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

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

4. ION ORBITS (3.5 MeV He ++ ) IN ‘STANDARD’ COIL SET Velocity space vrvr vzvz Prompt (ring) 0.33 × 4π Prompt (points) 0.09 × 4π Scattered 0.58 × 4π 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 *D.Rose, private communication 20 orbits 100 orbits

5. DISTRIBUTION OF ION LIFETIMES Cusp:RingPoint (2) Total ions:55.6%44.4% Prompt ions:32.6%8.6% Mean lifetime = 4.73 transits transits 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 × 4 π Ring cusp, effective solid angle = 0.205 × 4 π 50 m Summary of 10,000 ion orbits CuspNumber#/steradcf. over 4πF Ring5,5602,1607962.7 Polar (ea)2,2209,55079612 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

7. ‘Duckbill’ dumps (10 o 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? WHERE WE STAND ON THE DUMP PROBLEM

8. Tetrahedron (4) Cube (6) Octahedron (8) Dodecahedron (12) Icosahedron (20) A QUASI-SPHERICAL M.I. SYSTEM WITH ONLY POINT CUSPS B in B out 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 Current Equivalent in spherical geometry * This requirement was pointed out by Robert L. Bussard (1928 -2007)

9. 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. Focusing solenoids Spherical windings Field lines

10. ATTENUATION OF PERKINS SPECTRA BY LEAD VAPOR IonEnergy %Number % D32.525 T21.824 He0.460.25 All16.52.2 Remaining after 15 mg.cm -2 of Pb = 0.63 Torr 0 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. THE LEAD VAPOR DUMP Roots pump Toroidal boiler T = 1100 o C Condensing surface T = 850 o C ‘Cold’ collar T = 500 o C 1.3 Torr 0 0.65 Torr 0 1.5 × 10 -5 Torr 0 2 × 10 -2 Torr 0 Ion dumps/ Baffles Lead vapor density Liquid return 56 kg.s -1

12. THE LEAD VAPOR DUMP – A set of numbers 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: 30 o Energy on baffle surface: 2.1 J.cm -2 Baffle material Range µm κ cm 2.s -1 ΔT (no cond) ΔT (w/cond) Fe10.80.07399227 Cu10.80.93504107 Mo11.80.45701218 Pd9.50.23656236 W8.80.44867173 Pb (liq)14.60.09854524 POWER at EACH DUMP IONS: 54.5 MW Pb CONDENSATION: 48.4 MW NEUTRONS: 8.6 MW Assumptions Uniform deposition over range depth Pulse shape exp(-t/ τ)

13. THE LEAD VAPOR DUMP – Version 2 (pace JDS) Add mist/ droplets to stop ALL ions 15 mg.cm -2 + 300 mg.cm -2 (pulsed) ◄◄ 56 kg.s -1 +188 kg.s -1 ►► 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 = 500 o C p Pb = 1.5 × 10 -5 Torr 0 (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 o C) 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. OCTACUSP REACTOR CONCEPT TARGET INJECTOR BEAMLINES & NEUTRON TRAPS LID OCTACUSP TUBES & DUMPS CONCRETE SHIELD/STRUCTURE Conflict in latitude resolved in longitude 60m

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!