is taken from page 15, 20% Zr, 2.2um, 12mm width, 30K, B perpendicular to the tape.
Extrapolation to higher fields is done by scaling a curve derived from the chart on page 18
Strategy
Tokamak Energy is founded on the emergence of two remarkable new technologies:
• Spherical tokamaks
• HTS
We are also making progress on the development of ‘thin’ neutron shielding materials
Our strategy is to pursue three engineering development areas in parallel:
HTS
High field spherical tokamaks
HTS spherical tokamak
(small, minimal shielding)
Neutron shielding
=> early deployment of commercial fusion power plants
HTS spherical tokamak
(reactor size, no breeding)
Prototype reactor
Comparative Strategy
16MW 1997
JET
2029
ITER
ST25(HTS)
ST25
START
MAST
other
STs R&D:
2050
DEMO
ST60(HTS)
2075
Each step
requires
significant
technology
development
Power Plant
ST140
ST Power Plant
HTS
technology
ST40(Cu)
High stress magnet
development
Neutronics
tE scaling (H-
Qfus(equivalent)
Materials
Shielding design
Tritium
Burning plasma
studies
factor)
10 YEARS TARGET
19
Note: ST140 is much smaller than JET
Major radius 1.4m vs 3.0m
Strategy – existing machines
ST25 – HTS
• HTS tape demonstrator
• Diagnostics, current drive and heating
development
• Long run times
ST25 – copper
• R = 25cm
• Diagnostics, current drive and heating
development
• Run time of a few seconds
Strategy – the next machine
ST40 – LN cooled copper
• Pulse length 1.5 ~ 8s (depending on field)
• Plasma centre field 3T (> 3x higher than other planned machines)
• In advanced state of design – mechanical engineering of such a high field
device requires care and ingenuity
• Lower cost and risk than HTS, but can still …
Provide proof of ST scaling laws
HTS progress
Rapid and relevant progress is being made ….
HTS progress
Data presented by Selvamanickam et al, University of Houston, HTS4Fusion Workshop
Pieve S. Stefano, Italy, September 11 - 12, 2015
Critical current (A/12 mm)
8000
7.5%Zr, 0.9 µm
30 K, B ⊥ tape
7000
15%Zr, 0.9 µm
6000
25%Zr, 0.9 µm
5000
20% Zr, 2.2 µm
4000
3000
Zr doping and thicker YBCO layers are yielding significant
performance improvements
2000
• temperature is relevant and B // c
1000
• thinner substrate materials are being developed
0
0
1
2
3
4
5
6
Magnetic field (T)
Supercond. Sci. Technol. 28, (2015) 072002
7
8
9
HTS progress
Data presented by Selvamanickam et al, University of Houston, HTS4Fusion Workshop
Pieve S. Stefano, Italy, September 11 - 12, 2015
4X Ic achieved in 20% Zr-added tape made in Advanced MOCVD System
Good performance and reduced field angle
dependence
⇒ Increased design flexibility
AC losses
Our aim is to make a DC machine but highly elongated
plasmas are vertically unstable
• Active feedback is required
• Further work on our new copper machine is required to
determine the necessary dB/dt, but we share the AC loss
problem with other devices – conductor progress is being
made ….
AC losses
Data presented by Selvamanickam et al, University of Houston, HTS4Fusion Workshop
Pieve S. Stefano, Italy, September 11 - 12, 2015
2
unstriated
100 Hz
ac loss (W/m)
• Filamentization of coated
conductors is desired for low ac
loss applications.
