A Preliminary Linac Design to Inject the CIAE 300 MeV Storage Ring

(Chinese Institute for Atomic Energy)

J. Stovall, K. Crandall, L. Young, J. Billen

30 September, 2013 RAL, UK

Winitial= 50 keV Wfinal = 3 MeV Ipeak = 50 mA Ppeak = 510 kW Duty = 2.5% L=3 m Status: running Producing neutrons

Tsinghua University (THU) RFQ

Winitial= 3 MeV Wfinal = 13 MeV Ppeak = 1.2 MW Drift tubes: 40 L=4.3 m Status: prototyping components

Tsinghua University (THU) DTL

The CIAE Plans to Construct a Proton Accelerator for Nuclear Physics Research

1. Wfinal : 300 MeV 2. Iave : 1 mA 3. Duty Factor : 2.5% (rf) 4. Ipeak : 50 mA 5. rf frequency : 325 & 975 MHz

The CIAE Injector Linac Design Requirements

Mode Pulse length

Frequency Macro-pulse length

Ipeak Application

Long-pulse

400 μs 50 Hz 400 μs 50 mA Fixed target

Short-pulse

400 ns 1.43 MHz 400 μs 20 mA Extraction gap

μ-pulse <1 ns 1 MHz 400 μs 1 mA Time-of-flight

Separated Drift-Tube Linac (SDTL) is only marginally more efficient

Annular-Coupled Structures (ACS) have a second π/2 mode that effectively reduces ZT2

A 3rd harmonic design will accelerate H+ & H- simultaneously

chopper

chopper

K

K K K K K K K K K K K K

DTL-1 DTL-2 DTL-3 DTL-4 CCL-1 CCL-2 CCL-3 CCL-4 CCL-5 CCL-6 CCL-7 CCL-8

RFQ-H-

RFQ-H+

H-

H+

CCL-8

H-

H+

Storage Ring

Fixed Target

Bunchers

Debuncher

325 MHz 3 MW

975 MHz 5.3 MW

Proposed design uses 3 different types of accelerating structures; RFQ, DTL & CCL

325 MHz 3 MW

By Increasing the length to 4 m the THU RFQ could deliver 4.5 MeV

• SNS replacement RFQ fabricated by Research Instruments, RI

• Increasing the injection energy would eliminate 12 drift tubes

• Beam quality would be better but chopping would become more challenging

• ESS RFQ is 4.7 m long and delivers 3.6 MeV

Effective (real-estate?) Shunt Impedance (scaled) of Candidate Structures

（By Dr. Ciprian Plostinar）

Drift Tube Linacs offer the best beam dynamics for intense beams

LAMPF PIGMI LINAC4 EMQs PMQs PMQs ~1972 ~1980 ~2011

Side-Coupled Structures (SCL or CCL) are used in hundreds of medical linacs

LAMPF SNS

CIAE Design Comparison CIAE Preliminary Design

Proposed Design

Structure Wfinal (MeV)

Length (m)

Power (MW) Klystrons

RFQ (2) 3 3 <1.0 1 DTL 63 30 8.2 4

SDTL 150 57 14.6 7 ASC 300 65 13 total 155 25

Structure Wfinal (MeV)

Length (m)

Power (MW) Klystrons

RFQ (2) 3 (4.5) 3 (4) <1.0 (?) 1 DTL 75 25 (24) 9.4 4 SCL 300 109 28.1 8 total 137 38.5 13

Proposed design operates with dual ions and multiple energies

Current independent Dual ion acceleration H+ & H-

simultaneous operation Multiple beam energies 100 – 300 MeV (~7 steps)

325 MHz klystrons (RFQ & DTL) VKP-8325A Pk= 3 MW

975 MHz klystrons (SCL) CPI-TBD Pk= 5.3 MW τ=450 μs duty factor=2.25%

1. Minimize number of different structures • marginal differences in P among structures • parallel design efforts are expensive • economy of scale in fabrication

2. Minimize number of (different) rf amplifiers • maximize klystron power • use π/2 mode to accommodate long structures

3. Maximize power per module • consistent with beam dynamics constraints • consistent with Pwindow

• Pwindow=Pklystron-Pwaveguide-Pcontrol • P325=3 MW-10%-15%=2.37MW • P975=5 MW-10%-15%=3.95MW

