hif (heavy ion fusion) gas desorption issues*

The Heavy Ion Fusion Virtual National Laboratory

HIF (Heavy Ion Fusion) Gas Desorption Issues*

A.W. Molvik1,2

With contributions from F.M. Bieniosek1,3, J.J. Barnard1,2, E.M. Bringa2, D.A. Calahan2, C.M. Celata1,3, R.H. Cohen1,2, A. Friedman1,2, M.A. Furman3, J.W. Kwan1,3, B.G. Logan1,3, W.R. Meier2, A. Sakumi4, P.A. Seidl1,3,W. Stoeffl2, S.S.

Yu1,3, 1 HIF-VNL, 2 LLNL, 3 LBNL, 4CERN

Workshop on Beam-Induced Pressure Rise in RingsBrookhaven National Laboratory

December 9-12, 2003

The Heavy Ion Fusion Virtual National LaboratoryMolvik, BNL-1203, 2

OUTLINE

• Introduction to Heavy Ion Fusion (HIF)

Recent Robust Point Design (RPD) – a self-consistent,

detailed, and conservative HIF power plant design

• Why are we concerned about pressure rise in a linac?

• Pressure rise issues at several Hz

• Measurements of gas desorption & electron emission

• Hypotheses on sources of gas and electrons


3 - 7 MJ x ~ 10 ns ~ 500 Terawatts

Ion Range: 0.02 - 0.2 g/cm2 1- 10 GeV

Beam charge (3-7 MJ/1-4 GeV) few mCoul

Target Requirements establish accelerator requirements for power plant driver

0.7 cm

1.5 cm


Artist’s Conception of an HIF Power Plant on a few km2 site

120 beams Multibeam

Accelerator


Efficiency increases as current increases

Multiple beams withinsingle induction core

Induction Acceleration is used for efficiency

B


The First Wall Protected by Neutron-thick Molten Salt FLiBe, FLiBe is a low Z salt low activation Green fusion energy

Crossing jets form beam ports

Vortices shield beamline penetrations

Oscillating jets form main pocket

(One Half Cut Away)

But vapor density ~ 1013 cm-3 too high for accelerator


The Robust Point Design beam line – pumps and blocks chamber vapor from accelerator

9 0 01 7 0 0

3 4 0 02 0 0 0

F o c u s M a g n e t S h i e l d i n g S t r u c t u r e F l i n a b e L i q u i d

J e t G r i d

P o c k e t

V o i d

5 0 0 2 9 0 0

CL

T a r g e t

S c h e m a t i c L i q u i d J e t G e o m e t r y

N e u t r a l i z i n g P l a s m a

I n j e c t i o n

L i q u i d V o r t e x

E x t r a c t i o n

> 2 0 0 0

L i q u i d V o r t e x

I n j e c t i o n

B a r e T u b e F l i n a b e V o r t e x

( < 4 0 0 ° C )

P l a s m a /

M a g . S h u t .( 6 0 0 - 6 5 0 ° C )

T a r g e t I n j e c t i o n


Building block pulse shape – illustrative of conservative approach in Robust Point Design

Beam and Pulse Shape Requirements

48 foot pulse beams:T = 3.3 GeV, EF = 1.76 MJ

72 main pulse beams:T = 4.0 GeV, EM = 5.25 MJ

120 total beams:ED = 7.0 MJ

The Heavy Ion Fusion Virtual National Laboratory

Robust PointDesign (2.8 B$)

Beam

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

20 30 40 50 60 70 80 90 100

Fill factor, %

Total driver cost, $B

Beam pipe

Rpipe

aavg

amax

Fill factor = amax/RpipeIBEAM results:

(fixed number of beams, initial pulse length, and quadrupole field strength)

Clearance

range being explored

~$1B

System studies show that driver cost reduced at high fill factor [fill factor may be limited by beam-induced desorption]

Electron Cloud Effects (ECE) may also limit HIF Fill factor


Gas desorption (or ECE) may be an issue in HIF linacs

• Economic mandate to maximally fill beam pipe

• Linac with high line charge density (Beam potential > 1 kV)

{ionized gas ions expelled to wall, og ~ 10 }

• Induction accelerator – pulse duration up to ~20 µs at injection,

down to ~0.2 µs at higher energy [Time for desorbed gas to reach

beam], ~5 Hz rep. rate [time to pump desorbed gas?], multiple

beams in parallel, frequent acceleration gaps, large neutral

desorption coefficients at pipe wall (~103 - 104 in present HIF-VNL,

CERN, and GSI heavy-ion accelerators)

