The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

OPTICAL MATCHING OF SLOWLY EXTRACTED BEAM
WITH TRANSPORT SYSTEM
T. Furukawaa,b #, K. Nodab, S. Shibuyac, T. Fujimotoc, M. Muramatsub, E. Urakabeb, S. Satob,T. Uesugib,M. Kanazawab, M. Torikoshib, H. Kawaia, E. Takadab and S. Yamadaba) Graduate School of Science and Technology, Chiba Univ., Chiba, JAPANb) National Institute of Radiological Sciences, Chiba, JAPANc) AEC, Chiba, JAPANAbstract
The matching of the optical function between the ringand the transport line plays important role in order tocontrol the size of pencil beam precisely in the scanning
irradiation for cancer therapy. For this purpose, theoptical matching of the slowly extracted beam with thetransport system has been investigated at HIMAC. As aresult, it was verified that the beam size of the slowly
extracted beam could be controlled owing to the opticalmatching. This technique will also make it easier todesign the complicated transport system such as a gantry.

1 INTRODUCTION
Heavy-ion beams have attracted growing interest forcancer treatment due to their high dose localization at theBragg peak as well as high biological effect there.
Recently, therefore, heavy-ion cancer treatments havebeen successfully carried out at various facilities aroundthe world. On the other hand, beam-scanning irradiation
method, such as spot and raster scanning, has beendeveloped around the world in order to achieve a highirradiation accuracy, even for an irregular-shaped target.
The emittance of the extracted beam along with its
position and angular stability are critically important for
creating pencil beams used in scanning irradiation forconformal 3-D irradiation. The matching of the opticalfunction between the ring and the transport line plays
important role in order to control precisely the size ofpencil beam. For this purpose, the optical matching of theslowly extracted beam with the transport system has been
investigated at HIMAC (Heavy Ion Medical Accelerator
in Chiba) [1]. Since the phase-space distribution of theslowly extracted beam is a kind of a thin string, aconventional method [2] to measure the optical function
and the emittance by using the beam profile, in which
Gaussian distribution of the beam is assumed, is difficult
to apply for the slowly extracted beam.
At HIMAC, the RF-knockout slow extraction method[3-4] has been routinely used for therapy and theexperiments of physics and biology. In the RF-knockout
extraction, to keep the emittance constant during theextraction is easier than the ordinary slow-extraction
owing to the constant separatrix. However, theverification method of beam matching has not yet beendeveloped. Thus, the method to measure the outgoing
separatrix is proposed and verified by using thin rod.
Since the outgoing separatrix was measured, the optical
function of the extracted beam at the entrance of theextraction channel can be estimated. This result suggested
the mismatching of the dispersion from the design value.
Thus, the optical parameters of the transport line arereadjusted to match with the extracted beam. Finally, it
was verified that the optical matching of the slowly
extracted beam with the transport system.

2 OPTICAL CHARACTERISTICS OFSLOWLY EXTRACTED BEAM
In order to estimate the optical characteristics of theextracted beam, a simple method [5] to measure theoutgoing separatrix using two tantalum rods has been
proposed and verified at the HIMAC synchrotron. By
measuring the extracted beam intensity with inserting tworods into the ring, the intensity spectrum for different
position of second rod can be observed for each positionof rod1. As a result, one of the outgoing separatrix for theextraction can be measured. As shown in Fig. 1, the resultof the measurement is in good agreement with thesimulation result. This simulation is carried out by theparticle tracking code [6] for the RF-knockout extraction.
Since there is the electrostatic deflector (ESD), theparticles into 77~90mm will be extracted.

Fig. 1. Triangles represent outgoing separatrix for each
momentum of –0.3%, 0.0% and 0.3% from the bottom, asa result of the experiment. Closed circles are simulation
result of the separatrix. The area enclosed by dotted line
shows extracted beam.
____________________________________________________________# t_furu@nirs.go.jp

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Since the simulation result well reproduces theexperimental result, we can estimate the optical propertyof the extracted beam at the entrance of the extraction
channel by using the simulation result. Firstly, the twissparameters at the entrance of the extraction channel areestimated by fitting the beam as an ellipse (enclosed bydashed curve in Fig. 1). Secondly, the dispersion function
is estimated by using the extracted beam center of theposition and angle for each momentum. Fig. 2 shows thesimulation result of the extracted beam at the entrance ofthe extraction channel for each momentum of –0.05%,0.0% and 0.05%, respectively. These results are
summarized at table 1, comparing with the original designvalue.

