Coupling of APT Transported Ion beam to Hybrid Target

D. R. Welch and D. V. Rose

Mission Research Corporation

C. L. Olson

Sandia National Laboratories

S. S. Yu

Lawrence Berkeley National Laboratory

July 1-2, 2002Presented at the ARIES Project Meeting at GA, San Diego, CA

Research supported by the DOE through PPPL and the HIF VNL

5 TorrXe

25-50 kA

Assisted Pinched Transport can reduce chamber focus requirements and reduce driver costs - Back up to NBT

Laser

Hybrid TargetIPROP simulation starts

Channel current Ic

B

• Potential well increases as Ic(r/rc)2

• Minimum Ic=43 kA to capture beam, where here IA 18 MA– For slow changes (conserving beam

emittance), as rc decreases the beam radius will decrease as rc1/2

43 kA net current required to capture beam at 1/2 cm radius

A

m c

I

kA

r

rc c z

c2

2

1 7

I

I

r

r

r

r rc

A

ci

c f

c i

c i sM

1

2

2

2 22

R

L

rs rcf

Nominal parameters: rs=1 cm, rci=2 cm, rcf= rb=.5 cm M=.03 rad

rci

Adiabatic section

rbM

IPROP is used to model beam/plasma interaction with initial discharge conditions

• IPROP is a quasi 3D EM hybrid code

• 2 T fluid model for the plasma, PIC beam ions

• Ohm’s Law, Je = (pe/ne-vim+ E + vB)

• Spitzer, e-neutral resistivity

• Ionization X-section falls as 1/Z2 • Moliere scattering, Bethe slowing down

• 50-kA discharge

• 5-Torr, 3 eV ambient Xe

• 0.5 torr reduced density within discharge

Initial Discharge Conditions

87% energy transport calculated for APT with 50 kA discharge - main pulse only

10 m

ballistic transport

Pb+72

30 cm

• 4-GeV, 6 MA Pb+72 ions, 1-mrad divergence

• 10-m ballistic transport to discharge

• Calculated m = 5 s limits net current growth to 30 kA over 8-ns pulse

• halo grows from self-field interaction

Discharge radius

halo

Previous IPROP APT calculation

APT Coupling of foot and main pulses to D. Callahan’s Hybrid Target– 125-ns long IPROP simulation – Discharge parameters

• 5-Torr ambient, 0.5-Torr channel Xe

• 25-50-kA discharge

– Beam parameters (1 side) - 3.45 MJ• 3.0-GeV, 12.5-kA, 25-ns Pb foot pulse

• 4.5-GeV, 66.5-kA, 8-ns Pb main pulse

• Xe beam was also simulated

• 1-2 milliradian divergence

– Hybrid Target has 5-mm radiator

New simulations have 2 distinct beams - halo is weaker

• Peak electrical current is lower than previous sims 4.8 vs. 6 MA, energy is 4.5 vs. 4 GeV

• Foot and Main pulses well focused• Nose of main pulse must “catch” tail of foot pulse

Self fields of order those of 50-kA discharge

• Self fields reach 45 kG, highly rippled by 80 ns

• Some enhanced confinement?

• Beam interacts with ripple and a halo forms

26.7 ns

80 ns

Self Fields Degrade Transport• 50-kA Discharge

Full sim main

Full sim foot

No fields foot

No fields main

85% energy efficiency within 5 mm for nominal beam/discharge

• Energy transport within given Radius• Ideal case (no self fields) yields 94% efficiency

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1

Radius (cm)

Inte

gra

ted

En

erg

y F

luen

ce

No Self Fields

Full Sim

6% collisional loss

4% inductive loss

Efficiency falls for currents below 50 kA

• 75 kA discharge yields best transport within .5 cm• Efficiency falls from 87 to 80%

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1 1.2

Radius (cm)

Inte

gra

ted

En

erg

y F

luen

ce

25 kA

35 kA

50 kA

75 kA

Comparison of 25 vs. 50 kA Discharge

• Both pulses have larger halos for lower discharge current

25 kA foot

50 kA main

50 kA foot

25 kA main

Efficiency falls slowly with beam divergence

• < 1.5 milliradian is adequate for 50 kA discharge defines minimum beam emittance

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1 1.2

Radius (cm)

Inte

gra

ted

En

erg

y F

luen

ce

1 mr

1.5 mr

2 mr

Comparison of 1 vs. 2 mr beams

• Main pulse is clearly larger for larger divergence beam

• Head to tail improvement suggests self-fields help for 2-mr sim

2-mr foot

1-mr main

1-mr foot

2-mr main

Transport insensitive to beam ion• Xe+44 beam scaled to deliver same energy on target

– 1.8-GeV foot and 2.4-GeV main pulse

– Similar stripped electrical current and charge to mass ratio

• Roughly 85% transport for both within 5 mm radius

• 50-kA discharge

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1 1.2

Radius (cm)

Inte

gra

ted

En

erg

y F

luen

ce

Pb Beam

Xe beam

Xe vs. Pb transport in 50-kA channel• Differences are minor - Xe has slightly larger

halo but loses less energy

Xe+44

main

Pb+72

main

Xe+44

foot

Pb+72

foot

Conclusions

• APT scheme efficiently couples HIF beam to the Hybrid Target with no cliff edges– 85% coupling efficiency for nominal case

• Efficiency falls slowly as discharge current decreases

• Transport degrades for beam divergence (< 1.5 milliradians)

• Scheme is insensitive to ion species