Development of button-type pickup for SSRF ring??
ZHAO Guo-Bi(赵国璧),1YUAN Ren-Xian(袁任贤),1CHEN Zhi-Chu(陈之初),1 ZHOU Wei-Min(周伟民),1and LENG Yong-Bin(冷用斌)1,?【期刊名称】核技术（英文版）【年(卷),期】2014(025)006【总页数】6
【关键词】Keywords:SSRF,BPM,Position sensitivity,Cut-off frequency,Distributed capacitance
In building Shanghai Synchrotron Radiation Facility(SSRF),in order to reduce or eliminate the unnecessary sources of beam motion,the precise and stable beam position measurement system in the feedback system was required.In this study,we focused on theoretical analysis of the electrode of beam position monitor(BPM). Simulations,including analytic derivations of the propagation impedance and the distribution inductance,were performed.The BPM was designed based on the results and the acceptance measurements of the impedance and the inductance by using the time domain reflection(TDR)with a network analyzer.It has been proved in years of operations that the BPM system meets the requirement of the resolution of sub-micron.
I.INTRODUCTION
Shanghai Synchrotron Radiation Facility(SSRF)has a storage ring of a 432m in
circumference,operating at 3.5GeV[1].During its commissioning and routine operation,the button-type BPMs play a key role.They ensure its stable high performance operation,and provide position information to the orbit feedback system,the transverse feedback system,and the safety interlock system,etc.,in required precisions and rates.Especially,the high precision closed orbit position feedback system tracks the position in a resolution of 1μm@2Hz bandwidth,the transverse feedback system needs the bunch-by-bunch position signal in resolution of 10μm@250MHz bandwidth,and the measurements of turn-by-turn signal need the resolution and data rate in between the above two.For the BPM detector itself,the main technical difficulty of implementation is to ensure the bunchby-bunch signal with high signal-to-noise ratio(SNR)while avoiding the beam impedance mismatch and the higher order mode(HOM)excitation.In this paper,according to the beam parameters of SSRF,we discuss detailed technical issues involved in the BPM design.
II.DESIGN OBJECT
In different cases of SSRF’s commissioning and routine operation,the button-type BPM provides position information in different precisions and rates.Table 1 shows the major beam parameters of SSRF accelerator.
The requirements for the button-type BPM differ slightly in different stages of accelerator operation:in the preliminary commissioning stage,higher beam charge sensitivity(larger button size)and higher position sensitivity are required dueto small beam charge and large orbit distortion;while in the user operation stage,lower beam charge sensitivity(smaller button size)and lower position sensitivity are required due to high average beam current and high position resolution needs.To meet the requirements and to work with beam parameters in Table 1 before providing appropriate signals to the electronics system,the button-type BPM should achieve the specifications in Table 2.
To meet the goals,the BPMs shall be of appropriate sensitivity,size,and inner
structure,i.e.they shall be large enough to provide strong signals,but as small as possible to avoid HOM,with appropriate button separation to achieve desired sensitivity,and with an inner structure to transmit the signal. BPM simulations were performed with the MAFIA code for an optimized structure with a balance between the transfer and coupling impedances.
III.PHYSICAL DESIGNS
The BEPCII BPM simulation[5]is an object lesson in modeling the SSRF BPMs with the MAFIA code(Fig.1).
A BPM electrode is mounted on inner surface of the beam pipe,The beam pipe geometric structure decides the BPM geometric structure except the BPM electrode size and location.The cross section of the SSRF storage ring beam pipe is octagonal,and the four button electrodes are symmetrically arranged on the upper and lower plane.
The BPM position sensitivities should be as high as possible,and the sensitivities in the horizontal and vertical plane should be as close as possible.The BPM sensitivity is determined by the signals picked up by different electrodes when the beam is off center.We adopt the sensitivity function[6] as
where Siand diare the offset in the x or y direction,the subscript i denotes either x or y direction;A and B are the signals picked up by the right top and right bottom monitors, respectively.
For BPMs at SSRF,the optimized electrode diameter is 10mm.The infinite element simulation results are given in the Table 3.G is the distance between the axes of two electrodes on the same horizontal plane;Kxand Kyare position sensitivities in horizontal and vertical planes,respectively;ρ is the normalized line density of induced charge at the electrode center,which is decided by the vacuum chamber structure and the electrode size and position;g= ρ2πB is the shape factor(B is the distance between the electrode center and geometric center of the vacuum chamber),which means the deviation from
distribution of induced charge at the electrode center to the circular chamber.As can be seen,by adjusting the relative position of the electrodes,the BPM sensitivities agree in the horizontal and vertical directions.
The beam position measurement can be influenced by signal intensity of electrode output.The signal intensity must be big enough to meet the input power requirements of the signal processing electronics,and should not be too strong to break the vacuum sealing when BPMs work under the large current. Finally,we chose G=16mm.
Button pickup needs to work properly in single bunch mode with low current of 0.02mA and in multi-bunches mode with high current of 300mA.The requirement of signal
processing electronics is that the input signal power is larger than-85dBm and smaller than 5dBm in the bandwidth of 9MHz.Combined with the insertion loss(around 5dB)of signal cables,this requirement is larger than-80dBm and smaller than 0dBm for pickup output.Fig.2 shows the output voltage and frequency spectrum of electrode with 10mm diameter and single bunch charge 1nC.
