Accelerators (<1 MeV/n) for Low-Energy Measurements Workshop on Underground Accelerators for

1. 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

2. 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

3. 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

4. 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

5. Tandem Configuration • Higher beam energies • Ion source at ground • Requires negative ion source which limits current and ion species +V • Strip to q+ in high voltage dome • E/A = V  (q+1)/A but

6. 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

7. Ultra high precision energy… TUNL, ca 1980??

8. 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

9. 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

10. 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

11. Then there’s always…

12. 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

13. 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

14. LUNA: Pace-setter in the field LUNA Collaboration, INFN, Gran Sasso

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

16. 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

17. 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

18. 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?

19. 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