Alexander Aleksandrov Spallation Neutron Source Oak Ridge, USA 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 2.5MeV laser-based longitudinal profile monitor

SNS Accelerator Complex 1 GeV LINAC Accumulator Ring: compress 1-msec long pulse to 700 nsec 2.5 MeV Warm LINAC Front-End RTBT HEBT Injection Extraction RF Collimators 945 ns 1 ms macropulse 1ms Liquid Hg Target 1000 MeV Cold LINAC 186 MeV Front-End: Produce a 1-msec long, chopped, H- beam

SNS Beam Time Structure 1 GeV LINAC Accumulator Ring: compress 1-msec long pulse to 700 nsec 2.5 MeV Warm LINAC Front-End RTBT HEBT Injection Extraction RF Collimators 945 ns 1 ms macropulse Current mini-pulse Chopper system makes gaps 1ms Liquid Hg Target 1000 MeV 1 ms <1 msec Cold LINAC 186 MeV 38mA 38A Front-End: Produce a 1-msec long, chopped, H- beam

SNS Beam profile diagnostics types 1 GeV LINAC Accumulator Ring: compress 1-msec long pulse to 700 nsec 2.5 MeV Warm LINAC Front-End RTBT HEBT Injection Extraction RF Collimators 945 ns 1 ms macropulse 1ms Liquid Hg Target 1000 MeV Cold LINAC 186 MeV Front-End: Produce a 1-msec long, chopped, H- beam Interceptive: conventional wire scanners H- ions protons Protons Non-Interceptive: conventional wire scanners Non-Interceptive: laser 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

SNS AR IPM system requirements for fully accumulated 1.5x1014 ppp beam Value or Range Measured Profile Plane Transverse Horizontal and Vertical Longitudinal Resolution 1µs (Single Turn) Time Resolution 20 ns System Bandwidth 17.5 MHz Beam Size Measurement Accuracy ±10% of RMS Beam Size Dynamic Range 100 Maximum Beam Trajectory Deflection 0.5 mrad Maximum Allowed Magnet Multipole Component < 1% at 12.78 cm Radius Pressure 10-9-10-8 mbar ~ 2-3mm

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

High Voltage Electrode Design Graphical representation of the electrode optimization method and parameters. 120kV potential difference is required to satisfy requirements for Spatial resolution when collecting ions Temporal resolution when collecting electrons

High Voltage Electrode Design Quarter model of IPM chamber and electrode with electrostatic surface electric field simulation results for detailed optimized dimensions.

Secondary emission suppression grid

Movable Chaneltron® Detector

SNS IPM Internal layout

Protection from circulating beam RF

Magnetic field for electrons collection 300 Gauss

SNS IPM mechanical design

SNS IPM mechanical design Large size => Large cost

Detector test with beam set up To verify RF shield and detector performance

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.

Some beam test results Low Pass filtered electron signals as a function of Channeltron voltage for a stored beam. BCM screenshot for 100 μs of beam storage.

Some beam test results IPM test chamber electron data for a fixed Channeltron bias as a function of electrode voltage.

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

2.5 MeV bunch longitudinal profile measurement MCP and Faraday Cup 80MHz CW Ti-Sapphire Laser H- H0 Delay scan H- beam 90 deg magnet Laser wire layout Signal from the detector Stripped by laser Stripped by residual gas Background subtracted Linac RF BENDING MAGNET MCP + FARADAY CUP MOUNTING FLANGE HV FEEDTHROUGH Electron collection system layout Measured at nominal current bunch RMS length of 8.5⁰ is smaller than design value of 15⁰ Cannot measure at low current because of poor signal to background ratio MCP bandwidth is too narrow to detect signal at laser frequency (80MHz) Courtesy of A. Menshov

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

New electrostatic collector with fast scintillator Ese = 600 eV Photomultiplier tube Fast scintillator screen H- 10kV Laser beam High voltage electrode Ese = 1800 eV 10kV Increase collection efficiency Suppress background by detecting signal at laser frequency (80MHz) EJ-212 Courtesy of A. Menshov

As of today results: Observe strong background from residual gas ionization Do not see laser induced signal in the background Do not see 80MHz laser induced signal Scintillator before installation Scintillator after few hours of operation

Thank you!

Full picture is more complicated Background from rebuncher cavities Background from beam Laser induced signal

Currently installed LW SCANNER DETECTOR SHIELDING SCREEN