January 12, 2018

Vinit Kumar

(on behalf of Accelerator Physics design team for ISNS)

RRCAT

Progress on Accelerator Physics Studies for Indian

Spallation Neutron Source

Interesting facts about spallation source and reactors

Neutron production via spallation was known before fission was discovered.

Building suitable accelerator was more difficult than reactor!

“At 1 GeV, 1 MW accelerator spallation source produces approximately the same

(time averaged) number of neutrons as a 10 MW research reactor.” (a rule of thumb)

SNS is pulsed, ~ 50-100 ms pulses @ 50-100 Hz peak flux = 100 times average

flux. Thus 1 MW SNS will produce same peak flux as a 1000 MW cw reactor!

Spallation source safer than reactor. Unlike a reactor, no proliferation issue with

spallation source

Developing an SNS will also help in the necessary R&D for sub-critical reactor for

energy production and transmutation of nuclear waste.

ISNS primary parameters

Parameter Value

Proton beam power on target 1 MW

Proton beam kinetic energy on target 1 GeV

Average beam current on target 1 mA

Average linac macropulse current 10 mA

Linac beam macropulse duty factor 10%

Protons per pulse on target 1.25 × 1014

Proton pulse length on target 680 ns

Ion type (Front end, Linac, HEBT) H-

Ion type (Ring, RTBT, target) proton

Ring circumference 262 m

Ring filling time 2 ms

Ring revolution frequency 1.0 MHz

Schematic of the accelerator for the proposed ISNS

1 GeV, 10 mA pulsed injector linac and accumulator ring

LEBT MEBT

35 keV

325 MHz

RFQ

3 MeV

1 GeV

Accumulator Ring

HEBT

Spallation

Target Pulsed neutrons

Peak flux ~ 1016 cm-2s-1 RTBT

IS

1 GeV

12 MeV 55 MeV 166 MeV

425 MeV

bg = 0.81, 650 MHz

Elliptic Cavity bg = 0.61, 650 MHz

Elliptic Cavity

325 MHz

bg=0.11 SSR0

325 MHz

bg=0.22 SSR1 325 MHz

bg=0.42 SSR2

Details of the pulse structure from linac

2 ms 2000 turns injection into accumulator ring.

Proposed ISNS layout

Length of different sections

LEBT 1.9 m

RFQ 3.49 m

MEBT 3.68 m

Linac 171 m

HEBT 179 m

Accumulator

Ring

262 m

RTBT 150 m (evolving!)

Design criterion: maximization of transmission,

and minimization of the emittance growth. Get

nearly equal emittance in both planes to avoid

fourth order resonance.

Beam dynamics codes: PARMTEQM, LIDOS,

TOUTATIS, TRACEWIN

Design approach: Constant intervane voltage

and average bore radius along the length of

RFQ.

Beam Dynamics Design of 325 MHz RFQ

Macropulse current 15 mA

Input energy 35 keV

et,rms,n (input) 0.35 mm-mrad

Output energy 3 MeV

et,rms,n (output) 0. 31 mm-mrad

el,rms (output) 0.31 MeV-deg

Transmission efficiency 94%

Design Parameters

Geometrical parameters of RFQ quadrant

Breakout Angle, αbk 200

Vane-Blank Half

Width, Bw

8 mm

Vane-Blank Depth, BD 30 mm

Vane Shoulder Half

Width, WS

15 mm

Vane Base Half Width,

Wb

20 mm

Vane angle 1, α1 200

Vane angle 2, α2 200

Corner Radius, Rc 10 mm

Vane height, H 103.38 mm

Vane half width, W 42.82 mm

Paramete

rs

Value

(RMS end)

Value

(exit fringe-field

end)

g 7.09 mm 4.93 mm

h1 101.86 mm 101.86 mm

h2 30.00 mm 30.00 mm

h3 72.25 mm 67.25 mm

d 49.34 mm 42.18 mm

t 10.00 mm 10.00 mm

b 20.00 mm 20.00 mm

Electromagnetic Design of 325 MHz RFQ

Vane cut-back details

Quad. mode frequency

(with tuner insertion =

11 mm)

