RF design and study of a 325MHz 7MeV APF IH-DTL
for an injector of a proton medical accelerator
Xuan Li 1,2,3•Yue-Hu Pu 1,2,3,4•Fan Yang 4•Xiu-Cui Xie 2,4•Qiang Gu 1,2,3•
Jian Qiao 2,3•Ming-Hua Zhao 2,3
Received:28January 2019/Revid:28April 2019/Accepted:29April 2019
ÓChina Science Publishing &Media Ltd.(Science Press),Shanghai Institute of Applied Physics,the Chine Academy of Sciences,Chine Nuclear Society and Springer Nature Singapore Pte Ltd.2019
Abstract A compact interdigital H-mode drift-tube linac (IH-DTL)with the alternating-pha-focusing (APF)method,working at 325MHz was designed for an injector of a proton medical accelerator.When fed in with a proper RF (radio frequency)power,the DTL cavity could estab-lish the corresponding electromagnetic field to accelerate the ‘‘proton bunches’’from an input energy of 3MeV to an output energy of 7MeV successfully,without any addi-tional radial focusing elements.The gap-voltage distribu-t
ion which was obtained from the CST ÒMicrowave Studio software simulations of the axial electric field was com-pared with that from the beam dynamics,and the errors met the requirements within ±5%.In this paper,the RF design procedure and key results of the APF IH-DTL,which include the main RF characteristics of the cavity,frequency nsitivities of the tuners,and coupling factor of the RF power input coupler are prented.
Keywords Proton therapy ÁProton medical accelerator ÁInjector ÁDrift-tube linac (DTL)ÁInterdigital H-mode (IH)ÁAlternating-pha-focusing (APF)
1Introduction
美术绘画Over the past few decades,various types of cancer have become a primary cau for human mortalities.Particu-larly,in current day China,on an average,ven patients are diagnod with malignant tumors every minute [1].With the rapid development of particle accelerator tech-nologies and related medical treatment techniques,usage of energetic particle beams such as protons,heavy ions,and electrons supplied by the accelerators is considered to be one of the most efficient and practical ways to treat cancers.Particularly,proton accelerators are getting more and more attention nowadays due to the unique Bragg peak of proton beams,which helps to deliver a more effective treatment and a higher quality of life for the cancer patients.
The Advanced Proton Therapy Facility (APTRON)is a dedicated proton therapy facility located in the Ruijin hospital proton therapy center,Shanghai Jiaotong Univer-sity School of Medicine.It is constructed by Shanghai APACTRON Particle Equipment Co.Ltd,which is a joint-stock company held by Shanghai Institute of Applied Physics (SINAP)and two other stakeholders.A schematic layout of APTRON is prented in Fig.1.There are three treatment terminals—a fixed beam room,an eye treatment room,and a 180-degree rotating gantry room.The accel-erator of the facility consists of a 7MeV linear accelerator as the injector and a synchrotron accelerator with proton energy ranging from 70MeV to 250MeV.The
This work was supported by the National Key Rearch and Development Program of China (No.2016YFC0105408).&Yue-Hu Pu
puyuehu@sinap.ac
1
Shanghai Advanced Rearch Institute,Chine Academy of Sciences,Shanghai 201210,China
2
Shanghai Institute of Applied Physics,Chine Academy of Sciences,Shanghai 201800,China
3
University of Chine Academy of Sciences,Beijing 100049,China
4
Shanghai APACTRON Particle Equipment Co.,Ltd,Shanghai 201800,
China
synchrotron has a circumference of 24.6m and is com-pod of eight dipoles and twelve quadrupoles [2–4].It is equipped with the resonant slow beam extraction method.The 7MeV injector is an important component of the APTRON facility and consists of a Duo-Plasma-IS,a low energy beam transport (LEBT)ction,and two linacs arranged one after the other [5–8].The first linac is a radio frequency quadrupole (RFQ)linac and the cond is a conventional Alvarez-type drift-tube linac (DTL).As in a conventional DTL,a synchronous pha of nearly -30°is chon to offer longitudinal focusing force as well as large acceptance.However,this negative pha array inevitably defocus the beam transverly or radially.To overcome the defocusing force,a ries of quadrupoles are empl
oyed to focus the beam transverly.There are two types of quadrupoles:electromagnets and permanent magnets;however,both of them make the conventional DTL heavy,bulky,and costly.Conquently,the development of a compact injector plays a key role in constructing a cost-effective proton therapy facility.
With a view to providing a compact injector to the facility,we propod a scheme that replaces the conven-tional Alvarez-type DTL with a new DTL in the mass production pha.The new DTL adopts the interdigital H-mode (IH)structure working at 325MHz to operate compatibly with the current RFQ.The alternating-pha-focusing (APF)method is employed for focusing the beam in the IH-DTL.The design work for the beam dynamics of this DTL has been already demonstrated [2].Two more key aspects of the injector in the propod scheme,namely the
RF design and the study of the APF IH-DTL cavity,are prented in this paper.
