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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. Outline.

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Accelerators (<1 MeV/n) for Low-Energy


Workshop on Underground Accelerators for

Nuclear Astrophysics

October 27-28, 2003

Jose Alonso, Rick Gough

Lawrence Berkeley National Laboratory


  • Types of accelerators suitable for low-energy nuclear astrophysics applications

  • Other system components

  • Existing and possible new configurations

  • Important questions to be addressed


Types of accelerators
Types of Accelerators

  • 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

Power supply types for dc accelerators
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

Tandem configuration
Tandem Configuration

• Higher beam energies

• Ion source at ground

• Requires negative

ion source which limits

current and ion species


• Strip to q+ in

high voltage dome

• E/A = V  (q+1)/A


Van de graaff pelletron
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

Traditional linac injectors
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


• 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

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

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

Electrostatic quadrupole esq focusing
ElectroStatic Quadrupole (ESQ) Focusing

Basic ESQ


  • Provides strong focusing for high beam current

  • Suppresses secondary electrons

  • Reduces longitudinal average voltage gradient to accommodate insulators

ESQ module for 4 parallel beams

Luna pace setter in the field
LUNA: Pace-setter in the field

LUNA Collaboration, INFN, Gran Sasso

Surface laboratories
Surface Laboratories


  • Bochum

  • Notre Dame


  • … others?

  • ~1 MeV electrostatic

  • Spectrometers

  • Careful attention to unavoidable backgrounds

Possible hi solution for underground lab
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

  • 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

E / A = 9 / 40 x 2.5 = 0.56 MeV / amu

Ecr in electrostatic accelerators
ECR in Electrostatic Accelerators


Hahn-Meitner Institute


ECR Ion Source in HV terminal

JAERI Tandem

Tokai Research Establishment, Japan

Ar8+ 2eµA at 112 MeV

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?

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