Aspect Ratio Optimization of

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

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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, HCD), 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
• NBIn 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

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Example Nb3Sn SC Design Criteria

• Temperature Margin from the max predicted T at
  any point to the local current sharing T.
• Tmarggt1K with FP plasma
• Stability (Heat transfer to Helium)
• Hot Spot Temperaturelt150K
• 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 lt CunonCu lt 1.5 RRR 100 Tcom
18K Bc28T Jc (12T,4.2K,?-0.25) 650 A/mm2
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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 theres an interplay between
  H, shaping, q, nGW,

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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

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Machines with different R/a Elevation View
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Machines with different R/a TF Magnet section
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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, VDEs, 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)

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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

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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 (HHf(q95) ?)
• All these effects are crucial for the
  optimisation of steady state tokamak and when
  included point to reduction of value of optimal A

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Important (but often forgotten) Engineering Issues

• Access to plasma for HCD (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)

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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.