• Prequired=PSF+Pfeatures+Pbeam • Prequired≤Pwindow

Design philosophy

CERN & ESS Drift Tube Geometry Scaled from 352 to 325 MHz

0

5

10

15

20

25

0

0 5 10

0 5 10

0 5 10 15

β=0.08 β=0.21 β=0.28 β=0.375

SNS CCL Cavity Geometry Scaled from 805 to 975 MHz

0

1

2

3

4

5

6

7

8

9

10

0 2

0 2

0 2 4

0 2 4

β=0.40 β=0.49 β=0.53 β=0.65

• The web may be too thin to accommodate cooling

Design procedure assumes polynomial fits to structure properties

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7β

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

T

DTL CCL

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7β

6

7

8

9

10

11

12

E peak

(MV/

m)

normalized to 1 Mv/m

DTL

CCL

Epeak normalized to E0=1.0 Transit time factor

Wi : 3 4.5 MeV β : 0.0798 0.0976

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7β

20

30

40

50

60

70

ZT2 (M

Ω/m

)

CCLDTL

W=75 MeV

W=81 MeV

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7β

0.25

0.35

0.45

0.55

0.65

P beam

/Pto

tal

DTL

CCL

ZT2 Beam loading

Transition energy based on structure efficiency

DTL design has ramped φs and E0 laws 4.5 MeV injection may have a flat field

φs Separatrix

E0 Peak surface electric field

DTL design philosophy maximizes power per tank

Tank No of Cells

Length m

Wfinal MeV

Power MW

1

2

3

4

57

24

20

18

7.24

5.53

5.73

5.93

22.29

40.36

57.99

75.00

2.309

2.305

2.361

2.384

total 126 24.44 9.36

Pwindow = 2.37

Transverse design relies on PMQs

SNS ID=25.4 mm L =35 mm G =36 T/m

PL-7 ID=12.7 mm L =25.4 mm G =175 T/m

THU ID=20.0 mm L =40 mm G =82 T/m

CERN PMQ provides 65 T/m & leaves room for cooling

PMQ assembly using rectangular

magnet blocks

PMQ gradient as a function of router for rinner=12 mm

Electrical discharge is enhanced by colinear E & B fields

EMQ & flat face

High field PMQ & 2° face

PMQ & flat face

Moretti effect

Bfringe penetrates the face of the drift tube

E0 & Bfringe in the first gap are limited by Moretti & multipactor criteria

0 1 2 3 4B (T)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Etre

sh (E

k)

Etresh=1.5-.67B+.18B2-.019B3

Moretti criteria; breakdown threshold in the presence of a dc magnetic field

Multipactor resonance diagram

We propose a modified drift tube geometry

0 5 10 15 20 25 30 35 40 45 50 55 60r (cm)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

E peak

(EK)

reference contour

proposed contour

rc

ro

Reference and proposed drift tube shapes

Surface field distribution on the face of the reference and proposed drift-tube

designs as a function of radius

• Increasing Winitial to 3.5 MeV “solves” both problems

2 drift tube options

CERN Linac4 drift tube showing flat face & PMQ in very close proximity to face

SNS drift tubes

THU Drift Tube

+0-0 quad law relaxes alignment tolerance and accommodates steering & diagnostics

PMQ gradients σ0t and σ0l avoid resonances

Kt and Kl at 50 mA are balanced, smooth & continuous Tune depression is significant

Quad law meets stability criteria

Equipartitioning ratio shows that that there is no free energy

available to cause emittance growth

Hofmann diagram shows that we have avoided coherent

resonances

Beam size increases by 70% in the DTL

Transverse and longitudinal beam size for 5*εrms & 50 mA

• Fixed width separatrix in Tank 1 • ESS increases aperture in tanks

3 & 4 with minimal power consequence

Zero- and full-current transverse acceptance

Design is relatively insensitive to misalignments

Expected centroid excursion due to random drift tube misalignments Expected filling factor in the DTL

Ipeak= 50 mA εx&y = 5*εrms initial radial displacement: 300 µm (12 mil) drift tube alignment tol.: 175 µm (uniform) ( 7 mil) expected alignment tol.: 75 µm (Gaussian) ( 3 mil)

SNS DTL Diagnostics

Tank 1

0

0

Tank 2 60

BeamQV QH QH QVQV QHQH QVQV QHQHQV

QV QV QH QH QV QHQH QVQV QHQV QV QHQV

QH QV QV QH QH QV QV QHQH QVQV QH

QV QV QH QH QH QHQV QV QV QHQH QVQV QV

QH

10

20 30 40

50

QH

QH

10 20

DV DV DH DH FC

WS

BLM

48 0

Tank 3

Tank 4

34 0

QH QH QV QHQHQV QV QV QH QH QV QHQHQV

QV QHQV QV QHQH QVQV

QHQH QV QV QHQH QV QV QHQH QH

QVQV QH QV QV QH QH QV QV QH QH

QV QV QH

QH

QVQH

4030

10 20

30 10

DV

DV

DV

DV

DH DH

DH

DH FC

WS

FC

WS

BLM

BLM

DTL Tank BPM / Phasedetector in DT

Current Monitor(Toroid)

