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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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 1 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 2 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 3 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 4 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators • Strip to q+ in high voltage dome 5 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 6 L AB Tucson, Oct 27-28, 2003 Ultra high precision energy… VandeGraaff ~ +10MV Analyzing magnets Stripper A -1 - - - - - + + + + + Position monitor Target Overall energy regulation і 10 -6 ~5 kV TUNL, ca 1980?? B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 7 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 8 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 9 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 10 L AB Tucson, Oct 27-28, 2003 Then there’s always… B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 11 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 12 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 13 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 14 L AB Tucson, Oct 27-28, 2003 Surface Laboratories • • • • • • • • LENA - TUNL Bochum Notre Dame ISAC, TRIUMF … others? ~1 MeV electrostatic Spectrometers Careful attention to unavoidable backgrounds B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 15 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 16 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 17 L AB Tucson, Oct 27-28, 2003 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? B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 18 L AB Tucson, Oct 27-28, 2003 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 B ERKELEY R.A. Gough, J.R. Alonso: Workshop on Underground Accelerators 19 L AB Tucson, Oct 27-28, 2003