In the following ctions,the principle and design pro-cedure are prented first.This is followed by the design,simulation,and analysis of the main parameters which include the cavity,gap-voltage distribution,tuners,and the coupler.Finally,some concluding remarks on the design are provided.
2Principle and procedure
2.1Overview of the APF IH-DTL
Interdigital H-mode (IH)as an idea was first propod in the 1950s by Blewett [9].One of the key features of the IH structure is that its capacitive load is very high compared with a conventional DTL [10–12].Conquently,for the same operating frequency,the IH structure has a smaller radius than a conventional DTL.This feature of the IH-DTL is an obvious advantage due to a significant reduction in the cavity size.
The alternating-pha-focusing (APF)DTL was also first propod in the 1950s by Good [13].In principle,an APF involves a quence of negative and positive accel-erating phas that appear alternately between groups of adjacent tubes.When the particles go through the dif-ferent gaps and tubes along a longitudinal axis,the nega-tive pha ctions focus on the particle bunches longitudinally and defocus them radially.Converly,the positive pha ctions focus on the particle
于谦明朝石灰吟
bunches
Fig.1Overview of the APTRON facility
135 Page 2of 13X.Li et al.
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radially and defocus them longitudinally.After passing through veral sinusoid-like periods in the quence of alternating phas,the particles will be simultaneously focud,both longitudinally and radially.Figure 2shows the principle behind the APF method.Both longitudinal and radial stability are achieved with just an RF acceler-ating field.Hence,there is no requirement of any additional focusing elements inside the drift tubes (DTs).This,in turn,leads to not only a reduction in the size of the DTs,but also savings in the cost of construction and mainte-nance of the whole cavity [14].
Unlike the Alvarez-type DTL,which excites the TM 010mode,the IH-DTL cavity excites the TE 111mode.Figure 3shows the basic principle of the IH-DTL and its structure.It consists of a ries of DTs,which are supported by stems connecting alternately to the opposite walls of the cavity.2.2Design procedure
The basic requirement of the RF design for the APF IH-DTL is getting the right voltage distribution between adjacent tubes along the longitudinal axis as required by the beam dynamics.As per the principle of the IH-DTL cavity,once a proper RF power is fed into the cavity,a longitudinal electric field is established directly in the structure shown in Fig.3;however,it cannot accelerate the particles efficiently.Furthermore,there is no focusing element in the cavity to focus the protons,and the motion of protons inside the APF IH-DTL is only determined by the electromagnetic (EM)field distribution.As
mentioned in [10],the EM field distribution would depend strongly on the whole structure of the cavity,including the radii of the cavity,the geometries of the ridge,the ridge tuner (RT)and the stems,the diameters of the DTs,RF power feed cou-pler,etc.Any geometrical changes of the components could lead to distribution variation of the EM field.Therefore,it is necessary to employ an efficient simulating procedure for designing the above components.In this study,we ud CST ÒMicrowave Studio (MWS)as the 3D EM solver to establish the model,carry out the simulations,and optimize the structure [14,15].
A flowchart depicting the design procedure that we developed for the APF IH-DTL cavity is shown in Fig.4.
3Design and simulation
3.1General parameters and considerations
Besides achieving the right gap-voltage distribution,another important objective of the RF design is to have a high shunt impedance,which leads to a low RF power dissipation on the cavity walls.Additionally,the cavity should operate stably under the high RF power condition,which requires the peak surface electric field E s inside the cavity to be lower than a value described as a multiplica-
tion of the Kilpatric limit E K [13,14,16].Following the aforementioned earlier studies,we have empirically taken E s to be B 1.6E K in our APF cavity design.Then,a 3D model of the APF cavity is constructed,and the electro-magnetic field and the cavity wall loss are analyzed by using the CST ÒMWS software.3.2Cavity design
According to Eq.(1),the frequency of the cavity is mainly determined by the inductance and the capacitance of the whole IH-DTL structure.The inductance mainly depends on the cross-ctional area enclod by the cavity walls,the stems,the DTs,and the two ridges.Corre-spondingly,the capacitance mainly comes from the gaps between the DTs,gap between the two ridges,the stems,and the RF cavity walls [10].f /1ffiffiffiffiffiffiffiffiffiffiL e C e
p ;
骆怎么组词
ð1Þ
where f is the operating frequency,L e and C e are the effective inductance and capacitance,respectively.Becau the velocity of protons at the exit of the cavity is 1.52times that at the entrance of the cavity,it gives ri to an increa in the length of the DTs and the gaps among the DTs.This results in a maldistribution of the capacitance from the entrance to the exit.As a conquen
ce,the E-field would peak near the entrance of the cavity and drop to zero near the exit of the cavity.Furthermore,the APF method introduces another kind of longitudinal distribution mod-ulation of the DTs [17],which makes the situation further complicated.
