IMPROVEMENTS TO MAGNETIC INTERVENTION A.E. ROBSON (Consultant, NRL) in collaboration with D. ROSE...
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Transcript of IMPROVEMENTS TO MAGNETIC INTERVENTION A.E. ROBSON (Consultant, NRL) in collaboration with D. ROSE...
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!