Injection Study of the ProTom-Radiance 330 Synchrotron with a 1.6 MeVRFQ Linac

Fuhua Wang , MIT Bates LaboratoryJay Flanz , MGH, Burr Proton Therapy Center, Harvard University Medical School

Robert W. Hamm, R&M Technical Enterprises, Inc.

1. Introduction: Proton therapy, Radiance 330 Proton Therapy system

2. Injector update objectives3. RFQ linac and injection beam line4. Synchrotron acceptance and attainable charge intensity

17/26/2011The 19th Particles and Nuclei International

Conference July 24th-29th, 2011 Cambridge, MA, USA

Proton Therapy

Clinical requirements•Penetration depth (energy): 4-40 cm (70-250 MeV)•Daily dose ~ 2 Gray (J/Kg) be delivered in 1-2 minutes ( protons/min + delivery) •Conformity: <± 2% to the treatment plan

2

Dose vs. depth for therapeutic proton, x-ray, electron beams and fast neutron (Sumitomo brochure)

A spread-out Bragg peak (SONP)( Al. Smith, Phys. Med. Biol. 51,2006, R491-504 )

Range-shifter wheel

Scatterer

Target

Patient

Collimator Compensator

Targett

Fixed energy beam

3

Proton Therapy - Treatment DeliveryEugen B. Hug (PSI)

Pencil beam scanningPassive scattering in practice

Radiance 330™

4

•70-250 MeV for treatment. Up to 330 MeV for proton-tomography.

•True 3D, pencil scanning (dynamic energy and intensity modulation) with minimal neutron exposure and residual radiation background.

•Small beam emittance and momentum spread. Small footprint, low-cost system.

McLaren Proton Therapy Center

x

Y

Beam writing on Exit windowat MIT-Bates site

Beam exit window

Ionization chamber

Patient position system

Radiance 330™

5

2. Injector update objectives

• Increase synchrotron charge intensity (~ 3 times), so dose rate of 2Gy/min on large field size ( 40cmx30cm) can be achieved.

• To ensure operational reliability and minimal maintenance.

Two options:•Improved tandem

•Radio Frequency Quadrupole (RFQ ) linac

6

sin( )2v tω ϕ− +

sin( )2v tω ϕ+

This study:•RFQ linac, injection line design•Synchrotron acceptance : aperture limits, space charge limits, RF capture

A long rf electric quadrupole with a sinusoidal varying voltage on its electrodes.

The longitudinal modulated electrode tip results in a acceleration longitudinal field (capable of a few MeV of acceleration).

The 1.6 MeV, small momentum spread RFQ linac

3. RFQ linac and injection beam line R&M Technical Enterprises, Inc.

Operation frequency 425 MHz

Final beam energy 1.6 MeV

Output beam FWHM momentum spread at 6 mA 5 keV

Beam within ± 2keV at 6mA 2.4 mA

Beam injection repetition rate 0.1-20 Hz

Structure length 1.29 m

Small momentum spread design: •A “prebuncher” designed into the front end of the RFQ vanes .Pre-bunching of the injected beam : uses the introduction of an axial field just after the input radialmatching section, followed by an unmodulated “drift” region for a short distance before the normal RFQfunctional sections. (J. Staples, Proc. 1994 Linac Conf., 775(1994).• The RFQ output energy spread is reduced from its normal value of ± 1% FWHM to less than ±0.2%.

7

8

X Emittance

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

-0.15 -0.1 -0.05 0 0.05 0.1 0.15

X (cm)

X' (r

ad)

Y Emittance

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

-0.15 -0.1 -0.05 0 0.05 0.1 0.15

Y (cm)

Y' (r

ad)

Z Emittance

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30

phi (deg)

E (M

eV)

Energy Distribution - ProTom RFQ - 6mA

0

50

100

150

200

250

300

350

400

450

500

550

600

-0.0

20

-0.0

18

-0.0

16

-0.0

14

-0.0

12

-0.0

10

-0.0

08

-0.0

06

-0.0

04

-0.0

02

0.00

0

0.00

2

0.00

4

0.00

6

0.00

8

0.01

0

0.01

2

0.01

4

0.01

6

0.01

8

0.02

0

Energy (MeV)

Freq

uenc

yRFQ final design: Output beam phase space and energy distribution at input current of 6 mA

I ≥ 2mA, |∆E|< 2 keV

9

Initial beam phase space used in this study (from Trace-3D)has more flat energy spread in the center I ≥ 1.1 mA, for |∆E|<2KeV

