ITER Halo Currents Question for US PFC Community

1. ITER Halo Currents Question for US PFC Community M. Ulrickson For the ITER Team August 6, 2010

2. Halo Current Generation During a disruption the plasma moves toward the first wall (divertor also sometimes). Flux surfaces carrying current are broken by the contact with the wall. The current flows through the first wall and shield block to the vessel to another shield block and first wall to reconnect with the flux surface. The halo current scrape-off length is found to be wider than the power scrape-off length on existing devices (even wider than the far scrape-off layer power width. The DINA code is used to simulate halo currents in ITER. The portion of the first wall receiving halo current depends on the motion of the plasma.

3. Predicted Halo Current on ITER The halo current data from DINA has been integrated over each blanket module as a function of time. Since DINA is a 2D code the values are representative of a whole toroidal row of modules. The halo current to a single module is found by dividing by the number of modules in a row (18 or 36), multiplying by the toroidal peaking factor (2) and an uncertainty factor (1.3). The variation of halo current flow over a FW panel is determined by mapping flux surfaces onto the shaped FW and using the observed halo scrape-off length. The average wetted area is about ¼ of the FW surface area.

4. VDE_Up_Slow_Fast_II

5. VDE_Up_Slow_II

6. VDE Down_Slow_Fast_II & Slow_II

7. VDE_Up_Slow

8. MD_up_Lin

9. Load Contribution • Halo Currents • Flow radially and poloidally to reach the vessel attachments • Toroidal halo current flow generates some loads but they are small compared to the radial and poloidal flow interaction with the toroidal field. • Radial flow generates net poloidal force and toroidal moment (because of the thickness of the SB). • Poloidal flow generates radial force and poloidal moments because of the distribution of halo current on a FW panel. • The halo forces are directly proportional to the halo current.

10. VDE up Linear III

11. VDE up Exponential III

12. Halo Current Density Estimate Given the maximum halo current to a row (4.5 MA) and 18 modules per row and the peaking factor shown above. The maximum estimated halo current on a module is 400 kA The wetted area is about 0.25 m2 Hence the current density to the FW is 1.6 MA/m2 The lower end of the scale is 400 kA/m2

13. Question for the PFC Community Given the halo current density on the FW, if the plasma has even a modestly low temperature (1 eV), the power flow to the FW can be quite substantial (MW/m2). Since the duration of the halo event can be 10’s of msec to seconds, the heating of the FW can be quite substantial. Given the long scrape-off length for halo current, components that are shadowed by being recessed from the FW horizon may also see substantial power due to halo current. When I asked people on the ITER STAC call to find data on the power due to halo current, no data was cited but there was a general feeling no significant temperature rise on the FW could be attributed to halo flow. What would be the prediction of the power flow due to halo currents using our edge codes?

14. Discussion • Strategy for answering the question • Step 1 • Step 2