IMPROVEMENTS TO MAGNETIC INTERVENTION

1. IMPROVEMENTS TO MAGNETIC INTERVENTION A.E. ROBSON (Consultant, NRL) in collaboration with D. ROSE (Voss Scientific) HAPL 17 (NRL) October 30 – 31 2007

2. WHY M.I. ? • IONS DON’T REACH THE OPTICS • eliminating the need for 40 separate ion deflectors • 2. IONS DON’T HIT THE CHAMBER WALL • butmaterials problems remain, transferred to external dumps • Q: Is it worth the trouble?

3. THE SIMPLE CUSP HAS PROBLEMS • Large chamber, awkward shape • Weight of upper half, plus atmospheric pressure = 1000’s of tons • Excessive power density on polar dumps Chamber design (Sviatoslavsky) Schematic (Sethian)

4. Prompt (points) 0.09 × 4π Prompt (ring) 0.33 × 4π vz vr Scattered 0.58 × 4π ION ORBITS (3.5 MeV He++) IN ‘STANDARD’ COIL SET • Ion orbit calculations ignore the distortion of the B-field by the ions • They are ‘zeroth order’ approximation to the full picture* • They are easy to do 100 orbits 20 orbits Velocity space *D.Rose, private communication

5. transits Mean lifetime = 4.73 transits DISTRIBUTION OF ION LIFETIMES Only the prompt ions escape in proportion to their initial solid angles in velocity space

6. TAKE ION ORBITS OUT TO 50m “to see where they go” (JDS) Polar cusp, effective solid angle = 0.0185 × 4π Ring cusp, effective solid angle = 0.205 × 4π Summary of 10,000 ion orbits F is the ratio of the fluence/sterad in the cusp to the fluence/sterad in an isotropic expansion The fluence/steradian in the polar cusp is ~ 12 × the fluence/steradian in a simple spherical chamber 50 m

7. WHERE WE STAND ON THE DUMP PROBLEM • ‘Duckbill’ dumps (10o half-angle)can reduce the surface power density by ~ 5, IF the ion flux is evenly distributed. • This makes duckbills feasible for the ring cusp (Raffray, Sviatlovsky), but not for the polar (point) cusps. • We need a radically different technology for the point cusps. ‘Armored’ surfaces will not suffice. • If we can develop this technology, can we devise M.I. systems consisting only of point cusps?

8. Current B out B in A QUASI-SPHERICAL M.I. SYSTEM WITH ONLY POINT CUSPS Tetrahedron (4) Cube (6) Octahedron (8) Dodecahedron (12) Icosahedron (20) Of the five regular polyhedra (Platonic solids), only the octahedron has an even number of faces at each vertex*, allowing all cusps to be point cusps Equivalent in spherical geometry * This requirement was pointed out by Robert L. Bussard (1928 -2007)

9. Field lines Focusing solenoids Spherical windings THE OCTACUSP • The aim of the octacusp is to convert the isotropic expansion of the target into eight identical beams • This 2-D section through four ports illustrates the basic principle, but it gets more complicated in 3-D, as Dave Rose will show in the following talk.

10. ATTENUATION OF PERKINS SPECTRA BY LEAD VAPOR Remaining after 15 mg.cm-2 of Pb = 0.63 Torr0 Pb vapor over 20m To stop all D, T & He needs 334 mg.cm-2 To stop the fast protons needs 585 mg.cm-2

11. ‘Cold’ collar T = 500 oC Toroidal boiler T = 1100 oC Condensing surface T = 850 oC Roots pump Ion dumps/ Baffles Liquid return 56 kg.s-1 2 × 10-2 Torr0 1.3 Torr0 0.65 Torr0 1.5 × 10-5 Torr0 Lead vapor density THE LEAD VAPOR DUMP

12. THE LEAD VAPOR DUMP – A set of numbers POWER at EACH DUMP IONS: 54.5 MW Pb CONDENSATION: 48.4 MW NEUTRONS: 8.6 MW Entering tube: all species Mean energy: 370 keV Total energy: 10.9 MJ Reaching baffle: D,T only Mean energy: 2.75 MeV Total energy: 1.62 MJ 8 escape holes take 5% of chamber surface: Ions confined for 20 transits (mean) τ~ 8µs Dump baffle diameter: 7m Vane angle: 30o Energy on baffle surface: 2.1 J.cm-2 Assumptions Uniform deposition over range depth Pulse shape exp(-t/τ)

13. 15 mg.cm-2 + 300 mg.cm-2 (pulsed) +188 kg.s-1 ►► ◄◄ 56 kg.s-1 THE LEAD VAPOR DUMP – Version 2 (pace JDS) Add mist/ droplets to stop ALL ions Added complexity only justified if dump materials problems remain

14. THE LEAD VAPOR DUMP ACTS AS A VACUUM PUMP Roots pump Target ions swept out by vapor stream > 100,000 l/s ‘COLD’ COLLAR at T = 500oC pPb = 1.5 × 10-5 Torr0 (residual pressure in chamber) Cf. diffusion pump Combined pumping speed of 8 dumps ~ 800,000 l/s

15. THE LEAD VAPOR DUMP - Summary • GOOD • Pb filters the low-energy ions and all the He ions. • The energy fluence reduced by ~ 84%, particle fluence by ~ 98%. • Only high-energy hydrogen isotopes (sputtering coef. < 10-3) hit dump surfaces. No He retention. • PROBLEMATICAL ? • Pulse of ions from target fully ionizes Pb vapor. Need to examine heat transfer processes, including radiation, and plasma effects (which may be beneficial). • High temperature ( > 1000 oC) needed in boiler to get adequate Pb vapor pressure. • High power needed for Pb flow: looks like a heat pipe. Can we use this principle to get all the heat out of blanket?

16. LID BEAMLINES & NEUTRON TRAPS OCTACUSP TUBES & DUMPS Conflict in latitude resolved in longitude TARGET INJECTOR CONCRETE SHIELD/STRUCTURE 60m OCTACUSP REACTOR CONCEPT

17. SUMMARY • The Octacusp aims to convert the isotropic expansion of the target into eight identical directed beams • The 3-D geometry is more complicated than the 2-D geometry of the simple cusp and there are aspects that we don’t fully understand (yet). • Getting the field lines to go where we want may involve additional coils, whose placement may be constrained by the laser beamlines. • Using a condensable vapor (e.g. lead) to absorb the ion beams may have significant advantages over solid dumps and is particularly appropriate for the octacusp. More work is needed to establish feasibility. • THIS IS WORK IN PROGRESS, COLLABORATORS WELCOME!