accelerators (powerpoint ppt presentation

R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators

B ERKELEY L ABTucson, Oct 27-28, 20031

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

Workshop on Underground Accelerators forNuclear Astrophysics

October 27-28, 2003

Jose Alonso, Rick Gough

Lawrence Berkeley National Laboratory



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



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: E/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

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



AcceleratingTube Grading

Rings

High VoltageDome

Charge ExchangeCellNegative

Ion Source

PositiveIon Beam

+V

HV PowerSupply

Tandem Configuration

• Higher beam energies• Ion source at ground

• Requires negative ion source which limits current and ion species

but

+V

• Strip to q+ in high voltage dome

• E/A = V (q+1)/A



Van de Graaff / Pelletron S-Series NEC Pelletron (1 - 5 MV)

Pelletron charging principle

Open air systems for lower beam energies (1 - 500 keV)

National Electrostatics Corporation



Ultra high precision energy…

A- - - - -+ + + + +StripperVandeGraaff ~ +10MVAnalyzing magnetsPosition monitorTarget~5 kVOverall energy regulation ≥ 10-6-1

TUNL, ca 1980??



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 E/E problematic

for this application

2.5 MeV H– RFQ built by LBNL for SNS

500 kV open-air injector at Livermore



Dynamitrons

- 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

• Dynamitron from Boeing Radiation Effects Lab shown w/cover removed

• 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

25 mA protons

2.5 MV CW ESQ accelerator for BNCT (spin-off application)

2 MV pulsed ESQ accelerator for fusion energy (base program)

coaxial transformer power supply

0.6A K+



Then there’s always…



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

Provides strong focusing for high beam current

Suppresses secondary electrons

Reduces longitudinal average voltage gradient to accommodate insulators

ESQ module for 4 parallel beams

Basic ESQ module



LUNA: Pace-setter in the field

I II

Terminal potential 50 kV 400 kV

Technology Electrostatic Electrostatic (Cockcroft-Walton)

Ripple 5x10-4 (25eV) 4eV

Long-term stability 1x10-4 (5eV) 5eV/hr

Measured E 72eV

Source Duoplasmatron(E ~ 20eV)

RF

Ions 3He, 400µAp, d

p, 750µA4He

LUNA Collaboration, INFN, Gran Sasso



Surface Laboratories

• LENA - TUNL• Bochum• Notre Dame• ISAC, TRIUMF• … others?• ~1 MeV electrostatic• Spectrometers• Careful attention to unavoidable

backgrounds



Possible HI Solution for Underground Lab

• 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

Requirement: 50 eµA up to 0.5 MeV/nucleon protons to argon

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



ECR in Electrostatic Accelerators

ISLHahn-Meitner InstituteBerlinECR Ion Source in HV terminal

JAERI TandemTokai Research Establishment, Japan

Ar8+ 2eµA at 112 MeV



• 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 - I



• 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

Important Questions for Accelerator Design-II

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