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ABSTRACT: A 2 kA cathode was successfully developed for the DARHT-II injector [1]. Since the DARHT injector cannot be baked and there may be virtual leaks, the local pressure near the cathode was not ideal even though the system pressure was in the 10<sup>-8</sup> Torr range. In a series of experiments using quarter-inch size button cathodes, we showed that gas poisoning was a significant factor in this pressure range. Furthermore we found that the 311-XM (doped with scandium and has an M coating) cathode was less affected by gas poisoning than the 612-M, corresponding to a lower effective work function. Water vapor was found to be the worst contaminant among the various gases that we have tested. With a 6.5rdquo diameter 311-XM cathode, the DARHT-II injector produced > 2 kA corresponding to a current density of 10 A/cm<sup>2</sup>.
Vacuum Electronics Conference, 2008. IVEC 2008. IEEE International; 05/2008
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A W Molvik,
H Kollmus,
E Mahner,
M Kireeff Covo,
M C Bellachioma,
M Bender,
F M Bieniosek,
E Hedlund,
A Krämer,
J Kwan,
O B Malyshev,
L Prost,
P A Seidl, G Westenskow,
L Westerberg
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ABSTRACT: During heavy-ion operation in several particle accelerators worldwide, dynamic pressure rises of orders of magnitude were triggered by lost beam ions that bombarded the vacuum chamber walls. This ion-induced molecular desorption, observed at CERN, GSI, and BNL, can seriously limit the ion beam lifetime and intensity of the accelerator. From dedicated test stand experiments we have discovered that heavy-ion-induced gas desorption scales with the electronic energy loss (dE_{e}/dx) of the ions slowing down in matter; but it varies only little with the ion impact angle, unlike electronic sputtering.
Physical Review Letters 03/2007; 98(6):064801. · 7.37 Impact Factor
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ABSTRACT: Over the past quarter century, advances in hydrogen negative-ion sources have extended the usable range of hydrogen-isotope neutral beams to energies suitable for large magnetic confinement fusion devices. Recently negative halogen ions have been proposed as an alternative to positive ions for heavy-ion fusion drivers in inertial confinement fusion, because electron accumulation would be prevented in negative-ion beams, and if desired, the beams could be photodetached to neutrals. This article reports an experiment comparing the current density and beam emittance of Cl+ and Cl− extracted from substantially ion-ion plasmas with that of Ar+ extracted from an ordinary electron-ion plasma, all using the same source, extractor, and emittance scanner. At similar discharge conditions, the Cl− current was typically 85%–90% of the positive chlorine current, with an e−/Cl− ratio as low as 7 without grid magnets. The Cl− current was as much as 76% of the Ar+ current from a discharge with the same rf drive. The minimum normalized beam emittance and inferred ion temperatures of Cl+, Cl−, and Ar+ were similar, so the current density and optical quality of Cl− appear as suitable for heavy-ion fusion applications as a positive noble gas ion of similar mass. Since F, I, and Br should all behave similarly in an ion source, they should also be suitable as driver beams.
Review of Scientific Instruments 02/2006; 77(3):03A501-03A501-6. · 1.37 Impact Factor
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M. Kireeff Covo,
A. W. Molvik,
A. Friedman, G. Westenskow,
J. J. Barnard,
R. Cohen,
P. A. Seidl,
J. W. Kwan,
G. Logan,
D. Baca,
F. Bieniosek,
C. M. Celata,
J.-L. Vay,
J. L. Vujic
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ABSTRACT: The cost of accelerators for heavy-ion inertial fusion energy (HIF) can be reduced by using the smallest possible clearance between the beam and the wall from the beamline. This increases beam loss to the walls, generating ion-induced electrons that could be trapped by beam space charge potential into an "electron cloud", which can cause degradation or loss of the ion beam. In order to test the physical mechanism model of ion- induced electrons production we have measured the impact of K+ ions with energies up to 400 KeV on stainless steel surfaces near grazing incidence, using the ion source test stand (STS-500) at LLNL. The electron yield will be discussed and compared with experimental
measurements from 1 MeV K+ ions in the High-Current Experiment at LBNL.
Physical Review Special Topics - Accelerators and Beams 01/2006; · 1.52 Impact Factor
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ABSTRACT: An anode stalk support insulator in a magnetically insulated transmission line was designed and modeled. One of the important design criteria is that within space constraints, the electric field along the insulator surface has to be minimized in order to prevent a surface flashover. In order to further reduce the field on the insulator surface, metal rings between insulator layers were also specially shaped. To facilitate the design process, electric field simulations were performed to determine the maximum field stress on the insulator surfaces and the transmission line chamber.
