Challenges in Tokamak Optimization for Burning Plasma: Aspect Ratio Study

1. Aspect Ratio Optimization of Burning Plasma Tokamaks Pietro Barabaschi ITER International TeamGarching Prepared for IEA Workshop on Optimization of High-b Steady-State Tokamaks February 14-15, 2005 General Atomics

2. Introduction • The ITER EDA.. developed -needed design solutions, -enabling technologies, -and knowledge base • BUT, the Tokamak is a complex system and for its optimisation it requires a detailed understanding of all interplays of design drivers (and the devil is in the details!) • We do not have the basis and criteria to design (or even more to optimise!) a fusion reactor today, most notably we are missing: • Plasma burn demonstration • Adequate understanding of Plasma • Practical viability of fusion • Beta (device optimisation) • Reliability, Availability, Maintainability (R.A.M.) • Materials • Divertor power exhaust • Higher performance structures/SC • Until we understand and develop all these points the cost optimisation of a reactor may not be realistic

3. Tokamak Design: Machine parameters • For a SC Tokamak, given: -Desired Plasma “performance”: Q, burn time, # of shots -Plasma Boundary conditions: q, ngwmax, bNmax, k, d, HH -Physics Criteria: t, ngw, PLH, Beta -Engineering Criteria: Stress, loads, SC criteria, times and solutions for maintenance, Access to Plasma (diags, H&CD), Nuclear criteria, Design solutions… • Only Aspect ratio (or Peak Field in magnet) is left ‘free’ • However, allowable k and d are function of R/a for divertor space, plasma shape and position control. Access to plasma is function of ripple requirements and R/a • NB:In the case of Steady State tokamaks also the safety factor may be an optimisation parameters. • A System code is normally used to study options combining physics rules with engineering design knowledge. It can only be used if a more detailed design of a particular type of machine has been done and the experience / knowledge has been implemented

4. Example : Nb3Sn SC Design Criteria • Temperature Margin from the max predicted T at any point to the local current sharing T. Tmarg>1K with FP plasma • Stability (Heat transfer to Helium) • Hot Spot Temperature<150K • 1 and 2 determine the amount of SC strand, 3 determines the amount of additional copper and overall conductor size • Main significant system interactions with: Magnet (local cost, System size and cost, VV (stress due to Fast Discharge)  Strand 1 < Cu:nonCu < 1.5 RRR 100 Tcom= 18K Bc=28T Jc (12T,4.2K,-0.25%) = 650 A/mm2

5. Plasma Performance Criteria q95 • BUT…. • All 3 main scaling relationships have little real physics basis!! Optimisation have big limitations and uncertainties! • It is likely that there’s an interplay between H, shaping, q, nGW, …

6. Typical results from the System Code Study • The main machine parameters and the cost change with increasing aspect ratio in the following way: • The toroidal Field, the Magnetic Energy increase with Aspect Ratio • The minor radius and the Plasma Current decrease with Aspect Ratio • However, the cost of the machine stays ~constant over most of the Aspect Ratio range investigated

7. Machines with different R/a – Elevation View

8. Machines with different R/a – TF Magnet section

9. System Analysis: Design Drivers • Radial Build: D  10cm R  18cm C  60kIUA • Shielding (heating, damage, reweldability) • CS Magnet • TF Magnet • Assembly and tolerances • Elongation: k95  0.1 R  17cm C  80kIUA • BUT …(Stability, VDE’s, SN-DN control, Divertor space, flexibility) • Triangularity: d95  0.1 R  10cm C  100kIUA • BUT …(SN-DN control, Divertor space, Sawtooth R, Magnet loads) • Safety Factor: q95  0.1 R  5cm C  50kIUA • BUT …(HH degradation, Disruptions loads, Magnet loads) • Confinement: H  0.1 R  12cm C  130kIUA (Note: DC/DR not constant)

10. A Power Reactor Where is a Power Plant quite different from an experiment? • Additional problems • In physics : • Beta(density, peaking), • Steady State • In engineering : • Remote Maintenance • Current drive efficiency • Reliability • Availability • Power exhaust • Materials • With some simplifications • In physics : • Experimental Flexibility • In engineering: • Disruptions/VDEs? • Diagnostics • Heating methods • Fatigue

11. Aspect Ratio related issues • Relation between shape and density „limit“ is far too simplified. Effects of shaping must be included. • Beta limit is NOT an invariant of aspect ratio • limit = f(A) • At low A the relative distance between plasma and wall is less (RWM) • Achievable shaping is NOT an invariant of aspect ratio • Natural elongation increases at low A • Available space for divertor at given d increases at low A • Relative (and absolute) distance between plasma and PF magnet reduces at low A. This impact shape control (in particular in SN) as well as plasma vertical controllability. • In steady state tokamak q95 is also a „free“ optimisation parameter (HH=f(q95) ?) • All these effects are crucial for the „optimisation“ of steady state tokamak and when included point to reduction of value of optimal A

12. Important (but often forgotten) Engineering Issues • Access to plasma for H&CD (and maintenance of internals) . In particular tangential for NBI (ans shinethrough issues). Check carefully at High A!! • Space available for divertor • Thermal and EM loads on Blanket a function of A (PF almost constant but TF not!) • Cost and replacement reqs of internals is a function of complexity (thermal and mechanical loads) • TF discharge parameters is important for VV stresses (and cost) at high A. • Space for water cooling and pipe extraction Again all the above issues, when included, tend to benefit low A (note: for Cu devices with limited pulse duration – e.g. FIRE – High A benefits largely from reduction of required pulse length with reduced a)

13. Conclusions • We cannot optimise a steady state tokamak today. • First we must understand the underlying PHYSICS and gain experience in construction, operation, and MAINTENANCE of a large super-conductive tokamak. • Warning: Simple scaling optimisation of steady state tokamak typically points to relatively high A (e.g. 3.5 - 4) due to low current requirements and high IBS fraction. This may be well be an illusion. Real engineering and more accurate physics basis may well reverse this conclusion.