Motion of the ablation cloud in torus plasmas R.Ishizaki, N.Nakajima and M.Okamoto National Institute for Fusion Science US-Japna Workshop PPPL, Princeton, NJ Mar , 2006

1.It is observed in LHD experiments that the ablation clouds are discretely created and drift to the low field side. We will make comprehensive understanding of those physics. 2.The ablation cloud dose not approach the core plasma even if the pellet is injected from the highest field side. It will be clarified what makes the difference between tokamak and LHD. LHD Pellet Cloud Tokamak High field sideLow field sideHighest field side Motion of the ablation cloud in torus plasmas BBB Cloud Pellet Drift to low field side Reconstructed cloud Exp. Injection speed : 10 3 m/s Drift speed : 10 4 m/s Discrete period : ~10  s

Topics : Drifting motion 1. P.B.Parks and L.R.Baylor, Phys. Rev. Lett. 94, (2005). Theory, no resistivity, constant B-field 2. R.Samtaney et al., Comput. Phys. Commun. 164, 220 (2004). Ideal MHD simulation, pellet is point source with ablation model 3. V.Rozhansky et al., Plasma Phys. Control. Fusion 46, 575 (2004). No resistivity, constant B-field, pellet is point source, mass and moment equations 4. H.R.Strauss and W.Park, Phys. Plasmas 7, 250 (2000). Ideal MHD simulation, pellet is plasmoid Topics : Discrete motion 5. P.B.Parks, Plasma Phys. Control. Fusion 38, 571 (1996). Theory, striation Recent works on motion of the ablation cloud. In order to make comprehensive understanding on drifting motion and discrete creation of ablation cloud, MHD simulation including ablation processes but not ablation model will be carried out. Ablation rate is not constant within creation time. Creation time of ablation cloud

1.Motion of high density plasmoid induced by heat flux in tokamak. 2.In order to clarify physics of the drifting motion, the motion is investigated in simple situation where an initial perturbation is uniform in toroidal direction and a bulk plasma is 1/R vacuum field and uniform pressure and not including heat flux. Roadmap. Future work Drifting motion in straight helical and LHD plasmas. Suggestion to obtain a good performance in LHD. Introduction of ablation with atomic processes from a solid pellet to a plasmoid. Comprehensive understanding of drifting motion and discrete creation of the cloud.

Geometry and basic equations. Perfect conductor Z R

Introdunction of the heat flux model enables the code to treat the ablation processes without any point sources. Maxwellian electron heat flux K 2 : Modified Bessel P.B.Parks, Phys. Plasmas 7, 1968 (2000). The model has already used in construction of the ablation model. n and T are assumed to be constant. The energy feedback from the plasmoid to n and T will be included in the future work.

Initial conditions Initial plasmoid Tokamak plasma R Z B2B2

The plasmoid is expanding along the B-field and simultaneously drifts to the low field side. Tokamak1/R vacuum field Uniform pressure

Drifting motion is induced by tire tube force. Drifting motion is disturbed by poloidal field in tokamak. Peak pressure reaches more than 100 times of bulk pressure.

Since 1/R vacuum field is stable according to linear theory, small perturbations have oscillation. On the other hand, large perturbations have different behavior because force balance is not satisfied. Conditions 1.1/R Vacuum field, uniform pressure 2.Initial perturbation, no heating source 3.Uniform in toroidal direction Oscillation Drift Essential points in drifting motion 1.Curvature of B-field 2.Large perturbation

Summary. 1.3D MHD code including the heat flux model is constructed in order to evaluate the motion of the plasmoid with ablation processes. 2.It is verified that the plasmoid is expanding along B-field and simultaneously drifts to the low field side. The drifting speed is about 0.05 Alfven velocity which is fairly comparable to experimental data. 3.The drifting motion is induced by tire tube force. On the other hand, the drifting motion is disturbed by poloidal field in tokamak. 4.If the perturbation is small, it has just oscillation. Then, the essential points in the motion are curvature of B-field and large perturbation. Future work 1.The motion of the plasmoid will be clarified in straight helical and LHD plasmas, and it will be clarified what makes the difference between tokamak and LHD on the motion of the plasmoid. 2.We will make comprehensive explanation of the drifting motion and discrete creation of the plasmoid by including ablation with atomic precesses.

The plasmoid is expanding along the B-field and simultaneously drifts to the low field side. Tokamak1/R vacuum field

Characteristic parameters. Cloud Bulk Exp. Injection speed : 10 3 m/s Drift speed : 10 4 m/s Discrete period : ~10  s R. Ishizaki et al, Phys. Plasmas 11, 4064 (2004).

アブレーション雲は低磁場側へ加速前に圧力の大きな振動が見られる。これは密 度に同期する一方で、速度とは半周期ずれている。