325 MHz

Structure power loss 385 kW

Total RF power 415 kW

No. of sections 3

Coupling b/w sections Direct,

gap = 0.1 mm

No. of ports 4 (RF), 8(Vacuum)

48 (Tuners)

Tuning Range 316 – 344 MHz

Dipole mode

cut-off frequency

315 MHz

Mode stabilization

scheme

Dipole rods

Cross section details 8 Vacuum ports

4 RF coupling ports

48 Tuner ports

Extensive error studies (geometrical as beam dynamic) have been performed [ID-245]

3D EM simulation of RFQ with vane modulation

Tuning done to take care of field tilt due to vane modulation

Confirmed that field profile from CST is same as the one used in beam dynamics code

Input

ax,y -1 to -3 (-2.5)

bx,y 0.12 to 0.24 mm/

mrad (0.12)

en,rms 0.30 mm-

mrad

Output

ax,y 1.2533

bx,y 0.036 mm/

mrad

en,rms 0.35 mm-

mrad

• Beam dynamics studies done using

BEAMPATH and TRACEWIN.

• Space charge compensation will be

needed to control the emittance

growth for operation at 15 mA.

• A 60 mm long inclined plane deflector

will be used for pre-chopping of the

beam in LEBT .

Physics Design of Low Energy Beam Transport*

3 mA 15 mA

Parameters SSR0

(βg=0.11)

SSR1

(βg=0.22)

SSR2

(βg=0.42)

Ep/Eacc 4.2 3.8 3.5

Bp/Eacc [mT/(MV/m)] 6.4 5.9 5.8

R/Q (ohm) 165 290 304

G (ohm) 62 87 118

Eacc,max (MV/m) 9.4 10.4 11.2

Beam aperture dia (mm) 30.0 30.0 50.0

Cavity Radius (mm) 223.0 253.4 273.4

Cavity length (mm) 200.0 301.6 520.0

Cryogenic load (W) 2 2.8 10.2

Multipacting growth rate (ns-1) 0.05 0.065 0.065

Electromagnetic Design of 325 MHz SSRs*

Geometrical optimization done to (i) minimize Ep/Eacc and Bp/Eacc, and (ii) maximize

R/Q. HOM studies are done using CST-MWS. Multipacting studies done using CST-PS

Quadrupole Asymmetry parameter

Q = ∆𝒑𝒙 𝒓,𝟎 𝒄−∆𝒑𝒚 𝒓,

𝝅

𝟐𝒄

(∆𝒑𝒙 𝒓,𝟎 𝒄+∆𝒑𝒚 𝒓,𝝅

𝟐𝒄)/𝟐

For other higher order

multipole…….

∆𝒑𝑹𝒄 𝒓, 𝜶 = 𝑨𝟎 𝒓 + 𝑨𝒏

∞

𝒏=𝟏

𝒓 𝐜𝐨𝐬 𝒏𝜶 + 𝑩𝒏 𝒓 𝐬𝐢𝐧 𝒏𝜶

S.N. Multipole

Amplitude (An/rn-1 )

SSR0 SSR1 SSR2

1 1st (keV) 0.01209 -0.0648 0.01805

2 2nd(keV/mm) -0.4243 0.9582 -0.24405

3 3rd(keV/mm2) 0.0114 0.00142 0.0014

4 4th(keV/mm3) 0.00133 0.00104 0.00015

5 5th(keV/mm4) 0.00011 4.26E-05 8.27E-06

6 6th(keV/mm5) 6.1E-06 1.18E-05 -1.1E-06

Estimation of quadrupole asymmetry in SSRs

Poster ID-246

SSR0 vs. DTL

ID-254, 256

• Preliminary design of a 3-10 MeV normal conducting DTL has been completed.

