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The Heavy Ion Fusion Science Virtual National Laboratory
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PRoy LINAC06
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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PRoy LINAC06
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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PRoy LINAC06
Beam compression: Why?
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PRoy LINAC06
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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PRoy LINAC06
Beam compression: How?
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PRoy LINAC06
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
0
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0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 25 50 75 100
Distance (cm)
RM
S R
ad
ius
(c
m)
Perfect Neutralized
Vacuum transport:un-neutralized
CAP neutralized
Time
Cu
rre
nt
NTX
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PRoy LINAC06
Neutralized Transport Experiment (NTX)
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PRoy LINAC06
Neutralized transport experiment completed with quantitative agreement between simulation &experiment
FWHM: 2.71 cm
Un-neutralized
FWHM: 2.14 mm
Neutralized (CAP+RF)
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2 2.5 3 3.5
FLU
EN
CE
(a.u
.)
R(mm)
CAP ONLY
100% neutralized
CAP and RF
Measurement
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2 2.5 3 3.5
FLU
ENC
E (a
.u.)
R(mm)
CAP ONLY
100% neutralized
CAP and RF
Theory ( LSP)
CAP source (Plasma plug) RF source (Volume plasma)
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PRoy LINAC06
Neutralized Drift Compression Experiment
(NDCX)
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PRoy LINAC06
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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PRoy LINAC06
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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PRoy LINAC06
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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PRoy LINAC06
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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PRoy LINAC06
Plasma is essential for high beam compression
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PRoy LINAC06
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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PRoy LINAC06
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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PRoy LINAC06
Spot size increases at maximal compression due to time dependent defocusing at gap
At peak compression
Experiment
Simulation
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PRoy LINAC06
Pulse Line Ion Accelerator (PLIA)
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PRoy LINAC06
First beam dynamics validation experiment for the Pulse Line ion Accelerator (PLIA)
At peak compression
Experimental setup
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PRoy LINAC06
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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PRoy LINAC06
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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PRoy LINAC06
BACK UPBACK UP
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PRoy LINAC06
NDCX Depends on the simultaneous pulsing of Marx, quadrupoles, tilt core, plasma channel & plasma gun
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PRoy LINAC06
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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PRoy LINAC06
Typical NTX ion beam is focused to the final drift section for neutralization and compression (24 mA Ib)
0
5
10
15
20
25
30
230 250 270 290 310 330
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
n=0.050 -mm-mr (measured), from source temperature alone n 0.030 -mm-mr