Beam line Physics design: bunching cavity included

10

Optics : transverse phase space matching to the synchrotronA single gap rf cavity is placed to rebunch the beam • Eliminate vertical emittance growth (space charge)• Improve energy distribution

11

4. Synchrotron acceptance and attainable charge intensity• Synchrotron accepatance (at injection): transverse phase space, dispersion

Mismatching will cause emittance dilution leading to large betatron amplitudes.Limits here: Injection aperture limit, dispersion mismatching (0m-line, ~3.4m synchrotron)

• Charge limits at low energy (space charge):Laslett tune shift Longitudinal microwave instability

• Ramping and RF capture (energy acceptance, adiabatic capture)

I= 5.7 mA, end of inj. lineI=3.9 mA, after one turnPARMILA tracking

Injection aperture limit, phase space and dispersion mismatching

12

Beam profiles after one and ¾ turn in the synchrotron

Energy profile

x

y

13

Space charge limit -Laslett tune shift. Comparision: Protom 1.6 MeV, 0.9MeV & Loma Linda** Protom Protom Loma Linda

rp(m) 1.54E-18 1.54E-18 1.54E-18R(m) 2.28 2.28 3.18N 5.30E+10 3.00E+10 1.70E+11T(MeV) 1.6 0.9 2γ 1.00171 1.00096 1.00213β 0.05833 0.04377 0.06519ax(cm) 1 1 3

by(cm) 0.5 0.5 0.3νx 0.825 0.825 0.58

∆νx 0.13973 0.14076 0.10756∆νx (allowed) 0.155 0.155 0.1

Laslett tune shift (space charge limit)

: HWHM,babaa

LNr

yx

x

ppx νγβπ

ν)(2 322 +

=∆

** G. Coutrakon et al., J. Med. Phys. 21(11) (1994), p.1691.

14

longitudinal microwave instability (space charge limit)

Keil-Schnell stability criterion *:

eIp

pE

nZ c

22

||2

∆

≤ηπβ

3221,10g1.6MeV,TFor

3770Zradius, beam:,chamber vac.of radius:),ln21(0g where 2 2

00||

≈≈=

=+==

n

Z

ohmsaba

bZngnZ

βγ

The capacitive longitudinal coupling impedance

Microwave instabilities are observed at ZAO-synchrotron:

•Show up at all intensity levels.

•No instant beam losses observed in the intensity level of ~2-3x109 protons

* A. Hofmann, “Overview of beam instabilities”, AIP conference Proceedings 496, p.13-14

Delay: 61 µs (clean start , ~ 10 turns after injection)

15

Microwave instability observation at 0.9 MeV (injection)

Toroid: ~0.12 mA

Toroid~0.35mA

Delay: 200 µs (~ 150 turns, varing high frequency signals)

Observed microwave instability at different beam intensities, but no instant beam losses.

16

Long. Microwave Instability limit. Comparision: Protom 1.6, 0.9 MeV & Loma Linda**Protom Protom Loma Linda

N 2.10E+10 1.20E+10 1.50E+11T(MeV) 1.6 0.9 2γ 1.001705 1.000959 1.002132β 0.058325 0.043768 0.065189L 14.345 14.345 19.981Rev. time 8.20397E-07 1.09325E-06 1.02238E-06I(A) 4.10E-03 1.76E-03 2.35E-02E 939.8723 939.1723 940.2723ZL/n 3000 4000 4000η (slip factor) 0.490 0.489 1.950∆p/p (rms) 0.0011 0.0011 0.0014±∆T(keV) 3.57 2.03 5.53

(simulation)∆p/p (rms) limit (%) ~0.0011 0.14

(measured)

γt 0.820 0.820 0.583

αc 1.487 1.487 2.940

η=(1/γt 2̂-1/γ 2̂) 0.4904 0.4889 1.9444

Capacitive longitudinal coupling impedance (space charge force)g0=1 1 1 1Z0 377 377 377Z=g0Z0/2βγ^2 3220.89 4298.52 2879.32

Longitudinal microwave instability limits (cont.)

Ramping and RF capture – 3D tracking RF capture efficiency: ~52%

17

18

System Beam progress Current (mA)

Protons

RFQ T=1.6 MeV 6

Injection Line With one bunching cavity

At injection 5.7

Synchrotron Space charge limit at 1.6 MeV (2.1x1010)

Injection aperture- transverse phase space limit (1.6MeV)

3.9 2x1010

|∆T|< 3.2KeV, |∆p/p| <1x10-3

(1.2x1010)

RF capture: 52% Attainable intensity

1x1010

Maximum charge intensity in Protom-Synchrotron with the 1.6MeV RFQ injectorfrom simulations