Particle Accelerator Conference, 2005. PAC 2005. Proceedings of the; 06/2005
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J.-L. Vay,
P. Colella,
J. W. Kwan,
P. McCorquodale,
D. B. Serafini,
A. Friedman,
D. P. Grote, G. Westenskow,
J.-C. Adam,
A. Héron,
I. Haber
[show abstract]
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ABSTRACT: Plasma simulations are often rendered challenging by the disparity of scales in time and in space which must be resolved. When these disparities are in distinctive zones of the simulation domain, a method which has proven to be effective in other areas (e.g., fluid dynamics simulations) is the mesh refinement technique. A brief discussion of the challenges posed by coupling this technique with plasma particle-in-cell simulations is given, followed by a presentation of examples of application in heavy ion fusion and related fields which illustrate the effectiveness of the approach. Finally, a report is given on the status of a collaboration under way at Lawrence Berkeley National Laboratory between the Applied Numerical Algorithms Group (ANAG) and the Heavy Ion Fusion group to upgrade ANAG’s mesh refinement library Chombo to include the tools needed by particle-in-cell simulation codes. © 2004 American Institute of Physics.
Physics of Plasmas 04/2004; 11(5):2928-2934. · 2.15 Impact Factor
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ABSTRACT: A energy analyzer system is being built for the DARHT-II accelerator similar to the energy analyzer used on the Astron accelerator. This system consists of a scattering wire, magnetic bend, and signal detector. The wire thickness of 40 mil carbon and the scattering angle of 11 degrees is chosen for good signal to noise ratio. The dipole bend angle is 60 degrees, with a 30 cm radius of curvature. The image-plane focal distance is chosen for the required energy resolution. The energy resolution and acceptance are 0.1% and ±5% with a time response of 10 nsec. The wire must survive the 2 usec 2 kA, 18.4 MeV DARHT-II beam. The MCNP code was used to study the wire scattered properties. The scattered beam fills the available 1×2 cm dipole aperture. The dispersion normal to the beam direction is 0.43 cm/%. The detector is a PIN diode array which determines the beam position on the chip. This array consists of 40 2.5×0.1×0.25 mm bins with a gain in excess of 10000. The system will be installed in the space between the debris blocker and the cruncher solenoid up-stream from the shuttle dump.
Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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ABSTRACT: In Heavy-Fusion and in other applications, there is a need for high brightness sources with both high current and low emittance. The traditional design with a single monolithic source, while very successful, has significant constraints on it when going to higher currents. With the Child-Langmuir current-density limit, geometric aberration limits, and voltage breakdown limits, the area of the source becomes a high power of the current, A∼Ia8/3. We are examining a multi-beamlet source, avoiding the constraints by having many beamlets each with low current and small area. The beamlets are created and initially accelerated separately and then merged to form a single beam. This design offers a number of potential advantages over a monolithic source, such as a smaller transverse footprint, more control over the shaping and aiming of the beam, and more flexibility in the choice of ion sources. A potential drawback, however, is the emittance that results from the merging of the beamlets. We have designed injectors using simulation that have acceptably low emittance and are beginning to examine them experimentally.