• What is the right choice – DTL or SSR0? ~ 4 m long DTL vs. ~ 7 m long SSR0 section ~ 1 RF feed (550 kW) vs. 12 RF feed (~10 kW each) ~ better beam dynamics in DTL

EM Design of bg = 0.61 and bg = 0.81, 5-cell, 650 MHz SCRF cavity*

Parameter

bg = 0.61 cavity bg = 0.81 cavity

Mid-

cell

End-cell

(entry)

End-cell

(exit)

Mid-

cell

End-cell

Riris (mm) 44.00 44.00 44.00 43.9 43.9

Req (mm) 195.591 195.591 195.591 196.92 196.92

L (mm) 70.336 71.55 71.24 93.4 94.8

A (mm) 52.64 52.64 52.25 75.08 75.08

B (mm) 55.55 55.55 55.55 69.0 69.0

a (mm) 15.28 15.28 15.28 13.04 13.04

b (mm) 28.83 28.83 28.83 20.78 20.78

Cell Helium Vessel

Stiffener Ring

Parameter bg = 0.61 bg = 0.81

Eacc (MV/m) 15.4 18.6

Epk/Eacc 2.36 2.15

Bpk/Eacc

[(mT/(MV/m)] 4.56 3.77

kc 0.8% 0.8%

R/Q (W) 328 556.5

Cryogenic load 16 W 21 W

Geometrical parameters

RF parameters

Geometrical parameters optimized

to minimize Bpk/Eacc and Epk/Eacc,

and ensure that there are no trapped

HOMs and no multipacting.

EM design and multipacting studies performed

for of fundamental power couplers ID-250

Studies on the requirement of purity of niobium

ID-269

A magneto-therrmal analysis of breakdown of RF superconductivity has been performed, taking the realistic material parameters into account. The optimum material parameters that maximize the achievable acceleration gradient obtained from this analysis.

Studies on multipacting in elliptic cavities

• Developed a computer code, which simulates this process accurately, but takes less

computational time and requires less computer memory.

• A simplified model has also been worked out for the fast calculation of multipacting

growth rates.

• It has been shown that the cavity can be made mutipacting-free by geometry

modification.

• Total Power loss due to HOM is given by:

𝑃 = 𝑅𝑛

2 𝐻𝑛𝑚

𝑚

2

𝑑𝑠

𝑛

• Voltage amplitude of the mth mode :

𝑉𝑚 =𝜔𝑛𝜔𝑚

𝜔𝑛2 − 𝜔𝑚

2 − 𝑖𝜔𝑛𝜔𝑚𝑄𝑚

• Beam passing thru the axis of the cavity will produce monopole modes given by:

𝑬𝑛𝑚 =𝜔𝑛 𝜔𝑚

𝑖 𝜔𝑛2 − 𝜔𝑚

2 + 𝑖𝜔𝑛𝜔𝑚𝑄𝑚

𝐼 𝑛

2 𝑈𝑚

𝑅

𝑄 𝑚

𝑬 𝑚

𝑯𝑛𝑚 =𝜔𝑛 𝜔𝑚

𝑖 𝜔𝑛2 − 𝜔𝑚

2 + 𝑖𝜔𝑛𝜔𝑚𝑄𝑚

𝐼 𝑛

2 𝑈𝑚

𝑅

𝑄 𝑚

𝑯 𝑚

*Y. Yakovlev eta al., FERMILAB-CONF-11-117-TD

Studies on heat generation due to resonant excitation of HOMs

These formulae are derived in ID-253

Optimized lattice configuration of 1 GeV injector linac

Section Energy (MeV)

