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FAST — Scientific Rationale

The FAST conceptual study has been proposed as possible European ITER [1] Satellite facility with the aim of preparing ITER operation scenarios and helping early DEMO [2] design and R&D.
Insights into ITER operation regimes and physics of reactor relevant plasmas can be obtained by experiments for the integrated study of:

  1. plasma wall interactions with very large power load;
  2. plasma operation open problems (i.e. Edge Localized Modes (ELMs), plasma control,…);
  3. non linear dynamics, that are relevant for the understanding of alpha particle behaviours in burning plasmas, by using fast ions accelerated by heating and current drive systems [3-5].

The possibility of investigating integrated burning plasma physics issues in an ITER satellite facility largely relies on the similarity argument based on the existence of three dimensionless parameters in the equations governing quasi-neutral, collisional, finite-β plasmas; i.e., ρ*, β and ν* [6]. These parameters are the ion Larmor radius in units of the torus minor radius (ρ*), the ratio of plasma to magnetic pressure (β) and the ratio of connection length to the collisional mean free path.

For fixed equilibrium geometry and profiles, the similarity argument corresponds to having one free quantity to choose among B (magnetic field), R (major radius), n (density) and T (temperature). The choice underlying the FAST scientific rationale is to relax idea of maintaining identical ρ*, β, ν* and fixing T with the consequent ρ*∝R-1/2 scaling.
This choice ensures systematic derivation of FAST plasma scenarios in which ρ* is within a factor 31/2 from ITER values as an upper bound [5].
For fixed ratio of the fast particle (FP) slowing down time (τSD) to the energy confinement time (τE), fixing T implies fixing plasma performance, since τSD / τE ∝ T5/2/N, with N=nTτE.
Fixing T corresponds to controlling edge physics conditions and Plasma Wall Interactions (PWIs) as well.
As anticipated in [4,5], the FP population must satisfy the following criteria:

  1. EH must satisfy the condition EH > Ecrit∝T, with Ecrit the critical energy, corresponding to dominant electron heating by FPs (»70% by fusion as in ITER);
  2. the fast ion induced fluctuation spectrum must be preserved in mode number
    (ρ*H ~ ρ*H,ITER) and normalized frequency (H/ ωA)~( ωH / ωA)ITER, with ωH the FP characteristic frequency [9] and ωA =vA/qR the Alfvén frequency);
  3. the strength of the waveparticle interactions must be preserved (given β, βH ~ βH,ITER ⇔ (τSD / τE) ~ (τSD / τE)ITER).

Selecting T≅13keV, and by assuming the perpendicular supra-thermal 3He minority tail ∝ exp (-EH/TH) due to Ion Cyclotron Resonance Heating (ICRH) in D plasmas, it is possible to show that TH ≅750keV [5] gives »70% of collisional power transfer from FPs to electrons.
Following [3], one can further relate T and R imposing that τSD / τE ~ const and using a confinement scaling law, such as ITER98y2, for expressing N∝(Ip/Ra)5/2 with a=1/3 [3].
By using the standard scalings [3-8], N∝(T1/2/Ra-1/2)5/2 , this implies T∝R1/3, Ip∝R2/3, B∝R-1/3, τres∝R3/2 (resistive time), ν*∝R-2/3 PADD∝R5/6, and N∝R5/6 [5].
These scalings encompass physics integration as discussed above and in Ref. [5]; meanwhile they allow us to elucidate how physics integration reflects on macroscopic design parameters as a function of system size. A larger device would be favored by a moderate improvement.
Note that, given wave-number spectrum invariance, normalized frequency invariance w.r.t. ωA and/or ωTi automatically implies normalized frequency invariance w.r.t. magnetic and diamagnetic drift frequencies, towards ITER relevance (ρ*∝R-1/2 and ν*∝R-2/3); however, at the same time, it would be more demanding for additional power (PADD∝R5/6) and discharge duration (τres∝R3/2) for achieving long pulse operation.
Engineering constraints would also become increasingly more severe (B2R2∝R4/3) and cost would levitate (€∝B2R3∝R7/3). For instance, a device of R=3m would be marginally closer to ITER than FAST (R=1.82m); meanwhile it would require B=6.3T, Ip=9.1MA and PADD=45MW to achieve the same integrated physics as in the FAST H-mode reference scenario, it would need to operate with a pulse length of 360s at PADD=60MW to reproduce the AT Full NICD scenario at 170s, and it would be more than 3 time as expensive.

In brief, FAST aims at helping preparation of ITER scenarios and the development of new expertise for DEMO design and R&D in an integrated fashion, simultaneously addressing many aspects of non linear dynamics that are relevant for the understanding of alpha particle behaviors in burning plasmas and their interaction with plasma turbulence and turbulent transport, exploiting advanced regimes with long pulse duration with respect to the current diffusion time and up to full non-inductive current driven (NICD), testing technical innovative solutions for the first wall/divertor directly relevant for ITER and DEMO, and providing a test bed for ITER and DEMO diagnostics as well as an ideal framework for model and numerical code benchmarks, verification and validation in ITER and DEMO relevant plasma conditions.

The prerequisites to be satisfied, in order to reproduce the physics of ITER relevant plasmas, yield the following set of FAST parameters [5]:

  1. plasma current, IP, from 2 MA (corresponding to full NICD) up to 8 MA (corresponding to maximized performance);
  2. auxiliary heating systems able to accelerate the plasma ions to energies in the range of 0.5÷1 MeV;
  3. major radius of about 1.8m and minor radius around 0.65m;
  4. pulse duration from 20s for the reference H-mode scenario, up to 170 s (~ 40 resistive times τres) at 3MA/ 3.5T.
FAST Operating Scenarios FAST operating scenarios table

References

  1. [1] Holtkamp N. et al 2008 Proc. 22nd Int. Conf. on Fusion Energy 2008 (Geneva, Switzerland 2008) (IAEA: Vienna) CD-ROM file [OV/2-1] and http://www-naweb.iaea.org/napc/physics/FEC/FEC2008/html/index.htm
  2. [2] Nishitani T. et al 2008 Proc. 22nd Int. Conf. on Fusion Energy 2008 (Geneva, Switzerland 2008) (IAEA: Vienna) CD-ROM file [FT/1-3] and http://www-naweb.iaea.org/napc/physics/FEC/FEC2008/html/index.htm Nucl. Fusion submitted
  3. [3] Romanelli F. et al 2004 Fusion. Sci. Technol. 45 483
  4. [4] FAST-Team Technical Report ENEA/FPN-FAST-RT-07/001
  5. [5] Pizzuto A. et al 2008 Proc. 22nd Int. Conf. on Fusion Energy 2008 (Geneva, Switzerland 2008) (IAEA: Vienna) CD-ROM file [FT/1-5] and http://www-naweb.iaea.org/napc/physics/FEC/FEC2008/html/index.htm Nucl. Fusion submitted
  6. [6] Kadomtsev B.B. 1975 Sov. J. Plasma Phys. 1 295
  7. [7] Lackner K. 1990 Comments Plasma Phys. Control. Fusion 13 163
  8. [8] Lackner K. 1994 Comments Plasma Phys. Control. Fusion 15 359
  9. [9] Cardinali A. et al 2008 Proc. 22nd Int. Conf. on Fusion Energy 2008 (Geneva, Switzerland 2008) (IAEA: Vienna) CD-ROM file [TH/P3-6] and http://www-naweb.iaea.org/napc/physics/FEC/FEC2008/html/index.htm Nucl. Fusion submitted