Particle Accelerator Conference, 2003. PAC 2003. Proceedings of the; 06/2003
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ABSTRACT: The successful operation of a Two-Beam accelerator based on extended relativistic klystrons hinges upon decreasing the cumulative dipole BBU growth from an exponential to a more manageable linear growth rate. We describe the theoretical scheme to achieve this, and a new experiment to test this concept. The experiment utilizes a 1-MeV, 600-Amp, 200-ns electron beam and a short beamline of periodically-spaced rf dipole-mode pillbox cavities and solenoid magnets for transport. Descriptions of the beamline are presented, followed by theoretical studies of the beam transport and dipole-mode growth. Comment: 3 pages, 3 figures. Submitted to XX Int'l. LINAC Conference
08/2000;
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ABSTRACT: During normal DARHT II operation, the beam exiting the accelerator
will be well characterized by its nominal design parameters of 20-MeV,
2000-Amperes, 2-μsec-pulse length, and 3 cm-mr unnormalized
emittance. Normal operation will have the beam delivered to a beam dump
via several DC magnets. A 2-way kicker magnet is used to deflect
portions of the beam into the straight ahead beamline leading to either
a diagnostic beamline or to the converter target beamline. During start
up and or beam development periods, the beam exiting the accelerator may
have parameters outside the acceptable range of values for normal
operation. The Enge beamline must accommodate this range of unacceptable
beam parameters, delivering the entire 80 KiloJoule of beam to the dump
even though the energy, emittance, and/or match is outside the nominal
design range
Particle Accelerator Conference, 1999. Proceedings of the 1999; 02/1999
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S. Sampayan,
R. Buckles,
G. Caporaso,
Y.-J. Chen,
C. Crist,
S. Falabella,
T. Houck,
M. Krogh,
J. McCarrick,
R. Richardson,
D. Sanders,
J. Weir, G. Westenskow
[show abstract]
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ABSTRACT: For multi-pulse radiography facilities, we are investigating the
possible adverse effects of (1) backstreaming ion emission from the
bremsstrahlung converter target and (2) the interaction of the resultant
plasma with the electron beam during subsequent pulses. These effects
would primarily manifest themselves in a static focusing system as a
rapidly varying X-ray spot. To study these effects, we are conducting
beam-target interaction experiments on the ETA-II accelerator (a 6.0
MeV, 2.5 kA, 70 ns FWHM pulsed, electron accelerator) by measuring spot
dynamics and characterizing the resultant plasma for various
configurations
Particle Accelerator Conference, 1999. Proceedings of the 1999; 02/1999
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M.J. Burns,
B.E. Carlsten,
T.J.T. Kwan,
D.C. Moir,
D.S. Prono,
S.A. Watson,
E.L. Burgess,
H.L. Rutkowski,
G.J. Caporaso,
Y.-J. Chen,
S. Sampayan, G. Westenskow
[show abstract]
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ABSTRACT: The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility
will use two perpendicular electron linear induction accelerators to
produce intense, bremsstrahlung X-ray pulses for flash radiography. We
intend to produce measurements containing 3D information with
sub-millimeter spatial of the interior features of very dense
explosively driven objects. The facility will be completed in two phases
with the first operational by June 1999 utilizing a single-pulse,
19.8-MeV, 2 to 4-kA, 60-ns accelerator (activated in March 1999), a
high-resolution electro-optical X-ray imaging system, and other
hydrodynamics testing systems. The second phase will be operational by
Sept. 2002 and features the addition of a 20-MeV, 2 to 4-kA,
2-microsecond accelerator. Four short electron micropulses of variable
pulse-width and spacing will be chopped out of the original, long
accelerator pulse for producing time-resolved X-ray images. The second
phase also features an extended, high-resolution electro-optical X-ray
system with a framing speed of about 2-MHz. In this paper we present a
Figure-Of-Merit for a X-ray based flash radiography system to motivate
the selection of accelerator parameters. We will then present sub-system
performance measurements from Phase 1, the physics of the interaction of
our high-intensity beams with the X-ray conversion target, initial Phase
1 accelerator measurements (if available), and plans for operation. We
will also discuss designs and prototype testing results for the
2-microsecond Phase 2 accelerator, including prototype induction cells
and pulsed power, prototype kicker magnet performance to chop the beam,
and design considerations for a multipulse X-ray conversion target
Particle Accelerator Conference, 1999. Proceedings of the 1999; 02/1999
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ABSTRACT: INTRODUCTION Pulsed ferroelectric electron emission was observed as early as 1964 [1]. New materials have shown extremely high peak current densities (>100 A-cm -2 ) and high brightness (10 11 A-m -2 -rad -2 ) [2,3]. Further, these emitters can be operated at non-UHV pressures, do not require elevated operating temperatures or a pulsed laser system, making them a potentially very attractive alternative to conventional cathode technology. Pulsed electron emission from a ferroelectric results during a rapidly switched internal polarization change. The exact emission mechanism has not yet been unambiguously identified but is believed to result from the expulsion of charge stimulated by the large, uncompensated, bound charge generated during the induced polarization change [2,4,5]. Ferroelectric emission has many unique properties which we have previously reviewed [5]. They are as follows: (1) e
04/1998;
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ABSTRACT: Construction of a prototype RF power source based on the RK-TBA concept, called the RTA, has commenced at the Lawrence Berkeley National Laboratory. This prototype will be used to study physics, engineering, and costing issues involved in the application of the RK-TBA concept to linear colliders. The status of the prototype is presented, specifically the 1-MV, 1.2-kA induction electron gun and the pulsed power system that are in assembly. The RTA program theoretical effort, in addition to supporting the development of the prototype, has been studying optimization parameters for the application of the RK-TBA concept to higher-energy linear colliders. An overview of this work is presented. © 1997 American Institute of Physics.
