the heavy ion fusion virtual national laboratory neutralized transport experiment (ntx) p. k. roy,...

The Heavy Ion Fusion Virtual National Laboratory

Neutralized Transport Experiment (NTX)

P. K. Roy, S. S. Yu, S. Eylon, E. Henestroza, A. Anders, F. M. Bieniosek, W. G. Greenway, W. L. Waldron, D. B.

Shuman, D. L. Vanecek, B. G. Logan, LBNLD. R. Welch, D. V. Rose, C. Thoma, MRC

R. C. Davidson, P. C. Efthimion, I. Kaganovich, E. P. Gilson, A. B. Sefkow, PPPL

W. M. Sharp, LLNL

15th International Symposium on Heavy Ion Inertial FusionPPPL, Princeton University, New Jersey, USA

June 7-11, 2004


Overview

● Requirements of NTX for Driver

● NTX system and diagnostics

● Keys to a small spot

● Experimental results of neutralization.

● Sensitivity study

● Achievements/conclusion


NTX and HIF Driver


The HIF Driver Requires Beam Neutralization in Chamber Transport to Hit mm-sized Spot on Target

How and what Neutralized Chamber Transport can do:●Electrons from external source are entrained by the beam and neutralize the space charge sufficiently that the pulse focuses on the target in a nearly ballistic manner for a small spot.

●Present generation of drivers requires total of ~40 kA divided between perhaps hundred beams, each beam of 2 mm focal spot radius. Plasma Plug

(externally injected plasma)

Low pressure chamber (~ 10-3

Torr).

Final focus magnet

Target

Volumetric plasma

Convergingion beam

Chamber Wall

Driver needs:

Buncher Finalfocus

Chambertransport TargetIon source

& injector Accelerator

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Distance (cm)R

MS

Rad

ius

(cm

)

Perfect Neutralization

Vacuum

MEVVA Source


Neutralized Transport Experiment (NTX) Addresses Driver-Relevant Issues

Final Focus Chamber Transport

Final focusmagnet

Magnetic-transport section Neutralization drift sectionBeam source

Cathode arcPlasma plug

RF plasma source

Diagnostic box

Quadrupoles

2.4 m 1 m

Source chamber

A schematic of the NTX beam line setup

●Perveance is the key parameter for final focus and neutralization

●NTX covers range of perveance relevant to the driver (K≤10-3).


NTX System Setup and diagnostics Located at LBNL

GatedCamera


Keys to a Small Spot

●Low source emittance

●Optimal convergence angle

Source of emittance growth:●Geometric Aberration●Magnetic transport mismatch

●Efficient neutralization—plasma plug—volume plasma

r


Low emittance Source


Extraction of Uniform High Brightness Beam from NTX Injector

Increased SourceTemperature (~185W)Aperturing

Sm

oo

th s

ou

rce

surf

ace

Unapertured beam

Lower Temperature source(~160w)

Uneven source


Aperturing for High-brightness and High Perveance beam Designed by EGN code

EGN simulation of NTX diode &beam aperture

NTX beam scraper system


NTX Provides low emittance beam (300kV, 25 mA, 2-cm aperture).

Slit-integrated density profile and (x, x’) phase space of a high-brightness apertured beam (300kV, 25 mA, 2-cm aperture).


Magnetic final focus


Magnetic Final Focus Transport

Final Focus Chamber Transport

Final focusmagnet



RF plasma source

Diagnostic box

Quadrupoles

2.4 m 1 m

Source chamber

A schematic of the NTX beam line setup

●Perveance is the key parameter for final focus and neutralization

●NTX covers range of perveance relevant to the driver (K≤10-3).


Good agreement between Experimental and Theoretical Beam profile at Entrance of Final Drift Section

Simulation

Experiment191keV

401keV

265 keV

283 keV

(1.5 mA beam, 5 mm initial radius, Ne ~ 1.2x1011/cm3, 20mm-20mr)


Measured and Calculated Beam Profiles Agreed Well

265keV

283keV

Horizontal densityprofile

Vertical

Horizontal

Vertical

5mm diameter aperture, 265keV horizontal profile

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Experimental Optical 040206021

Theoretical

5mm diameter aperture, 265keV vertical profile

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Theoretical

5mm diameter aperture, 283keV horizontal profile

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Experim ental Optical 040206022