In order to get the maximum accelerating efficiency,a balanced uniform longitudinal E-field distribution along the beam axis should be achieved.To achieve the same,we modified the inner radii of the cavity as a function of the distance from the cavity entrance.This modification of the cavity inner radii was performed incrementally,in a few steps,rather than in a continuous manner.Along the whole cavity length of 1514.6mm,we divided the cavity into 15ctions longitudinally;each of the first fourteen ctions is 100mm in length;and the last ction at the cavity
exit
Fig.2Illustration of the principle of the APF.E is the accelerating electric field
RF design and study of a 325MHz 7MeV APF IH-DTL for an injector of a proton medical …Page 3of 13135
neighborhood is 114.6mm long.After veral iterative modifications and simulations of the radii of the 15ctions,we obtained the axial component of the electric field of the standard TE 111mode shape along the beam axis.The simulation results of the on-axis electric field obtained using CST ÒMWS are shown in Fig.5.The dotted red curve is the initial field strength distribution before the optimization,and the dashed blue curve corre-sponds to the results after adjustment of radii of the g-ments.The solid green curve is the on-axis electric field distribution after the optimization of the ridge shapes on both ends of the cavity.Further,flatness in the on-axis
field
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Fig.3Illustration of the principle of IH-DTL
structure
Fig.4Flowchart of the design procedure for the APF IH-DTL cavity
135 Page 4of 13X.Li et al.
纪录片拍摄distribution was obtained by adjusting the geometry of the ridge tuners (RT)or the ridge ends on the cavity,following the design procedure described in Fig.4,Then,an ideal TE 11(0)-like field distribution is achieved.
The next task is to tune the shape of the cavity to get the designed operating frequency of 325MHz required from the beam dynamics [2].Starting from the radii arrived at,in the previous step,we changed the radii of all the 15g-ments simultaneously by a given amount,until we got the frequency of 325MHz.This inevitably would influence the flatness of the axial component of the electric field along the beam axis.Hence,steps 3–5were iteratively carried out to retain the flatness.The relation between the cavity res-onant frequency and the cavity radii was analyzed during the optimization of the APF IH-DTL.Due to the irregular radii distribution longitudinally,we cho one of the fourteen radii to study the nsitivity of frequency,and the result is plotted in Fig.6.According to Fig.6,the corre-sponding nsitivity between frequency and radius is about -2.9M
Hz/mm.If the required frequency accuracy is 100kHz,the corresponding machining precision should be about 34l m.
Compared with a nsitivity of -0.8MHz/mm [13]between the resonant frequency and the cavity radius variation of a conventional Alvarez-type DTL,it appears that the resonant frequency of the APF IH-DTL cavity is very high,the reason being that the IH-DTL has a rela-tively small cavity radius when compared with an Alvarez-type DTL.Nevertheless,the higher nsitivity implies higher accuracy and better stability requirement on the cooling system of the cavity.
The peak surface electric field E s inside the cavity is another important constraint during the cavity design.In normal-conducting cavities like the IH-DTL,too high an E s
could lead to an electrical breakdown or sparking while operating with high RF power.According to the Kilpatrick criterion [13,18,19],the relationship between the E s and the cavity resonant frequency is expresd by the following two Eqs.(2)and (3):
f ðMHz Þ¼1:64E 2K e À8:5=E K
;
ð2ÞE s ¼bE K ;
ð3Þ
where f is the operating frequency (MHz),E K is known as the Kilpatrick limit (MV/m),and b is known as the bravery factor.According to some previous studies [5,14,17]and bad on the consideration of a conrvative design of the first APF IH-DTL,the typical value of the bravery factor b of our cavity was considered to be no more than 1.6.In our design,the value of E K was 17.86MV/m.Substituting the values of b and E K in Eq.(3),we obtained the E s in the 325MHz APF IH-DTL as 28.58MV/m.
According to the electric field distribution inside the IH-DTL cavity shown in Fig.7,E s primarily occurs on the outer edge of the DTs near the exit of the cavity (a model of which is shown in Fig.8a).Therefore,we should reduce the E s at this location.This can be remedied by blending the outer edge of the DTs and optimizing the blending radii to get an appropriate value of the E s .The relation between the bravery factor b and the blending radii is shown in Fig.9.Although a blending radius of 2mm could meet the requirement for b to be less than 1.6,to minimize the dissipation power on the surface of the walls,a blending radius of about 5mm was considered a better choice.This corresponds to a b of 1.53.However,considering that some DTs have a very short length of 16.5mm,
优文and the upper radius of the stems was only about 4mm,we determined that a blending radius of 4mm would ensure
consistency
爱六网Fig.5(Color online)Axial component of the electric field along the center of the beam axis.The red (Dotted),blue (Dashed),and green (solid)curves show electric field distributions before radii and RT modification,after radii modification,and after RT modification,respectively.The orange (Dash-dot)curve shows the dipole field component
RF design and study of a 325MHz 7MeV APF IH-DTL for an injector of a proton medical …Page 5of 13135