AIP Conference Proceedings. 03/1997; 398(1):842-851.
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[show abstract]
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ABSTRACT: Double pulse transport experiments were conducted on the front end
of the ATA accelerator to obtain data on the capability of a velvet
cloth cathode to produce two successive pulses. Pulses of approximately
3 kA were extracted from the cathode with interpulse spacings varying
from 150 ns to 2.8 μs using an anode-cathode voltage of about 1 MV.
Analysis of the current and voltage waveform data from the injector
indicate that the effects of cathode plasma on the second pulse of a
two-pulse burst is minimal
Particle Accelerator Conference, 1995., Proceedings of the 1995; 06/1995
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[show abstract]
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ABSTRACT: We have measured a current emission density of 36
A-cm<sup>-2</sup> over an 11.4-cm<sup>2</sup>-area
lead-titanate-zirconate (PZT) ferroelectric cathode with a pulsed
anode-cathode (A-K) potential of 50 kV. We have also observed currents
above those predicted by classical Child-Langmuir formula for a wide
variety of cases. Since a plasma within the A-K gap could also lead to
increase current emission we are attempting to measure the properties of
the plasma near the cathode surface at emission time. In other
measurements, we have observed strong gap currents in the absence of an
A-K potential. Further, we continue to make brightness measurements of
the emitted beam and observe spatially non-uniform emission and large
shot-to-shot variation. Measurements show individual beamlets with a
brightness as high 10<sup>11</sup> A m<sup>-2</sup> rad<sup>-2</sup>
Particle Accelerator Conference, 1995., Proceedings of the 1995; 06/1995
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P. Kwok,
P. Chen,
D. Cline,
W. Barletta,
S. Berridge,
W. Bugg,
C. Bula,
S. Chattopadhyay,
W. Craddock,
I. Hsu, [......],
M. Ross,
A. Sessler,
T. Shintake,
J. Spencer,
J.J. Su,
A.W. Weidemann, G. Westenskow,
D. Whittum,
R. Williams,
J. Wurtele
[show abstract]
[hide abstract]
ABSTRACT: The proposal to perform a series of plasma lens experiments at the
Final Focus Test Beam at SLAC has been described earlier. We report on
our progress towards validation of concepts involved in the experiments,
including the laser ionized plasma production test, development of the
supersonic gas jet as the plasma source, and study on focused beam size
measurement techniques. Most importantly, the effects of background
events due to plasma lenses in future linear collider detectors, such as
that in the NLC, are studied in details and are shown to be within
detector tolerances
Particle Accelerator Conference, 1995., Proceedings of the 1995; 06/1995
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G. Westenskow,
G. Caporaso,
Y. Chen,
T. Houck,
S. Yu,
S. Chattopadhyay,
E. Henestroza,
H. Li,
C. Peters,
L. Reginato,
A. Sessler
[show abstract]
[hide abstract]
ABSTRACT: We are designing an experiment to study physics, engineering, and
costing issues of an extended Relativistic Klystron Two-Beam Accelerator
(RK-TBA). The experiment is a prototype for an RK-TBA based microwave
power source suitable for driving a 1 TeV linear collider. Major
components of the experiment include a 2.5-MV, 1.5-kA electron source, a
11.4-GHz modulator, a bunch compressor, and a 8-m extraction section.
The extraction section will be comprised of 4 traveling-wave output
structures, each generating about 360 MW of RF power. Induction cells
will be used in the extraction section to maintain the average beam
energy at 5 MeV. Status of the design is presented
Particle Accelerator Conference, 1995., Proceedings of the 1995; 06/1995
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[show abstract]
[hide abstract]
ABSTRACT: A design study has recently been conducted for exploring the feasibility of a relativistic‐klystron two‐beam accelerator (RK‐TBA) system as a rf power source for a 1 TeV linear collider. We present, in this paper, the beam dynamics part of this study. We have achieved in our design study acceptable transverse and longitudinal beam stability properties for the resulting high efficiency and low cost RK‐TBA. © 1995 American Institute of Physics.
AIP Conference Proceedings. 05/1995; 335(1):817-836.
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[hide abstract]
ABSTRACT: The authors recently demonstrated the reacceleration of a bunched beam through an induction accelerator cell in support of the two-beam accelerator concept. They present the results of this experiment including amplitude and phase measurements of the extracted microwave power at 11.424 GHz. They also describe progress in achieving a two-beam accelerator microwave source design that is efficient and cost effective for linear collider applications.
07/1994