Theoretical

5mm diameter aperture, 283keV vertical profile

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Theoretical


Typical NTX Ion Beam is Focused to the Final Drift Section for Neutralization (24 mA Beam Current)

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Beam energy (keV)

Beam

cur

rent

(mA)

Data on040513&14

Experimental results and simulations of NTX beam profile and phase-space distribution at exit of channel


Neutralized drift


LSP predicted Neutralized Reduces Beam Spot

No neutralization 1.5 cm

Plasma Plug 1.4 mm Plasma Plug + Volume 1 mm

300 keV, 25 mA, 0.1 pi-mm-mrad, K+ beam


Unexpected Partial Beam Neutralization in the Final Drift Section was controlled by a Radial Bias Mesh

6 inch pipe 3 inch pipeUnwanted neutralization for “vacuum”

propagation in small pipe

Mesh

Drift tubeInner wall

Ion beam

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Mesh voltage (V)

Cu

rre

nt

(mA

)

Electron current collected in the radial mesh

Radial mesh suppresses beam-Generated secondary electron

Final focusmagnet



RF plasma source

Diagnostic box

Quadrupoles

2.4 m 1 m

Source chamber

Phase IIIPhase IIPhase I


Electron Suppression provided beam for controlled Neutralization

Simulation

Experiment 6 inch pipe

Experiment 3 inch pipe

Mesh (+1kV)

260 – 300keV


MEVVA Plasma Plug and RF Volume Plasma Source Neutralized Ion Beam for Small Spot Size

MEVVA plug

RFPlasma system

1 m

Cathode-arcPlasma sourceplasma plug

RF plasmasource

0.52 m

Diagnostic

a) b)

c)

Neutralization drift section


High Density Plasma Obtained from MEVVA Plasma plug and RF Plasma (~1011 cm-3)

Argon Plasma3.7<t<4.0mSN>1011 cm-3

P<10-5 Torr

Neutral Pressure and RF Plasma DensityEach point on the IV characteristic for MEVVA plug.

The ion saturation current is used for plasma density.

Density~1011 cm-3


Reduction of Spot Size Using Plasma Plug & Volume Plasma (24 mA beam, 20 mm initial radius)

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Mesh+250,31002060

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MEVVA30813016

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X(mm)

MEVVA & RF30813010

FWHM: 2.71 cm FWHM: 2.83 mm FWHM: 2.14 mm

Non-neutralized transport

Effect of plasma plugon spot size

Effect of plasma plug and volume plasma on spot size


100% Current Transmission Through Neutralized Drift Section (24 mA Beam)

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Beam energy (kV)

Be

am

cu

rre

nt

(mA

) At exit, no plasmaAt exit, with plasmaAt entrance


*Image taken after pinhole sample has drifted 1 meter

Vertical Pinhole Scan

Full 2-D Pinhole Scan

7 mm

Pinhole scan at Entrance to Neutralization* and Neutralized beam (6 mA beam, Ne ~ 2x1011/cm3)

Neutralized beam

rms size ~ 1.0 mm

rms size ~ 1.4 mm

Plasma density ~ 2x1011/cm3

Movable Pinhole Measurements of 4-D Phase Space


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Beam profile at focal plane for three neutralization methods(6 mA beam, 10mm initial radius)

MEASUREMENT

SIMULATIONS

With plasma plugWith plasma plugand RF Plasma 100% neutralization

MEVVA ONLY

100% neutralized

MEVVA and RF

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Fluence

MEVVA ONLY

100% neutralized

MEVVA and RF


Sensitivity Study: Beam Spot Size Dependency on Convergence Angle

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Convergence angle (mrad)S

pot r

adiu

s (m

m)

Non neutralized beam radius as a function of convergence angle calculated using WARP code

Measured neutralized beam radius as a function of convergence angle

at the end of final focus


Sensitivity Study of Neutralization

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MEVVA discharge voltage (keV)

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am

ra

diu

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mm

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Axial distance (cm)

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am

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diu

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242 247 252 257 262Beam energy (keV)

Be

am

ra

diu

s (

mm

)

Head to tail variationVariation with beam energy

Beam envelope variation with axial position

Beam envelope variation with plasma discharge voltage


Summary

●We have completed a detailed study of neutralized final transport.

● The experimental results are in good agreement with simulations.

● The NTX experiments have significantly increased our confidence for a variable driver final focused scenario.

the heavy ion fusion virtual national laboratory neutralized transport experiment (ntx) p. k. roy,...

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