Transcript No Slide Title

Accelerators (<1 MeV/n) for Low-Energy
Measurements
Workshop on Underground Accelerators for
Nuclear Astrophysics
October 27-28, 2003
Jose Alonso, Rick Gough
Lawrence Berkeley National Laboratory
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Outline
• Types of accelerators suitable for
low-energy nuclear astrophysics
applications
• Other system components
• Existing and possible new configurations
• Important questions to be addressed
– REQUIREMENTS
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Types of Accelerators
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For low energy, linacs are generally considered more
“straight forward” than circular machines
There are various schemes to apply kinetic energy:
- radio frequency (rf), induction, or static potential drop
A dc electrostatic accelerator is a potential-drop type of
linac with typical voltages up to several MV
- Offers easy and continuous energy variation
- Superior energy dispersion: DE/E ~10-4 compared to room
temp. rf linacs or RFQs (~10 -2 ), SCRF linacs (<10-3), or
cyclotrons (>10 -3 )
- Energy dispersion determined by dc power supply voltage
regulation
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Power Supply Types for DC Accelerators
 Van de Graaff (including pelletron) – low current but
capable of reaching terminal potentials > 10 MV
 Cockcroft-Walton – uses a ladder network to build
voltage up to ~1 MV
 Dynamitron – a “shunt-fed” type Cockcroft-Walton that
has higher current capability and provides voltages to a
few MV
 External transformer – high current capability but high
voltage limited by breakdown between windings
 Coaxial transformer – a high current (50 mA) and high
voltage (2.5 MV) design under development
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Tandem Configuration
Accelerating
Tube
High Voltage
Dome
Negative
Ion Source
Charge
Exchange
Cell
Grading
Rings
Positive
Ion Beam
+V
+V
• Higher beam energies
• Ion source at ground
HV Power
Supply
but
• Requires negative
ion source which limits
current and ion species
• E/A = V  (q+1)/A
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• Strip to q+ in
high voltage dome
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Van de Graaff / Pelletron
S-Series NEC
Pelletron (1 - 5 MV)
National Electrostatics Corporation
Open air systems for
lower beam energies
(1 - 500 keV)
Pelletron charging principle
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Ultra high precision energy…
VandeGraaff
~ +10MV
Analyzing magnets
Stripper
A -1
- - - - -
+ + + + +
Position monitor
Target
Overall energy regulation ³ 10 -6
~5 kV
TUNL, ca 1980??
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Traditional Linac Injectors
• Open air electrostatic systems used as traditional linac injectors
– require lots of space, largely being replaced by RFQs
• RFQs are compact and efficient
– tunability and low DE/E problematic
for this application
500 kV open-air
injector at Livermore
2.5 MeV H– RFQ
built by LBNL for SNS
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Dynamitrons
• Dynamitron from Boeing Radiation
Effects Lab shown w/cover removed
-
used to produce x-rays, protons,
electrons, and low-Z ions for TREE
& space radiation effects
-
pulsed or dc operation
-
energies from 0.2 - 2.8 MeV
-
< 10 mA of electrons
-
hundreds of microAmps of positive
ions
• Require high pressure gas ( SF6 )
• Dynamitron was used as HILAC
injector and is in use at Argonne for radioactive beam studies
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High Current Accelerator Development at LBNL
2 MV pulsed ESQ accelerator
for fusion energy (base program)
0.6A K+
2.5 MV CW ESQ accelerator
for BNCT (spin-off application)
 25 mA protons
coaxial transformer power supply
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Then there’s always…
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Types of Beam Focusing
Electric field lens
 Aperture lens – strength decreases with beam energy
 Electrostatic quadrupole (ESQ) – strength increases
with beam energy
Magnetic field lens (best at high beam energy)
 Magnetic solenoid lens
 Magnetic quadrupoles
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ElectroStatic Quadrupole (ESQ) Focusing
Basic ESQ
module
 Provides strong
focusing for high
beam current
 Suppresses
secondary electrons
 Reduces longitudinal
average voltage
gradient to
accommodate
insulators
ESQ module for 4 parallel beams
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LUNA: Pace-setter in the field
I
II
50 kV
400 kV
Technology
Electrostatic
Electrostatic
(Cockcroft-Walton)
Ripple
5x10-4 (25eV)
4eV
Long-term stability
1x10-4 (5eV)
5eV/hr
Terminal potential
Measured DE
Source
Ions
72eV
Duoplasmatron
(DE ~ 20eV)
3He,
400µA
p, d
RF
p, 750µA
4He
LUNA Collaboration, INFN, Gran Sasso
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Surface Laboratories
•
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•
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LENA - TUNL
Bochum
Notre Dame
ISAC, TRIUMF
… others?
~1 MeV electrostatic
Spectrometers
Careful attention to unavoidable
backgrounds
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Possible HI Solution for Underground Lab
Requirement: 50 eµA up to 0.5 MeV/nucleon protons to argon
• Low power, permanent magnet ECR ion source mounted on the
terminal of a 2.5 MV Van de Graaff could provide cw ion beams
from hydrogen to argon at 0.5 MeV/nucleon
• Demonstrated performance: commercial permanent magnet ECR
ion sources can produce Ar9+ at greater than 100 eµA
E / A = 9 / 40 x 2.5 = 0.56 MeV / amu
• Utilize lower charge states for lower energy ranges
• Beams from gaseous elements straightforward; beams from solids
more challenging but possible
• Integration of ECR and Van deGraaff technologies has been
demonstrated, but not available as commercial off-the-shelf item
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ECR in Electrostatic Accelerators
ISL
Hahn-Meitner Institute
Berlin
ECR Ion Source in HV terminal
JAERI Tandem
Tokai Research Establishment, Japan
Ar8+ 2eµA at 112 MeV
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Important Questions for Accelerator Design - I
• Maximum beam energies? (rest-frame, to determine accel. potential)
• Range of energies needed? (tunability, energy precision)
• Short / long term energy stability (high voltage control, ripple)
• Energy spread? (ion source temperature or RF accelerator design)
• Ion species needed?
• Purity of ion species?
– heavy ions with q/A = 0.5 likely to have contaminants
– molecular, charge-state ambiguities
• What beam currents are required?
• What are the beam current stability requirements?
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Important Questions for Accelerator Design-II
• Beam-on-target requirements? (spot size…)
• Duty factor (CW or pulsed? Is RF structure OK?)
• Noise constraints?
– could x-rays beyond some energy interfere w/ exp. signals?
– are accelerator-produced neutrons a background problem?
• Site constraints?
– space, access, power, utilities, special safety issues...
• Configuration flexibility?
– may be necessary to have more than one accelerator system to
meet all requirements
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