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Laboratório Associado de Plasma, Instituto Nacional de Pesquisas Espaciais

12227-010 São José dos Campos, SP, Brazil

Fluid model of electron cyclotron current drive

G.O. Ludwig

17th IAEA Technical Meeting on

Research Using Small Fusion Devices

Centro de Fusão Nuclear, Instituto Superior Técnico

22nd to 24th October 2007 Lisbon, Portugal

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Fluid model of electron cyclotron current drive (ECCD)G.O. Ludwig

Abstract

A macroscopic model was recently proposed for describing the dynamics of ECCD in tokamak plasma [1]. This model depends on the adoption of a suitable distribution function for the relativistic magnetized stream of current-carrying electrons [2], thus avoiding the use of complex quasi-linear 3-D Fokker-Planck codes. The model is readily applied to examine the current drive efficiency of high-power ECCD experiments [3,4,5,6], reproducing the main experimental features.

Fokker-Planck (F-P) equation

• F-P equation for streaming electrons neglecting trapped particle and radial transport effects

• The fluxes due to both an inductive electric field and diffusion by RF waves are given by

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Moments of the Fokker-Planck equation

• The rates of change of the kinetic energy and momentum give the equations of motion

where the collision frequency of energetic electrons with the background plasma is normalized to c

• The volumetric densities of driven power and force are given in terms of the driving fluxes by

• These quantities must satisfy the electrodynamic relation

which gives the rate at which RF energy is being converted to kinetic form

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Quasi-linear RF diffusion dyadic

• In cylindrical coordinates in momentum space the gyro-averaged quasi-linear diffusion dyadic is

• In the small gyroradius limit for the fundamental resonance of electron cyclotron waves one has

where the diffusion coefficient strength is expressed in terms of the perpendicular wave amplitude by

• For a narrow spectrum centered at n║, within a small range Δn║, the function Δ(ξ) is ~ a delta function

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RF power and force densities

• The average values are evaluated according to the transformation (ξ, β║)→(πp┴2, p║ )

• For Δn║→0 the leading terms in the expressions of the power and force densities are

reducing the evaluation of average values to single integrals along the resonance lines

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Resonance lines

• Resonance lines in velocity space normalized to the speed of light for ω=0.982Ωe

• The curves correspond to the refractive index n║=0, 0.15, 0.3, 0.5 and 0.9 from center to edge

• The dashed lines represent the trapped particle region for a mirror ratio 2/3, although the present model does not include its effect

• Trapped particle effects can be included on an average basis in the present zero-dimensional model

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Distribution function of a magnetized relativistic electron stream (Dory-Guest-Harris type)

• The distribution function of a weakly collisional, strongly magnetized relativistic electron stream is [2]

where the normalization coefficient is determined by the condition nℓ = ∫ fℓ d3p

• The intensive thermodynamic quantities satisfy the Lorentz transformation properties

• Taking ℓ = 0, T→∞ and <β║> = 0 the Jüttner distribution for an isotropic relativistic gas is recovered

• In the warm plasma limit the Chew-Goldberger-Low double adiabatic equations are recovered

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Macroscopic quantities in the rest frame of the stream (vanishing momentum density)

• Four parameters: <β║>, ℓ , T║, T → three macroscopic quantities in the rest frame (<β║>=0)

• Energy density, magnetization and enthalpy density of component ℓ (apply a boost to lab frame)

• Parallel and perpendicular pressures (P║, M/n and P┴ are Lorentz invariants)

• Take stream distribution function as a sum of ℓ = 0 (bi-Maxwellian) and ℓ = 1 (DGH-like) components

• Four parameters: <β║>, F, T║, T → three macroscopic quantities in the rest frame

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Macroscopic model of ECCD

• Using the above form of the distribution function both the energy and parallel momentum losses by collisions with the background plasma, and the average perpendicular velocity are evaluated

with <β┴> satisfying the electrodynamic relation (conversion of RF to kinetic energy)

• The electron stream in ECCD does not perform work and there is no heat addition if the RF power input is balanced by power lost through collisions, leading to the thermodynamic relation (Gibb’s law)

• The equations of motion constrained by the electrodynamic and thermodynamic relations give four equations depending on the parameters <β║>, F, T║ and T of the distribution function

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ECCD discharges

• The expression of the parallel force density shows that the RF driven current vanishes when n║→0

• For a fully RF driven tokamak plasma the inductive electric field is put equal to zero

• In the steady-state the global figure of merit is defined by

with

• Relative density of high-energy electrons as a function of n║ for ω = (0.975,0.982,0.985)Ωe

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Application to the DIII-D electron cyclotron current drive experiment [4, 5]

• Tokamak parameters: R=1.7m, a=0.6m, κ=1.8, B=2.0T

• Plasma parameters: ne=2.01019m-3, Te=2.7keV, Zeff=1.4, Ip=900kA

• ECCD experiment: fRF=110GHz (2nd harmonic, ω=0.982Ωe), WRF=1.5MW, IRF=75kA, ζRF=0.05

• Assumptions from ray-tracing: φT=20º, n║= sin(φT)=0.342, Δn║=0.02, D0=? (reference solution)