Cav

/mag

Focusing

SSR0 3-12.4 12/12 solenoid

SSR1 12.4- 54.5 20/10 solenoid

SSR2 54.5-165.6 24/6 solenoid

MB650 165.6-425 30/10 doublet

HB650 425-1091 42/7 doublet

(1) (20 + 10.5 + 20 + 10.5) cm ≈ 61 cm

(2) (9.53 + 30 + 9.53 + 30 + 9.53 + 30 + 9.53) cm ≈ 128 cm

SSR0 → 12 Sec → 7.32 m

SSR1 → 10 Sec → 12.8 m

(3) 2 × (9.58 + 50.64) cm + (9.58 + 30 + 9.58) cm + 2 × (9.58 + 50.64) cm ≈ 290 cm

SSR2 → 6 Sec → 17.4 m

(4) (35+20+35) cm + 42.48 cm + 3 × (100+42.48) cm ≈ 560 cm

MB 650 → 10 Sec → 56 m

(5) (35+20+35) cm + 29 cm + 6 × (134.5+29) cm ≈ 1100 cm

HB 650 → 7 Sec → 77 m

12 SSR2 cavities + 3 solenoids required to reach 100 MeV

ID-247

RF power requirements per cavity

SSR0

SSR1

SSR2

bg=0.61

bg=0.9

Should add a margin of at least 25% to take care of cavity detuning

Physics Design of MEBT*

R

F

Q

S

S

R

0

Section from RFQ to chopper

Beam Dump Chopper Plates

QF QD QF QF QF QD QD QD

Chopping section Matching section

RB RB RB QD QF QF

• 11 Quads + 3 re-buncher

cavities + Chopper plates

• Length ~ 3.86 m,

• Rms emittance growth < 4%

(3 planes)

• Matching variables: Quad

gradients and buncher

voltage

Some results of end to end beam dynamics simulations

Transverse and longitudinal beam sizes are within the acceptable limits

No significant emittance growth

Lattice footprints are in the safe zone of Hoffmann diagram

Horizontal (X) beam size

Vertical (Y) beam size

Intrabeam stripping in ISNS (H−) LINAC

Power loss due to intrabeam stripping

• It was discovered during the operation of SNS at Oak Ridge that reducing the beam size may sometimes have the disadvantage in terms of increasing the intra-beam stripping.

• For the 1 GeV injector linac, the calculation of beam loss due to intrabeam stripping has

been done. The power loss due to IBS is much less than the acceptable limit of 1 W/m.

Lattice design studies for Accumulator Ring for 1 GeV ISNS

• Design of the linear lattice performed for FODO as well as hybrid

• Studies on linear optics correction schemes performed.

• A code developed to study the longitudinal beam dynamics in the accumulator ring,

being benchmarked with ESME code.

Schematic of the proposed FODO lattice

Parameter FODO

lattice

Hybrid

lattice

Circumference 262.3 m

Periodicity 4

Linac emittance 0.5 mm-mrad

Painted emittance 103 mm-mrad

Dipole magnet length 2.25 m (sector type)

Bending angle 15

No. of dipole magnets 24

No. of quadrupole magnets 48 60

No. of quadrupole families 5 7

Revolution time 1 µs

RF frequency (h = 1) 1 MHz

RF frequency (h = 2) 2 MHz

Lattice parameters of the accumulator ring (FODO lattice)

Preliminary studies on sextupole scheme

Dynamic aperture for Hybrid lattice

3000 turns tracking was done: considering 2000 turns injection and 1000 turns for beam accumulation duration

0 20 40 60 80 100

0

20

40

60

80

100

120

140

Y (

mm

)

X (mm)

DA with sextupoles for (7.22,7.22)

DA alongwith multipolar components for (7.22, 7.22)

DA with sextupoles for (7.20, 6.20)

DA with multipolar components for (7.20, 6.20)

0 20 40 60 80

0

20

40

60

80

100

120

Y (

mm

)

X (mm)

DA with sextupoles for (7.22,7.22)

DA alongwith multipolar components for (7.22, 7.22)

DA with sextupoles for (7.20,6.20)

DA with sextupoles for modified (7.24,6.24)

DA alongwith multipolar components for (7.24, 6.24)

FODO lattice Hybrid lattice Beam sizes: 20 (H) × 70 (V) 21 (H) × 66 (V) (mm × mm)

×

Dynamic aperture simulations

ID-252

Preliminary Physics Design of HEBT

ID 231

Baem Injection studies

Studies on beam injection and phase space painting for FODO lattice are under progress.