The output power(P)in working frequency for the signal processing electronics iswhere,U is the output voltage of the corresponding spectrum and varies with ω0(the center frequency of signal processing electronics),Δω is the work bandwidth,R is 50? and Δt is the revolution time.With φ10 mm button BPM,spectral intensity of the signal processing electronics is 5.5×10-10V/Hz in center frequency in single bunch operation of SSRF.The output signal power(Fig.3)of electrode at different beam currents fully satisfies SSRF operation in different conditions.
HOM brings problems[7,8].The TE10mode of beam pipe may cause error in the beam position measurement.The pipe length and height should be considered(height=10mm, length＜175mm).The TE10mode within the cut-off frequency(＜ 499.654MHz)will reduce the BPM resolution. The higher slot will cause more transverse impedance.From
calculations of the TE10mode,the cut-off frequencies of the higher order modes within the
pipe structure are 2.449, 4.412,4.732,4.800,5.464,6.322,6.729,7.220,8.201 and
8.259GHz.Theyaremuchlargerthanthecenterfrequencyof signal processing(499.654MHz),so the TE mode,which will be coupled into the signal processing circuit-through BPM electrode,cannot be excited,i.e.,the TE mode will not affect the position measurement system.
Simulations were done on influence of the longitudinal wakefield,too.The results are shown in Fig.4.By integrating the wakefield energy loss,the energy loss factor of the BPM structure is k=0.0166V/pC.In general,the energy loss factor is particularly sensitive to the electrode size and the gap between the electrode and vacuum tube wall.To prevent large beam loss power,and considering the BPM machining and beam loss power,a 0.3-mm gap is chosen.
IV.TECHNICAL DESIGNS
For technological design and tolerance error,we mainly consider impedance matching of the ceramic sealing section and electrode capacitance.The induced BPM signals are transferred a vacuum seal ceramic section.Its structure must match the characteristic impedance of the signal transmission path,so as not to cause complex higher order mode and coupling problems[7,8].Two ways can be used to realize the impedance matching of the segment structure:to make gradual transition of both the inside and outside of the conductor structure,and to shield dielectric transition.Referring experiences of other accelerator labs,the ceramic sealing structure of shielding medium transition(Fig.5)is used.The ceramic welding in vertical direction is divided into two sections,and the ceramic section welded with outer conductor has a certain degree of freedom in the inner conductor direction,and vice versa,which avoids the welding or normal working thermal expansion damage to the ceramic structure.
In order to avoid the reflection of incident electromagnetic field in different transmission structure,the characteristic impedance matching should be considered.For the SSRF ceramic structures,when its characteristic length is about the same as transmitted electromagnetic wavelength,even smaller,its characteristic impedance can be analyzed by using TEM mode field structure.For upper part of the ceramic structure,its characteristic impedance,capacitance and other parameters,can be obtained by the equivalent dielectric constant.For the TEM model,assuming the unit length of the inner conductor with free charge q,then by Maxwell equation,the surrounding electric field is
where,r is the radial distance from the center of the inner conductor,and ε is the dielectric
constant of this point.After the electric field integral,the voltage difference between inside and outside conductor is
whereaisthethecenterconductorradius,bistheinnerradius of outer conductor,c is the Ceramic medium radius.
Therefore,we consider the mixed medium as a single medium,and its equivalent dielectric constant εeqis
Therefore,the characteristic impedance and distributed capacitance of the structure is given by
Figure 6 shows the results of TDR(Time Domain Reflection)test for the upper part ceramic structure before and after optimizing outer diameter of the inner conductor. Equivalent dielectric constant is used in the optimization. The optimized BPM characteristic impedance achieves a smoother transition.
The impedance of BPM wakefield is inversely proportional to its ground
capacitance,mainly decided by the electrode ground capacitance.Also,the distributed capacitance of the ceramic dielectric section has a greater contribution to the ground capacitance.For the coaxial structure,the ground capacitance can be calculated by:
where,ε is dielectric constant of the medium,H is length of the structure,and gap is the gap between internal and external conductors.For the BPM,the distributed capacitance of electrode is 954pF/m,equivalent dielectric constant of the upper part ceramic structure is 5.3,the corresponding characteristic impedance is 50.2?,the coaxial capacitance is 150pF/m,the lower part ceramic structure equivalent dielectric constant is 2.55,the corresponding characteristic impedance is 66?,the coaxial capacitance is 80pF/m,and the total capacitance is about 2.4pF.
Distributed capacitance of the electrode structure can be given by the rise time by TDR test,which is 244ps when BPM electrode terminal is open(10–90%),for the input impedance of 50?,or electrode capacitance of 2.2pF,it is consistent with theoretical expectations.Manufactureerrorisinevitable.Toguaranteethebuttoncapacitance,tolerance error of the gap between the round disc and the beam pipe is(300±25)μm,the error in button positioning is 0.1mm,the reading errors in manufacturer are 0.025mm and 0.1mm on X and Y direction,respectively. The error can be adjusted by mapping test or BBA.
V.FACTORY ACCEPTANCE TEST
Development of button-type pickup for SSRF ring