• Equilibrium solutions: 0.662 < D0 / (me2c2νc) < 2.24 → 0.134 < ΔAp/Ap < 0.151 ~published results [5]

• Assume normalized diffusion coefficient strength D0 / (me2c2νc) =1; scan n║ for fixed WRF

φT F <β║> T║0/(mec2) T0/(mec2) T┴

0/(mec2)

0o 0.628 0 0.252 0.0174 0.0232

20o 0.562 0.0303 0.100 0.173 0.0918

φT n/ne ΔAp/Ap PRF/(nmec2νc) F┴/(nmecνc) <β┴>

0o 0.0241 0.0447 2.97 16.2 0.183

20o 0.00901 0.141 2.53 6.93 0.361

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Application

• Electron distribution function truncated at 0.025 of its maximum value and contour plot of the distribution function for DIII-D (ω=0.982Ωe, Δn║=0.02, D0/(me

2c2νc)=1, WRF=1.5 MW)

φT=0o φT=20o

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Application

• Logarithmic plots of the distribution function (continuous line at pitch angle 0o, dashed line 90o and dotted line 180o)

φT=0o φT=20o

• φT=0o: contribution of high-energy electrons (~128 kV) in the tail of the distribution along the magnetic field lines; much lower energy in the perpendicular direction (<12 kV)

• φT=20o: about the same energy (~50 keV) in the parallel and perpendicular directions; small asymmetry between the forward and backward directions due to drift motion (<β║>≈ 0.03)

• The proposed form of the distribution function may be useful for fitting experimental results

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Application

• Stream parameters (grey circles ω=0.975Ωe, black circles ω=0.982Ωe, dots ω=0.985Ωe, line φT=20o)

• <β║> increases with n║; cooling in the parallel direction and demagnetization (T→∞ at maximum n║)

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Application

• Figure of merit (grey circles ω=0.975Ωe, black circles ω=0.982Ωe, dots ω=0.985Ωe, line φT=20o)

• Decrease in the stream density for large n║; compensated by increase in effective ΔAp (fixed WRF)

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Application

• RF power density (grey circles ω=0.975Ωe, black circles ω=0.982Ωe, dots ω=0.985Ωe, line φT=20o)

• RF power deposition limited by collisions; decrease in F┴ compensated by increase in <β┴>; increase in T┴ implies demagnetization of the high-energy stream (<12 → 60 keV for 0o→25o)

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Experimental results DIII-D [5]

• Figures 1 and 2 (left): ΔAp/Ap≈ 0.13; ζRF≈ 0.05 at φT=20o (experimentalists definition)

• Figure 3 (right): Fluid model results (WRF=1.5 MW; vertical line n║=0.342, φT=20o)

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Experimental results TCV [6]

• TCV model: similar to DIII-D model, but

larger stream density and

larger effective area of power deposition

(same WRF=1.5 MW in small volume)

• Figure 1 (right): Experimental results

12o<φT<30o; 12 keV<T┴<50 keV

• Figure 2 (above): Fluid model results for DIII-DφT=0o → T┴<12 keV; φT=20o → T┴=51.1 keV;φT=25o → T┴≈ 60 keV

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Conclusions

• Macroscopic model based on a suitable distribution function of the high-energy electrons

• Effective value of the diffusion coefficient strength determined from reference solution:

• given toroidal launching angle, input power and global figure of merit

• obtained poloidal area of power deposition consistent with experimental values

• Main features of ECCD reproduced for fixed input power and diffusion coefficient strength:

• refractive index scan gives consistent variation of the global figure of merit

• perpendicular temperature variation consistent with HXR measurements

• Proposed form of the distribution function may become an useful tool for fitting experimental results

• Provides simple model for studying the time evolution of ECCD and possible instabilities (Weibel)

Future work

• The model must be tested using results of ray-tracing codes

• Must include pitch-angle scattering due to Weibel instability (also true for F-P codes)

• Must include trapped particle effects

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Acknowledgment

• Work partially supported by the IAEA under contract BRA/12932

References

[1] G.O. Ludwig, “Macroscopic model of electron cyclotron current drive”, Plasma Phys. Control.Fusion (2007) submitted.

[2] G.O. Ludwig, “Relativistic distribution functions, fluid equations and equations of state for magnetized electron streams”, Plasma Phys. Control. Fusion 49, 1521-1549 (2007 ).

[3] O. Sauter et al., “Steady-state fully noninductive current driven by electron cyclotron waves in a magnetically confined plasma”, Phys. Rev. Lett. 84, 3322-5 (2000).

[4] R.W. Harvey, O. Sauter, R. Prater and P. Nikkola, “Radial transport and electron-cyclotron-current drive in the TCV and DIII-D Tokamaks”, Phys. Rev. Lett. 88, 2050001-1-4 (2002).

[5] C.C. Petty et al., “Detailed measurements of the electron cyclotron current drive efficiency on DIII-D”, Nucl. Fusion 42, 1366-1375 (2002).

[6] S. Coda et al., “Electron cyclotron current drive and suprathermal electron dynamics in the TCV tokamak”, Nucl. Fusion 43, 1361-1370 (2003).