neutralized drift compression and related experiments*

The Heavy Ion Fusion Science Virtual National Laboratory

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Neutralized Drift Compression and Related Experiments*

P. K. Roy1, W. L. Waldron1, S. S. Yu1, P. A. Seidl1, E. Henestroza1, J. Coleman1, A. Anders1, D. Baca1, J. J. Barnard2, F. M. Bieniosek1, R. J. Briggs3, C. Celata1, R. C. Davidson4, P. C. Efthimion4, S. Eylon1, A. Friedman2, E. P. Gilson4, W. G. Greenway1, D. P. Grote2, I. Kaganovich4, M. Leitner1, B. G. Logan1, H. Qin4, L.L. Reginato1, A. B. Sefkow4, W. M. Sharp2, C. Thoma5 and D. R. Welch5

1Lawrence Berkeley National Laboratory, 2Lawrence Livermore National Laboratory, 3SAIC, 4Princeton Plasma Physics Laboratory,

5Voss Scientific, for HIFS-VNL, USA

*This work was supported by the US-DOE


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Overview

● Beam compression: Why ?--For HIF Driver & Warm dense matter (WDM) experiments

● Beam compression: How ?1. Neutralized Transport Experiment (NTX)

(for transverse beam compression)2. Neutralized Drift Compression Experiment (NDCX)

(for longitudinal beam compression)

● New beam acceleration technique --Pulse Line Ion Accelerator (PLIA)--First beam dynamics validation experimental results


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Beam compression: Why?


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The HIF Driver/WDM experiments require beam compressions to hit mm-sized spot with ns pulse

Buncher Finalfocus

Chambertransport TargetIon source

& injector Accelerator

HIF Driver WDM

Beam ~40 kA

(~4GeV)

~200A

(~23MeV)

Beam

number

~120 1

Focal spot

2 mm 1 mm

Pulse length

~10 ns ~1 ns

HIF Driver

One concept of WDM facility: NDCXII


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Beam compression: How?


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Neutralized beam transport (NTX) & drift compression (NDCX) can provide ~mm spot with ~nsec pulse

●NDCX: In neutralized drift compression beam is longitudinally compressed by imposing a linear head-to-tail velocity tilt to a drifting neutralized beam and producing a pulse duration of several ns.

●NTX: In beam neutralization, electrons from a plasma or 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 to a small spot.

NDCX

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Distance (cm)

RM

S R

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(c

m)

Perfect Neutralized

Vacuum transport:un-neutralized

CAP neutralized

Time

Cu

rre

nt

NTX


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Neutralized Transport Experiment (NTX)


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Neutralized transport experiment completed with quantitative agreement between simulation &experiment

FWHM: 2.71 cm

Un-neutralized

FWHM: 2.14 mm

Neutralized (CAP+RF)

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100% neutralized

CAP and RF

Measurement

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100% neutralized

CAP and RF

Theory ( LSP)

CAP source (Plasma plug) RF source (Volume plasma)


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Neutralized Drift Compression Experiment

(NDCX)


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Concept of longitudinal beam compression

Induction module voltage waveform

Plasma neutralization

Compressed beam bunch has higher space charge density than uncompressed beam bunch section. This higher space charge can contribute to beam blow-up before reaching the target or diagnostic location. Therefore, the compressed beam must be neutralized with an appropriate plasma density. Typically, np/Znb>1, where np is the plasma density, and nb and Z are the ion beam density and charge state.


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The NDCX experimental setupNDCX uses many components of the former NTX

Pulsed cathodic arc plasma (CAP) source

Plasma column consists of a 1m long solenoid (~1kG & 7.6 cm diameter).

Basic concept of a module

Induction core with head to tail voltage ramp (imposing a velocity “tilt” on the beam)


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NDCX setup & induction module voltage waveform

Theory specifies the ideal voltage waveform required to produce an exactly linear longitudinal velocity ramp. The induction module voltage waveform is optimized to obtain a rather close approximation to the ideal waveform.


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3 Fast (ns) diagnostic systems for NDCX

-0.09

-0.08

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

1.55E-06 1.56E-06 1.57E-06 1.58E-06

Time [sec]

Sig

nal [V

]

Phototube(<1 ns)

Faraday Cup(~3ns)

Optical imaging(1ns) gating

Beam

5 cm

Beam

Sampling

A. Sefkow et al.PRSTAB 9,(2006)052801

F. Bieniosek et al.PAC ‘05


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Plasma is essential for high beam compression


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50-fold* compression measured

Phototube signal

with & without compression

Compression ratio

Obtained using phototube

Compression ratio

obtained using Faraday cup

Compression ratio

Obtained using LSP simulation

The maximum compression is observed by fine tuning the beam energyto match the voltage waveform and precisely positioning the longitudinal

focal point at the diagnostic location.

*Slightly different diagnostic and data reduction yield a factor of 60


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Beam stability test with 2-m drift section

2m

● As the drift length is increased, the compression is more sensitive to: -the degree of neutralization and -intrinsic longitudinal temperature.

● If there are any instabilities, e.g. two-stream, they may become evident with longer drift length.

using phototube

●Longitudinal beam temperature: ~1eV

●No evidence of two- stream degradation or collective instabilities


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Spot size increases at maximal compression due to time dependent defocusing at gap

At peak compression

Experiment

Simulation


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Pulse Line Ion Accelerator (PLIA)


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First beam dynamics validation experiment for the Pulse Line ion Accelerator (PLIA)

At peak compression

Experimental setup


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Beam energy modulation of -80 to +150 keV was measured using a PLIA input voltage waveform of -21 to +12 kV

At peak compression

Beam energy: Helix un-powered

Marx voltage

Longitudinaltime-energy Phase-space

Beam energy: Helix powered

Experiment Simulation

Voltage waveform at the exit of the helix

Experiment Simulation

Beam current amplification


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An example of how the PLIA can accelerate whole bunches, a short pulse was accelerated

At peak compression

Helix poweredHelix un-powered Marx voltage


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Summary● Radial compression of a neutralized ion beam achieved, increasing current density by a factor of 100 over un-neutralized beam in the Neutralized Transport Experiment (NTX) [One of many references: Roy et al., Phys. of Plasmas 11, 2890 (2004)].

● 50-fold longitudinal compression of a velocity-ramped, intense neutralized beam to 3ns has been demonstrated.

[Roy et al., Physical Review Letters, 95, 234801(2005)].

● Significant energy amplification has been achieved with a modest voltage pulse on the PLIA.

[Roy et al., Phys. Rev. ST Accel. Beams, 9, 070402(2006)].

-We are preparing for experimentally exploring simultaneous transverse and longitudinal compression experiments.

-Vacuum flashover, which presently limits the acceleration gradient to ≤150 kV/m, is being investigated.


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BACK UPBACK UP


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NDCX Depends on the simultaneous pulsing of Marx, quadrupoles, tilt core, plasma channel & plasma gun


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Beam bunching observed as tilt voltage waveform turned on

Induction module voltage waveforms produced by varying the timing of the modulators

The degree of bunching, as well as the pulse shape, is correlated with the voltage waveform.


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Typical NTX ion beam is focused to the final drift section for neutralization and compression (24 mA Ib)

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Beam energy (keV)

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(mA)

Data on040513&14

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

n=0.050 -mm-mr (measured), from source temperature alone n 0.030 -mm-mr

neutralized drift compression and related experiments*

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