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Aspect Ratio Optimization of

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Title: Aspect Ratio Optimization of


1
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
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, 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

4
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
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 theres 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, 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)

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 (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

12
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)

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.
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