Strength of bumper magnets are obtained on all working points, considering mismatch injection with preferred condition of ainj/binj = aring/bring

ID-230

Beam Injection studies

Major difficulty is the reduction of number of hits on foil per proton. The present optimized

scheme has lesser than 4 hits per proton on the foil (ORBIT simulations). Up to ~6-7 hits

per proton on foil, peak temperature on foil remains below 2000-2100 C (~2300 K).

1 2 3 4 5 6 7 8 9 10 11 121000

1200

1400

1600

1800

2000

2200

2400

Ma

xim

um

Tem

pe

ratu

re (

K)

Number of hits per proton on the foil

0 250 500 750 1000 1250 1500 1750 20000

1

2

3

4

5

6

7

8

9

10

Nu

mb

er

of

hit

s o

n f

oil

per

pro

ton

Turns

Beam in real space after injection painting (2000 turns)

Beam extraction studies

QF_A3

QD_A2 K1-K4 K5-K8 K9-K11

Septum

QF_A3

QD_A3

Quadrupole magnet

Kicker magnet

FODO lattice

QFD

QDA K3-K4 K6-K8 K9-K10

Septum

QF_A3

QDD

K1-K2 K5

QFD1

Hybrid lattice

•Bump of 150 mm is generated

at the location of the extraction

septum

•11 kicker magnets in FODO

lattice and 10 kicker magnets in

Hybrid lattice

•Extraction scheme is able to

work with any two kicker failure

•Maximum kick 4.5 mrad (field <

520 G for 500 mm kicker length)

Preliminary Physics Design of RTBT

Lattice parameters in RTBT

ID-232

Quadrupole AR orbit Ext

Septum (15)

Dipole Magnet

(15)

ID No. Abstract Title

ID-230 Beam Injection Studies for FODO lattice of 1 GeV proton Accumulator Ring

ID-231 Beam optics design studies of the ISNS High Energy Beam Transport line

ID-232 Preliminary optics design of Ring To Target Beam Transport line for a 1GeV Spallation

Neutron Source

ID-245 Error study of a 325 MHz, 3 MeV RFQ for ISNS

ID-246 Field perturbation due to azimuthal asymmetry in SSR cavities

ID-247 Lattice design and beam dynamics simulations for the 1 GeV ISNS SRF LINAC

ID-250 Electromagnetic design studies of fundamental RF power coupler for superconducting

RF cavities

ID-252 Nonlinear studies for lattice of 1GeV Proton Accumulator Ring for Indian Spallation

Neutron Source

ID-253 Analysis of Generation and Effect of Higher Order Modes (HOMs) in Superconducting

Cavities

ID-254 Beam Dynamics Studies on 325 MHz DTL using GenDTL and Tracewin

ID-256 Physics Design Studies of 10 MeV, 325 MHz Drift Tube Linac for the Indian

Spallation Neutron Source

ID-269 On the requirement of high purity level of material for niobium based SRF cavity

Future studies

• Possibility of using a DTL instead of SSRs in the beginning of the injector linac will be

explored with an in-depth study.

• Compact designs of LEBT and MEBT will be explored.

• Detailed beam dynamics studies with realistic 3D field, taking quadrupole asymmetry

and coupler kicks.

• Detailed studies on effect of error in beam injection and positioning and alignment of

lattice elements, along with cavity failure will be performed after finalizing the lattice.

• End to end beam dynamics studies starting from HEBT entrance to beam target

through AR will be performed.

• Detailed studies on beam loss/collimation and beam instability.