3 COMPARISON BETWEENSIMULATION AND EXPERIMENT
Using this new optics, the experimental test was carriedout. The transport line from the HIMAC upper ring to the
biological experiment room was used. This line isincluding the vertical beam line and 11 profile monitors.
In the experiment, the carbon ions with an energy of 400
MeV/u were extracted from the synchrotron through theRF-knockout slow-extraction method.

20

Horizontal(a)Beam size [mm]100-10Vertical-20020406080

100120s [m]

(b)

20

Table 1: Comparison of twiss-parameters and dispersion
function at the entrance of the extraction channel
Original design

Estimated bysimulation

βx5.0m

15m

αx0.0

1.0εx

10π mm mrad

1.5π mm mrad

Dx-0.5-0.5Dx′

0.00.1910

Beam size [mm]Fig. 2. Simulation result of extracted beam at the entranceof the extraction channel for each momentum of –0.05%,0.0% and 0.05% from the bottom.

0-10-20

Vertical
0The initial condition of the beam line is defined to bethe parameters estimated by the simulation. Using theseparameters, the optics of the beam transport line was recalculated in order to match the optical function. In thecalculation, the magnetic parameters were adjusted not toloss the beam under the present magnets and aperture.

Horizontal

20406080100120s [m]Fig. 3. Comparison of the beam envelope (2σ) between
the calculation and the measurement (triangles). (a) themomentum spread (2σ) is ±0.05% and (b) ±0.4%.
The beam size was measured at each monitor in thetransport-line. The both beam envelopes of horizontal andvertical were compared with the measured beam size asshown in Fig. 3 (a). The emittance (2σ) and themomentum spread were estimated to be (1.0, 2.0) [π mmmrad] and ±0.05%. In this calculation of the beamenvelope, the estimated dispersion function, as shown inFig. 4, was used. The achromatic condition wassuccessfully realized. Further, not only the dispersion, butalso the measured beam size is in good agreement withthe estimation at each monitor in the transport line. As

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

shown in Fig. 3 (b), even for the beam having wide
momentum spread (±0.4%), the measured beam size is ingood agreement with the estimation. It wasexperimentally verified that the extraction efficiency waskept at more than 95% even for the beam having suchwide momentum spread.
On the other hand, we can estimate the horizontal beamprofile at each profile monitor by using the simulation
result of the extracted beam and the transfer matrix of thetransport line. Owing to the matching of the opticalfunctions, this result is also in good agreement with themeasurement at each profile monitor. The typical result of
this is shown in Fig. 5.

(a)20The authors would like to express their thanks tomembers of the Department of Accelerator Physics andEngineering at NIRS for continuous encouragement. This
work has been performed as a part of Research Projectwith Heavy-Ion at NIRS-HIMAC.

REFERENCES[1] Y. Hirao et al, Nucl. Phys. A528 (1992) 541c.
[2] M. Giovannozzi et al, CERN PS (CA) 98-032, (1998).
[3] K. Noda et al, Nucl. Instr. and Meth. A 374 (1996)
269.
[4] T. Furukawa et al, Proc. 8th EPAC, Paris, 2002, p.
2739-2741.
[5] T. Furukawa et al, “New Approach Toward
Optimized Resonant Slow-Extraction”, Nucl. Instr.
and Meth. A, in press.
[6] T. Furukawa and K. Noda, Nucl. Instr. and Meth. A489 (2002) 59-67.

-2

6020

4060

80

100

2120s [m]4(b)0-2-42-6-20-100102010201020x [mm]0200(b)-2Counts(arb. unit)

Vertical dispersion [m](a)4-4x' [mrad]

Horizontal dispersion [m]4ACKNOWLEGMENT-4020406080100150100120s [m]500Fig. 4. Comparison of dispersion function between theestimation (line) and measurement (triangles), (a)horizontal and (b) vertical.

-20-100x [mm]30(c)4 SUMMARY
20Counts(arb. unit)

The optical parameters of the transport line werereadjusted to match the optical function with the extracted
beam. Finally, it was experimentally verified that theoptical matching of the slowly extracted beam with thetransport system. The measured beam profile anddispersion function is in good agreement with thesimulation at each monitor in the transport line. Further,the HIMAC transport system can deliver the beam having
wide momentum spread (±0.4%) without any beam loss.
This technique allows us to control the beam size
precisely as planned. Further, these results will contributeto the designer of the complicated transport system, suchas a gantry, together with the slowly extracted beam.

100-20-100x [mm]Fig.5. Comparison of the horizontal beam profile between
the simulation and the experiment. (a) Estimated phasespace distribution, (b) horizontal profile estimated by thesimulation and (c) the measured profile.