• Heavy-ions – stripping cross sections E-0.5, v E0; don’t win

at high energy like proton accelerator where E-1

• Large fraction of length occupied by quadrupoles (>50% at

injector end)


Heavy ions may hit wall multiple times, increasing desorption

TRIM Monte Carlo Code predicts• 60-70% scatter at 88-89 • 0.05-0.5% scatter at 0-45

Beam scrapers effective• Spread in angle ~0.2 rad.• Issues

- Spreads ion loss azimuthally- Causes electron emission- Scattering decreases slowly with

energy near grazing incidence.50,000 1.8 MeV K+ incident on SS at 45 deg., 0.5% scatter

0.00E+0

5.00E+5

1.00E+6

0 0.5 1 1.5 2Angle of scattered ions (rad)

Energy of scattered ions (eV)

450.5%

0 1 radian 20

12000 1.8 MeV K+ ions incident at 88 deg., 64% scatter

0.0E+0

5.0E+5

1.0E+6

1.5E+6

2.0E+6

0 0.5 1 1.5 2

Angle of scattered K+ ions (rad)

Energy of scattered ions (eV)

8864% Scat.

0 1 radian 20

2

MeV

SRIM-2003, K+ on SS target

1E-5

1E-4

1E-3

1E-2

1E-1

1E+0

1 10 100 1000K+ ion beam energy (MeV) .

Ion back-scatter coefficient

89 deg.85 deg.0 deg.


Gas buildup can limit peak beam current in rapidly pulsed accelerator

ˆ I b =0.5qeπrw fwpnovo

n0σiΓog +n0σxΓob +fhaloΓob

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

0.2τb s( )

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

πrw2 dno

dt=nbnoσivbπab

2Γog +nbnoσxvbπab2Γob

+nbvbπab2 fhaloΓob −2πrwfwpno

vo

4

Ib =qnbvbπab2

Ionize gas - og=10 Charge-exchange loss of beam

Halo loss Pumping: fwp = fraction of wall that pumps

Solve for Ib, convert to peak current with inverse duty cycle at 5 Hz.

small

nb,o {vb,o}beam, neutral density (m-3) {velocity (m/s)}i cross section for beam ionization of gas x charge-exchange of beam on gasab {rw }beam radius; {wall radius}og,ob desorption coefficient for expelled ion (from gas), beam ion.fhalo, fwp fraction beam lost per m, fraction wall open to cryo-pump.

Whereb = beam duration (≤20 µs)


Beam desorption coefficients necessary for HIF:

• HIF cold bore: each beam pumped by its own beam tube, limit applies to each beam [ need ob < 2 x104 for Ib ≥1 A].

• Warm bore: pumping between quad. magnets, limit applies to sum of beam currents in array [ need ob < 103 for Ib ≥ 100 A].

• Both limits relaxed if beam halo loss less than 10-4/m

Bea

m c

urr

ent

(A)


Measure electron emission and gas desorption from 1 MeV K+ beam impact on target

Gas, electron source diagnostic (GESD)

• Measure coefficient of electron and gas emission per incident K+ ion.• Calibrates beam loss from electron currents to flush wall electrodes.• Evaluate mitigation techniques: baking, cleaning, surface treatment…

Ion gauge

Target, angle

~2o-15o

Reflected ioncollector

ElectronSuppressor

Beam

Suppressor grid

Grid & target bias varied

Faraday cup

Beam

Tiltable target


GESD secondary electron yield (SEY) varies with cos()-1

L

L = /cos()

• Simple model gives cos()-1 - Delta electrons pulled from

material by beam ions (dE/dx)- Electrons from depth > (~

few nm) cannot leave surface- Ion path length in depth is L.

L = /cos()• Results depart from this near

grazing incidence where the distance for nuclear scattering is < L1

1. P. Thieberger,A. L. Hanson, D. B. Steski, et al., Phys. Rev. A 61, 42901 (2000).

0

50

100

150

76 78 80 82 84 86 88 90

Angle of incidence (deg.)

Coefficient of electron emission

SEY6.06/cosSRIM(22A)

Angle from normal (deg.)

