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

2. Outline SNS accelerator Accumulator Ring IPM requirements
SNS IPM design
IPM-relevant experience with SNS 2.5MeV laser-based longitudinal profile monitor

3. 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

4. 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

5. 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 ?

6. 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

7. 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 cm Radius
Pressure
mbar
~ 2-3mm

8. 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

9. 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

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

11. Secondary emission suppression grid

12. Movable Chaneltron® Detector

13. SNS IPM Internal layout

14. Protection from circulating beam RF

15. Magnetic field for electrons collection
300 Gauss

16. SNS IPM mechanical design

17. SNS IPM mechanical design
Large size => Large cost

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

19. 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.

20. 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.

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

22. 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

23. 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

24. 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

25. 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

26. 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

27. Thank you!

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

29. Currently installed LW SCANNER DETECTOR
SHIELDING SCREEN