IPM Experience at SNS Alexander Aleksandrov Spallation Neutron

IPM Experience at SNS Alexander Aleksandrov Spallation Neutron Source Oak Ridge, USA

Outline • SNS accelerator • Accumulator Ring IPM requirements • SNS IPM design • IPM-relevant experience with SNS 2. 5 Me. V laser-based longitudinal profile monitor 2 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

SNS Accelerator Complex Collimators Front-End: Produce a 1 -msec long, chopped, Hbeam Accumulator Ring: 1 Ge. V LINAC compress 1 -msec long pulse to 700 nsec Injection Extraction RF 1000 Me. V 186 Me. V 2. 5 Me. V RTBT HEBT Front-End Warm LINAC Cold LINAC Liquid Hg Target 945 ns 1 ms macropulse 3 Managed by UT-Battelle for the U. S. Department of Energy 1 ms Presentation_name

SNS Beam Time Structure Collimators Front-End: Produce a 1 -msec long, chopped, Hbeam Accumulator Ring: 1 Ge. V LINAC compress 1 -msec long pulse to 700 nsec Injection Extraction <1 msec RF 1000 Me. V 186 Me. V 2. 5 Me. V RTBT HEBT Warm LINAC Liquid Hg Target 38 A Chopper system makes gaps mini-pulse Current 945 ns Cold LINAC Current Front-End 38 m. A 1 ms macropulse 1 ms 1 ms macropulse 4 Managed by UT-Battelle for the U. S. Department of Energy 1 ms Presentation_name

SNS Beam profile diagnostics types Collimators Front-End: Produce a 1 -msec long, chopped, Hbeam Accumulator Ring: 1 Ge. V LINAC compress 1 -msec long pulse to 700 nsec Injection Extraction RF 1000 Me. V 186 Me. V 2. 5 Me. V RTBT HEBT Front-End Warm LINAC Cold LINAC Interceptive: conventional 945 ns wire scanners Liquid Hg Target H- ions protons Non-Interceptive: laser wire scanners ? 1 ms macropulse 5 Managed by UT-Battelle for the U. S. Department of Energy 1 ms Presentation_name Protons Non-Interceptive: conventional wire scanners

Credits • “Analysis, Prototyping, and Design of an Ionization Profile Monitor for the Spallation Neutron Source Accumulator Ring” by Dirk A. Bartkoski. – A Dissertation Presented for the Doctor of Philosophy Degree The University of Tennessee, Knoxville • Mechanical design by Kerry Ewald, SNS Research Accelerator Division 6 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

SNS AR IPM system requirements for fully accumulated 1. 5 x 1014 ppp beam. Requirement Value or Range Measured Profile Plane Transverse Horizontal and Vertical Longitudinal Resolution 1µs (Single Turn) Time Resolution 20 ns System Bandwidth Beam Size Measurement Accuracy 17. 5 MHz ± 10% of RMS Beam Size Dynamic Range 100 Maximum Beam Trajectory Deflection Maximum Allowed Magnet 0. 5 mrad < 1% at 12. 78 cm Radius Multipole Component 7 Pressure Managed by UT-Battelle for the U. S. Department of Energy 10 -9 -10 -8 mbar Presentation_name ~ 2 -3 mm

Simulation Tools • COMSOL general purpose PDE solver – Static electrical and magnetic fields in 3 -D • ORBIT particle tracking code with space charge – Generate ions/electrons – Calculate space charge forces from accumulated beam – Track ions/electrons in static and space charge e/m filed 8 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

High Voltage Electrode Design Graphical representation of the electrode optimization method and parameters. 120 k. V potential difference is required to satisfy requirements for • Spatial resolution when collecting ions • Temporal resolution when collecting electrons 9 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

High Voltage Electrode Design Quarter model of IPM chamber and electrode with electrostatic surface electric field simulation results for detailed optimized dimensions. 10 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Secondary emission suppression grid 11 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Movable Chaneltron® Detector 12 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

SNS IPM Internal layout 13 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Protection from circulating beam RF 14 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Magnetic field for electrons collection 300 Gauss 15 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

SNS IPM mechanical design 16 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

SNS IPM mechanical design Large size => Large cost 17 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Detector test with beam set up To verify RF shield and detector performance 18 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Some beam test results Good temporal resolution for ions Test chamber ion signal after background subtraction and inversion for the fully modified test chamber with isolated feedthroughs, detector shield with opening, shielded cabling, high-bandwidth amplifier, and grounded amplifier casing. Poor temporal resolution for electrons Test chamber electron signal after background subtraction and inversion for the fully modified test chamber with isolated feedthroughs, detector shield with opening, shielded cabling, high-bandwidth amplifier, and grounded amplifier casing. 19 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Some beam test results BCM screenshot for 100 μs of beam storage. 20 Managed by UT-Battelle for the U. S. Department of Energy Low Pass filtered electron signals as a function of Channeltron voltage for a stored beam. Presentation_name

Some beam test results IPM test chamber electron data for a fixed Channeltron bias as a function of electrode voltage. 21 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Conclusion of the design study • High Cost • No confidence in understanding of IPM operation in electron collection regime • Ion collection regime does not satisfy temporal resolution requirement - No Go - Invest in electron scanner instead 22 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

2. 5 Me. V bunch longitudinal profile measurement Laser wire layout 80 MHz CW Ti-Sapphire Laser 90 deg magnet Delay scan Linac RF Background subtracted H 0 Stripped by residual gas H- Stripped by laser MCP and Faraday Cup - H m bea HV FEEDTHROUGH MOUNTING FLANGE Electron collection system layout MCP + FARADAY CUP Signal from the detector Measured at nominal current bunch RMS length of 8. 5⁰ is smaller than design value of 15⁰ BENDING MAGNET Cannot measure at low current because of poor signal to background ratio MCP bandwidth is too narrow to detect signal at laser frequency (80 MHz) Courtesy of A. Menshov 23 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Space charge effect on electron collection Simulated motion of stripped electrons in the collection system Space charge at nominal current creates ~50% energy spread in addition to transverse deflection 24 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

New electrostatic collector with fast scintillator Photomultiplier tube H- Fast scintillator screen Laser beam 10 k. V Ese = 600 e. V High voltage electrode 10 k. V Ese = 1800 e. V Increase collection efficiency Suppress background by detecting signal at laser frequency (80 MHz) EJ-212 Courtesy of A. Menshov 25 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

As of today results: • Observe strong background from residual gas ionization • Do not see laser induced signal in the background • Do not see 80 MHz laser induced signal Scintillator before installation 26 Managed by UT-Battelle for the U. S. Department of Energy Scintillator after few hours of operation Presentation_name

Thank you! 27 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Full picture is more complicated Background from rebuncher cavities Background from beam Laser induced signal 28 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name

Currently installed LW SCANNER DETECTOR SHIELDING SCREEN 29 Managed by UT-Battelle for the U. S. Department of Energy Presentation_name