100 MeV Injector Linac for Indian Spallation Neutron Source

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100 MeV Injector Linac forIndian Spallation
Neutron Source

• S. A. PANDE

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The Spallation Neutron Source
Ion Source
RFQ
Linac
50 keV
4.5 MeV
100 MeV
1 GeV Proton Synchrotron
Spallation target
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Synchrotron Parameters

• Injection energy 100 MeV
• Extraction energy 1.0 GeV
• Circumference 212.4 m
• Radio Frequency 1.21 2.47 MHz
• Repetition rate 25 Hz
• Beam power 100 kW
• No of protons/pulse 2.5x1013

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Requirements From Injector Linac

• Output energy 100 MeV
• Particles H
• Pulse current 20 ma
• Pulse length 500 ?s
• Repetition rate 25 Hz

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Layout of the Linac
1 GeV Proton Synchrotron
HEBT Long. Trans. Phase Space Painting
50 keV
4.5 MeV
100 MeV
Ion Source
LEBT Chopper
RFQ
MEBT
DTL
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Low Energy Beam Transport (LEBT)

• Required for the following.
• Phase space matching of the beam from ion source
  to RFQ
• Putting diagnostics after the ion source
• Vacuum pumping provision
• Additionally, LEBT will house the chopping system
  in our case

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The Chopper System

• Why it is required to chop the beam?
• The 500 ?sec beam pulse will be sufficient to
  wrap 302 times around the synchrotron
• With harmonic no.(h) 2, there will be 2 RF
  buckets.
• Bucket height

?E
Synch. RF Period
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The Chopper (Contd.)

• The RF bucket height decreases at the ends (shown
  by yellow circles in the last slide) and
  particles falling in these regions will be lost
• These lost particles form a considerable amount
  of the injected beam (at 100 MeV!)
• Why not stop this beam being injected into the
  synchrotron or in the linac itself.

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Chopper (Contd.)

• This is done using the chopper system
• The chopper will be an electrostatic deflector
  assembly sweeping the beam across a circular
  aperture
• 34 of the beam will be stopped and 66 will be
  transmitted through the linac

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The Chopper (Contd.)

Chopped
Injected
Injected
??
??
? 120? of RF Period
Synch. RF Period
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Radio Frequency Quadrupole (RFQ)

• Choice of Parameters main considerations

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Choice of Parameters - RFQ

• Main considerations will be-
• To control the emittance growth
• Less power loss in the structure to enable
  efficient removal of heat
• Higher transmission efficiency to reduce the risk
  of structure activation
• Choice of Structure
• Higher efficiency, simplicity in heat removal
• ? Four Vane Cavity Structure

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Input/output Energy

• Input energy can be anywhere from 30 to 100 keV
• Higher output energy is preferred from injection
  point of view in the following accelerator
• The RFQ output energy range from 3 MeV to 7 MeV
  for similar projects around the world
• The output energy chosen is 4.5 MeV

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The Design Frequency

• Major factor Availability RF Power Source
• Higher power conversion efficiency f1/2
• ? Choice of higher frequency
• Dimensional tolerances f-1/2
• Power dissipation capability of the accelerator
  structure f-1
• ? Choice of lower frequency
• Considering CW operating mode, machining and
• alignment tolerances, we chose f 350 MHz

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Inter-vane Voltage

• Higher inter-vane voltage preferred for
• Better transverse focusing and better beam
  characteristics
• Better transmission efficiency
• Higher acceleration efficiency in turn shorter
  accelerator length
• Lower inter-voltage is preferred for
• Lower power loss in the structure
• Less probability of sparking

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Inter-vane Voltage (Contd.)

• Power dissipation in the structure ? V2
• Acceleration or energy gain ? V
• Being a CW accelerator, the last point is of
  crucial importance.
• The inter-vane voltage between 65-90 kV should be
  a good choice.
• We generated RFQ designs with
• Inter-voltages of 65, 70, 75, 80 and 85 kV
• The design with 65 kV is selected

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Radio Frequency Quadrupole (RFQ)

• Frequency 350 MHz
• Energy 4.5 MeV
• Beam current 25 mA
• Inter vane voltage 65 kV
• Particle H/H
• Total length 6.52 m
• Transmission efficiency 96.3
• Total power loss(structure) 428 kW
• Beam power 111.25 kW
• Max. surface E field (Emax) 26 MV/m
• Kilpatrick 1.4

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RFQ Design Parameters

• Modulation Parameter (m) 1 1.915
• Average radius (r0) 3.30 mm
• Synchronous phase (?s) -90 - -30 ?
• Transmission efficiency (?) 96.3
• Input emitt. ?t,rms(n) 0.20 ??m.rad
• Output emitt. ?t,rms(n) 0.20 ??m.rad
• Output emitt. ?z,rms(n) 0.10 deg.MeV
• Quality factor (Q0) 9000 

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RFQ Design Parameters

• Modulation Parameter (m) 1 1.915
• Average radius (r0) 3.30 mm
• Synchronous phase (?s) -90 - -30 ?
• Transmission efficiency (?) 96.3
• Input emitt. ?t,rms(n) 0.20 ??m.rad
• Output emitt. ?t,rms(n) 0.20 ??m.rad
• Output emitt. ?z,rms(n) 0.10 deg.MeV
• Quality factor (Q0) 9000 

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RFQ Cavity Characteristics

• Four vane structure
• Transverse cross section optimized with SUPERFISH
• Power loss 656 W/cm
• Power density 6 W/cm2
• 3D Study with MAFIA is in progress to decide
  about joining the multiple sections.

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100 MeV DTL as injector for SNS

• DTL is designed for 50 mA with a view to inject
  more current into synchrotron with increased
  injection energy.
• A 50 mA RFQ is redesigned with 85 kV intervane
  voltage, Length 5.5 m.
• Beam dynamics design is performed with PARMILA.
• Beam available from RFQ is traced through DTL
• MEBT design and matching through DTL is studied
  with TRACE3D

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Matched beam through DTL
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Beam transmission through DTL
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100 MeV DTL - Parameters

• Energy 100 MeV
• Beam current 50 mA
• Average E0 1.8 2.2 MV/mm
• Synchronous phase -60 -30 ?
• Length 74 meters
• Number of Tanks 7
• Tank diameter 52 50 48 cm
• Total power 6.76 MW
• Focussing lattice FODO
• Input ?Tr (n,rms) 0.2 ? mm mrad
• Output ?Tr (n, rms) 0.28 ? mm mrad
• ?Long (n, rms) 0.197 deg.MeV
• Energy spread (100) ?450 keV
• Phase spread (100) ?16.6 ?
• rms radius at O/P 1.27 mm

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Beam at the Output