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HTS – an Enabling Technology for Spherical Tokamak Fusion Reactors

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HTS – an Enabling Technology for Spherical Tokamak Fusion Reactors HTS – An Enabling Technology for Spherical Tokamak Fusion Reactors Paul Noonan Tokamak Energy UK Magnetics Society 22 September 2015 Outline 1. Energy 2. Fusion 3. Tokamaks and progress 4. Some alternatives 5. Why spherical tokamaks? 6. Why HTS? 7. Strategy 8. Some engineering challenges • Conductor • Protection • Cables • Joints • Radiation • The need for collaboration 9. Conclusion Energy The world needs abundant energy • Low cost • Environmentally benign • Politically acceptable Business as usual 1000 Death 1 800 SO2- 0.8 Concentration 600 0.6 400 0.4 [ppm] 200 0.2 Number of deaths deaths of Number SO2-Concentration SO2-Concentration 0 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 SO 2 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 - But all sorts of instabilities appeared.. Thompson & Blackman [1] (Imperial College 1946) was indeed Compact Fusion 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 Early announcement of success proved Hence D-D fusion would be achievable 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 The T-3 tokamak was claimed to be Kadomtsev 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 2 1.5 τ ~ R x B T To obtain a Gigawatt size power plant, leads to the ITER project 3 R = 6.2m, Vol ~ 850m , B T (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 nT τ at present HIT SI3 (Jarboe) Why Tokamak Energy? Tokamaks still have a very significant lead in nT τ … … 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 /P in 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 βββ ON START (achieved through NB Heating) 50 1996 6 1997 = β N 1998 40 30 .5 DIII-D, #80108 ) β , % = 3 it T β N lim on y 20 ro A tokamak of aspect ratio A=R/a where 1 < A < 2 is commonly conventional (T known as a ‘Spherical Tokamak’ tokamak The plasma eQuilibria appear spherical 10 β = plasma pressure/magnetic field pressure 0 0 2 4 6 8 10 M. Gryaznevich et al., ‘Achievement of record beta in the START Spherical Tokamak’, PRL 80 (18), 1998, p. 3972 normalised plasma current, I p/aB T A.Sykes et al, ‘H-mode operation in the START Spherical Tokamak’, PRL 84 , 2000, p. 495 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) A = 1.8 A=3.25 (modelling from JUST) 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 … ͤ ∗ ̓ ̼ = 2ͦ 2 4 … and Pfus ~ β Bt Vol 3.2T at 1.4m So ineffective as a reactor Use low temperature superconductors? Space is reQuired for a neutron radiation shield … shield plasma centre column 18T For the previous example field on a 0.5m diameter centre column is ~18T 2 4 … and Pfus ~ β Bt 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 3000 the available current density is starting to meet our reQuirements 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 HTS spherical tokamak HTS spherical tokamak High field spherical tokamaks Prototype reactor (small, minimal shielding) (reactor size, no breeding) Neutron shielding => early deployment of commercial fusion power plants Comparative Strategy 16MW 1997 2029 2050 2075 Each step reQuires significant technology development JET ITER DEMO Power Plant ST25(HTS) ST140 ST Power Plant ST60(HTS) ST25 START HTS MAST technology ST40(Cu) High stress magnet Materials other development Shielding design STs R&D: Neutronics Tritium Burning plasma Qfus(eQuivalent) tE scaling (H- studies factor) Note: ST140 is much smaller than JET 10 YEARS TARGET 19 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 8000 30 K, B ⊥⊥⊥ tape 7.5%Zr, 0.9 µm 7000 15%Zr, 0.9 µm 6000 25%Zr, 0.9 µm 5000 20% Zr, 2.2 µm 4000 Zr doping and thicker YBCO layers are yielding significant performance improvements 3000 2000 • temperature is relevant and B // c 1000 Critical current (A/12mm) current Critical • thinner substrate materials are being developed 0 0 1 2 3 4 5 6 7 8 9 Magnetic field (T) Supercond. Sci. Technol. 28 , (2015) 072002 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 • Filamentization of coated conductors is desired for low ac 1 loss applications. 5.1 x • Maintaining filament integrity loss ac (W/m) uniform over long lengths (no Ic multifilamentary 0 reduction) 0.00 0.01 0.02 0.03 0.04 0.05 0.06 B (T) ac rms • Minimum reduction in non superconducting volume 4 mm (narrow gap) and fine filaments Ag HTS • 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.
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