Gra

zing

inci

denc

e


GESD gas desorption coefficient varies more slowly than cos()- 1 not mainly from adsorbed gas layers

Model: • Gas desorption results from

electronic sputtering of gas film on surface plus dust and oxides on surface and impurities near surface.

• Film would result in cos()- 1 [not seen so other sources dominate.]

0

2,000

4,000

6,000

8,000

10,000

12,000

76 78 80 82 84 86 88 90Angle of incidence (deg.)

Gas desorption coefficientN_0/N_b

Ion Beam

Angle from normal (deg.)

Gra

zing

inci

denc

e

Similar results reported for 800 MeV Pb on SS at CERNE. Mahner, et al., PRST-AB 6, 013201 (2003)


Is SEY 1/cos because electrons originate in beam-ionized gas? – No

Bead-Blasted Target, 1 MeV K+, Bands show standard deviation, 7-14-03

0

5

10

15

80 85 90Angle from normal (deg.)

SEY Desorp(k-mol)

• Gas expands ~2-3 mm/µs, so fills 3 mm high beam in fraction of 5 µs FWHM.

• If electrons from beam-impact on gas, electron production 1/cos

• SEY=13 & 1/cos Electrons are from ion impact on surface at an average angle of 60 from normal.

• At 60 , ion reflection is reduced to ~3%.

Mitigation technique: rough surface reduces SEY x10, gas desorption x2, but harder to beam scrub.


Electronic sputtering can account for larger gas yields than physical sputtering

• Nuclear-elastic (knock-on) collisions physical sputtering

• Electronic component electronic sputtering.

• Sputtering from ion and electron bombardment of frozen gas is believed to be the source of tenuous atmospheres on moons of outer planets.*

• Electronic sputtering applies to insulators, not metals. But observed gases (H, C, O compounds) would have been insulators on surface.* R. E. Johnson, “Sputtering of ices in the outer solar system” RMP 68, 305 (1996).

From TRIM Code:

Measured sputtering yield for H+ and O+ incident on H20 at ≤80K

Nuclear

Electronic

dE/d

x (e

V/À

)

O+ Ion energy (keV) Ei/Mi (eV/amu)

Yie

ld (

Mol

ecul

es/io

n)

100 104 100 106

O+

H+


Electronic sputtering model is being tested by HIF-VNL

1E-3

1E-2

1E-1

1E+0

1E+1

1E+2

1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3

K+ energy (MeV)

dE/dx (MeV/mg/cm2)

total dE/dx

Nuclear

Electronic

Range of energies for HIF Driver

HIF-VNL

SRIM 2003 for K+ ions on stainless steel

GSI

GSI Collaboration offers opportunity to test model over wide energy range, including that of HIF Driver and others


Summary/conclusions

• HIF has attractive power plant prospects, but

- Desorption and ECE are major determinants of allowable fill factor

- Gas desorption coefficient appears marginal for cold-bore (for wall

characteristics studied), and may rule out a warm-bore approach.

• Electron emission scales with cos-1() – Understood

• Gas desorption scales more slowly with angle.

• Electronic component of dE/dx is prime candidate for

supplying energy to drive emission and desorption.

• Particle source for desorption not primarily adsorbed layers

of gas – dust, inclusions, and oxide layers are candidates.


Backup material


Beam hitting gas or walls creates electrons and gas – these can multiply

Beam on gas, Ib

K0

K2+

K+ Beam

1.0-1.8 MeV

2-5 kV potential

e-

i+

Beam loss to walls, Ibw

Fe

K+ Beam

K+

e-

n0

n0

These interaction products create opportunities for diagnostics along with problems for diagnostics and beams


Energy (GeV)

Range(g/cm2)

H He Li Ne Kr Pb

Rangefor ICFtargets

1

0.1

0.010.01 0.1 1 10

Heavier Ions Higher Kinetic Energy


The IBX mission is to demonstrate integrated source-to-focus physics

7m25 ns

40 m

15 m250 25 ns

$70 - 80 M TEC over 5 yrs + $10 M R&D

2 m250 ns 1.7 MeV

Ion: K+ (1 beamline)

Total half-lattice periods: 148

Total length: 64 m5 - 10 MeV

Injector

Accelerator

DriftCompression

Final Focus

Neutraliz

ation

Capability for pressure-rise issues• Vary fill factor with accelerated & tilted beam• Drift compression & final focus

hif (heavy ion fusion) gas desorption issues*

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