Wire Scanner

Faraday cup(Beam stop)

2” VAT Gate Valve

Quadrupole,Vertical

Quadrupole,Horizontal

Symbol NameNumberof units

6

Symbol Name Numberof units

Drift tube

74

73

5

10

6

5

5

Quadrupole(9.0cm) 1

FC

QV

QH

Half drift tube

WS

210

12

DH

DV

Dipole,Horizontal

Dipole,Vertical

12

12Energy degrader 5

Beam LossMonitor

BLM 6

SNS DTL

KK

Tank Bridge Coupler

Quadrupole Lens

Klystron

CCL Modules are comprised of 12 or 16 tanks

Tanks have 9 or 11 cells of equal length

CCL design maximizes power per module

• Structure operates in the π/2 mode • Lcell = βλ/2 • all cells in a tank have the same length • Lbridge coupler = 3/2 βλ

• We want all bridge couplers on the bottom • requires odd number of cells per tank

• We split the power to reduce the window loading • drive at the ¾ points to balance power • Requires no. of tanks per module be divisible by 4

• Prequired includes 15% for coupling slots, coupling cells & bridge couplers

• Pklystron = 5 MW • Pwindow = 3.95 MW

• LDTL period = 4βλ325 = LCCL period = 12βλ975

SCL RF Design

Module number

Tanks Per

module

Cells per tank

Length m

Wfinal

MeV Power

MW

1 16 9 11.45 86.7 1.23

2 12 11 10.90 108.9 3.02

3 12 11 12.06 137.2 3.94

4 12 11 13.15 167.2 3.94

5 12 11 14.13 198.7 3.95

6 12 11 15.01 231.3 3.95

7 12 11 15.80 264.8 3.95

8 12 11 16.52 300.1 4.14 total 100 1068 109.01 300.1 28.13

We ramp φs & E0 in modules 1 & 2 to match K0t & K0l from the DTL

E0 Peak surface electric field

φs separatrix

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7β

0.25

0.35

0.45

0.55

0.65

P beam

/Pto

tal

DTL

CCLBeam loading ≡ Pbeam/Ptotal Pbeam ~ E0 Pcavity ~ E0

2

Beam loading

First two modules are relatively inefficient

SCL Energy Gain per Tank E0

The EMQ Must Clear the Bridge Coupler

SNS EMQ incudes steering dipoles

The 12βλ +- +- quad law maintains current independence

+-+-

EMQ gradients are reasonable σ0t and σ0l avoid resonances

Kt and kl at 50 mA are smooth & continuous Tune depression is moderate

Stability criteria in the SCL are not rigorously met

Equipartitioning ratio should be unity

Hofmann diagram shows the design intercepts the tip of the

most sensitive resonance

• These properties are sensitive to the beam emittance • Emittances are estimates at this point

Beam Size & Acceptance in the SCL

Transverse and longitudinal beam size for 5*εrms

Zero- and full-current transverse acceptance

• separatrix in module 1 decreases by 25%

SNS CCL

End-to-end TRACE3-D simulation shows a space-charge driven mismatch at 75 MeV

50 mA

0 mA

Variable energy TRACE3-D simulations space charge decreases as beam debunches

300 MeV

200 MeV

109 MeV

3 MeV

3 MeV

3 MeV

Multi-particle input beam for LINAC code

Transverse

Radial distribution

Longitudinal

Water-bag distribution Winitial = 3 MeV Ipeak = 50 mA εrms = 0.27 π m-mR, norm

End-to-end 50 mA multi-particle simulation shows a φ mismatch at 75 MeV

R (cm) Φ- Φs W-Ws

3 MeV 300 MeV

End-to-end 0 mA multi-particle simulation confirms current independence

0 mA

3 MeV 300 MeV

R (cm) Φ- Φs W-Ws

109 MeV multi-particle simulation 50 mA

R (cm) Φ- Φs W-Ws

3 MeV 109 MeV

100 m long HEBT with 2 bunchers preserves the bunch length

100 m long HEBT with 2 bunchers can focus the beam to 100 ps

Bunch length ≈ 10°at 325 MHz ≈ .1 ns