• Maintaining filament integrity
uniform over long lengths (no Ic
reduction)
1
5.1 x
multifilamentary
0
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Bac rms (T)
• Minimum reduction in non
superconducting volume
(narrow gap) and fine filaments
4 mm
HTS
Ag
• Striated silver and copper
stabilizer (minimize coupling
losses)
Substrate
Cu
AC losses
Data presented by Selvamanickam et al, University of Houston, HTS4Fusion Workshop
Pieve S. Stefano, Italy, September 11 - 12, 2015
REBCO
Ag
Electroplated Cu
Buffer Stack
Oxide Layer
Hastelloy
Hastelloy
Selective Cu Electroplating
Laser Striation + oxygenation
Non-striated
12 mm wide Ag
sputtered tape
12-filament tape
with electroplated
Cu
12-filament ,
12 mm wide
tape
Laser
Striation
Oxygenation
+ ED Cu
2
7
I. Kesgin, G. Majkic, and V. Selvamanickam, “A simple, cost effective top-down method to achieve fully filamentized low AC
loss 2G HTS coated conductors” , Physica C. 486, 43–50 (2013)
AC losses
Data presented by Selvamanickam et al, University of Houston, HTS4Fusion Workshop
Pieve S. Stefano, Italy, September 11 - 12, 2015
Significant ac loss reduction in full-filamentized conductor with copper stabilizer
• Critical current of standard
conductor = 207 A
• Critical current of 12-filament
conductor after 10 µm copper
stabilizer = 200 A
0.02 T
14
ac loss (W/m)
• Critical current of 12-filament
conductor = 197 A
16
12
10
Multfilamentary
8
Standard
6
Multfilamentary + Cu
Standard + Cu
4
2
AC loss of 12-fiament conductor at 60 Hz
is 11 times lower than that of
unstriated conductor without copper
stabilizer and 13 times lower with
copper stabilizer, at higher fields
0
0
100
200
300
400
ac field frequency (Hz)
We don’t yet know what our requirements will be –
but it’s good to know progress is being made
HTS progress
These recent and rapid advances are relevant and very exciting for spherical tokamak development
Protection
1
= . 2
Even our compact tokamak is a large magnet and B is high
• e.g. TF coil system stored energy will be ~ 1GJ
=> Protection will be an issue
Copper will be required, but this increases the volume of the central column
⇒ need to minimise the amount
Protection
Slow propagation in HTS implies an external energy dump system
Terminal voltage is limited by insulation
Assuming:
•
•
•
•
~100kA TF coil set
2s detection and activation time
each limb dumped independently
As much copper as we can tolerate
We still get a significant temperature rise
There are many challenges, for example:
• Rapid detection and activation in a noisy environment
• Insulation?
• Non/semi insulated windings?
⇒ Much work to be done!
Cables
The need for high current implies the need for multi-tape cables
A number of organisations are making progress
CRPP: 60 kA, B^12.5 T, 7.8 K
Transposed, scalable to 50-100 kA, designed
for B^ > 12 T, AC tolerant, force flow.
Uglietti et al, EPFL – CRPP, HTS4Fusion 2015
NIFS: 67.4 kA, B|| 4.3 T, 45 K
Designed for segmented fabrication and B|| (no transposition, rotation of
conductor to match the field direction, 100 kA).
Bruzzone, EPFL – CRPP, HTS4Fusion 2015
Cables
Augieri et al, ENEA, HTS4Fusion 2015
Conductor On Round Core
van der Laan et al,
Advanced Conductor Technologies & University of Colorado
HTS4Fusion 2015
Cables
Data presented by Satoshi ITO et al
Dept. Quantum Sci. Energy Eng., Tohoku Univ., Japan, National Institute for Fusion Science (NIFS), Japan
HTS4Fusion Workshop, Pieve S. Stefano, Italy, September 11 - 12, 2015
Lap joint preparation of prototype STARS cable
Prototype STARS Conductor joint achieved 120 kA and 1.8 nΩ
~ 26W, which is small fraction of the total cryo-plant heat load
Irradiation
Studies by Eisterer et al, HTS4 Fusion conductor workshop, PSI, Jan 2014 show that at low
temperatures <60K, neutron bombardment improves tape performance at least up to 2.3 x 1022 n/m2
Caution
•
•
•
•
Recent good performance has been obtained with extensive artificial pinning – how do effects interact?
Performance is very sensitive to operating conditions (B and T)
Is the neutron spectrum relevant?
Is the effect dependent on temperature (samples irradiated at ~room temperature
Conclusion
Much work remains to be done, but there are grounds for optimism
•
•
•
•
•
•
ST25s will be used to work on diagnostics, heating and current drive. Other labs are doing similar things
ST40 will demonstrate scaling on small, high field, spherical tokamaks
Manufacturers are rapidly improving HTS performance at temperatures and fields of interest
Joints with suitable performance have been demonstrated
Various cable designs are being developed around the world
Significant knowledge gaps remain around HTS irradiation and magnet protection
A combination of in-house development, collaboration and a systematic approach will progressively
reduce technical risk at minimum cost and time
THANKS FOR YOUR ATTENTION
Questions?