IMPROVEMENTS TO MAGNETIC INTERVENTION

A.E. ROBSON (Consultant, NRL)

in collaboration with D. ROSE (Voss Scientific)

HAPL 17 (NRL) October 30 – 31 2007

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?

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

ION ORBITS (3.5 MeV He++) IN ‘STANDARD’ COIL SET

Velocity space

vr

vz

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

DISTRIBUTION OF ION LIFETIMES

Cusp: Ring Point (2)

Total ions: 55.6% 44.4%Prompt ions: 32.6% 8.6%

Mean lifetime = 4.73 transitstransits

Only the prompt ions escape in proportion to their initial solid

angles in velocity space

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

50 m

Summary of 10,000 ion orbits

Cusp Number #/sterad cf. over 4π F

Ring 5,560 2,160 796 2.7

Polar (ea) 2,220 9,550 796 12

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

• ‘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?

WHERE WE STAND ON THE DUMP PROBLEM

Tetrahedron (4) Cube (6) Octahedron (8) Dodecahedron (12) Icosahedron (20)

A QUASI-SPHERICAL M.I. SYSTEM WITH ONLY POINT CUSPS

B inB 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

CurrentEquivalent in spherical

geometry

* This requirement was pointed out by Robert L. Bussard (1928 -2007)

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

ATTENUATION OF PERKINS SPECTRA BY LEAD VAPOR

Ion Energy % Number %

D 32.5 25

T 21.8 24

He 0.46 0.25

All 16.5 2.2

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

THE LEAD VAPOR DUMP

Roots pumpToroidal boiler T = 1100 oC

Condensing surface T = 850 oC

‘Cold’ collar T = 500 oC

1.3 Torr0 0.65 Torr01.5 × 10-5 Torr02 × 10-2 Torr0

Ion dumps/ Baffles

Lead vapor density

Liquid return56 kg.s-1

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: 30o Energy on baffle surface: 2.1 J.cm-2

Baffle material

Range µm

κ cm2.s-1

ΔT(no cond)

ΔT(w/cond)

Fe 10.8 0.07 399 227

Cu 10.8 0.93 504 107

Mo 11.8 0.45 701 218

Pd 9.5 0.23 656 236

W 8.8 0.44 867 173

Pb (liq) 14.6 0.09 854 524

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)

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

THE LEAD VAPOR DUMP ACTS AS A VACUUM PUMP

Roots pumpTarget 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

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?

OCTACUSP REACTOR CONCEPT

TARGET INJECTOR

BEAMLINES & NEUTRON TRAPS

LID

OCTACUSP TUBES & DUMPS

CONCRETE SHIELD/STRUCTURE

Conflict in latitude resolved

in longitude

60m

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!