Study and improvements of a radially coupled coaxial Fast Faraday cup design towards lower intensity beams

2025-05-02 0 0 2.72MB 38 页 10玖币

侵权投诉

Study and improvements of a radially coupled coaxial Fast

Faraday cup design towards lower intensity beams

K. Mal1, S. Kumar1, G.Rodrigues1, R. Singh2

1Inter University Accelerator Centre, New Delhi 110067, India

2GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstrae 1,,Darmstadt, Germany

Abstract

A radially-coupled coaxial fast Faraday cup design was presented in [1] for high-intensity non-

relativistic proton beams. In this work, we discuss a modification of that design in the context of

a relatively lower intensity ion beam for longitudinal charge profile measurements. Particle-in-cell

and time domain electromagnetic simulations of the new design with a focus on avoiding field

dilution while generating enough signal for relatively lower intensity ion beams at the upcoming

High Current Injector Programme at IUAC, New Delhi [2,3] is discussed. Profile distortions from

secondary electron emission are estimated and strategies to suppress them are discussed.

Introduction

The operation of Faraday cups for intensity measurements is well established where the

suppression of the emitted secondary electrons is carried out using a superimposed electric field,

such that the emitted secondary electrons are retarded and recaptured [4] . For charge profile

measurements of longitudinal short bunches ns, it is critical to avoid impedance

discontinuities in the Faraday cup structure until frequencies upto few GHz. Modified Faraday cup

designs tailored to measure longitudinal charge distributions [4,7] are called Fast Faraday Cups

(FFC). Early FFC designs were tapered extension of coaxial cables allowing for full beam

deposition on the central conductor while maintaining Ω characteristic impedance [4,5].

Following that, alternative FFC designs based on radial coupling in the central conductor [1] of a

co-axial cable and microstrip based designs [6,7] have been used in various accelerator

laboratories. So far, most studies available in the literature on FFC are focused mainly on

electromagnetic characteristics of the FFC, i.e. targeting impedance mismatch aspects. However,

additional challenges for short bunched beam measurements in non-relativistic regimes are a) the

field elongation and b) distortion by the emission of secondary electrons. In this study, we will

present an adaptation of a Radially-Coupled Coaxial Fast Faraday Cup (RCFFC) [1] for beam

conditions available at the High Current Injector (HCI) Programme, which is presently under

commissioning stages at the Inter-University Accelerator Center(IUAC),New Delhi [2,3]. The

primary drawback of the original design [1] is the low ’signal-to-noise’ ratio due to the narrow

beam limiting aperture of mm along with the need for precise beam alignment with long

averaging times for the measurement. For larger ’signal-to-noise’ ratio, we increased the aperture

size and used a transition method [8] without curved structures for the transition from the N-type

connector to the cup region to achieve uniform impedance and low reflection. Second challenge is

the delayed signal induction due to emission of secondary electrons. The major modifications in

our adapted design to counter these aforementioned challenges are discussed in this contribution.

We also herewith discuss signal induction process, field dilution, secondary electron emission

aspects, fabrication challenges as well as thermal considerations on the simulated design.

EM simulations of a modified RCFFC

The characteristic impedance  of a coaxial transmission line is defined as follows [9] :

 





 (1)

where and are permittivity and permeability of the material, respectively,  and  are the radii of

the inner and outer conductor, respectively. It is important to note that a coaxial line is the upper-

frequency limit for pure Transverse Electro-Magnetic (TEM) mode operation and is referred to the

frequency at which the first non-TEM mode starts to propagate. The non-TEM mode with the

lowest cut-off frequency () is  given by the following relation [9]



 (2)

where  and  are the relative permeability and permittivity of the medium, respectively and c

is the speed of light. Therefore the width of the co-axial line cannot be arbitrarily increased. To

counter this aspect, a conical taper between a thin and a thick co-axial line is chosen. The

characteristic impedance  of a conical line is defined as follows [10]:

 





 (3)

where  and  are the angles formed by the inner and outer conductor of the conical line with

the symmetry axis (z-axis) (See Figure 2.). In order to minimise the reflection at the transition of

coaxial and conical lines, it is necessary to design in such a way that the characteristic impedances

of both the lines have the same impedance of Ω. However, equation 1 and 2 can never be equal

except when  and  are identical and can asymptotically approach each other for long lines

such that  and approach zero [10]. The electromagnetic simulation code, CST Microwave

Studio [11] was used to design the RCFFC assembly with the interfacing option to the

transmission line at the desired characteristic impedance of Ω. Figure 1. shows the cross-

sectional view of the CST simulation model of the RCFFC.

Figure 1. CST simulation model (cross-sectional view) of the RCFFC with the geometrical

parameters.

The RCFFC consists of a metallic cube with two N-type connectors positioned concentrically on

its side as shown in figure 1. Note that the collimating aperture of diameter mm is chosen based

on the relatively lower beam intensities available at the High Current Injector. A rod of diameter

mm was inserted between the central electrodes of the connectors and tapered down to mm,

which is the diameter of the standard N-type connector pin. The rod acts as a collector as well as

the transmission line’s inner electrode. The outer conductor of the conical line is bent at o.

The diameter of the collector hole was chosen to be mm to avoid hitting the beam directly.

The secondary and reflected particles stay mostly inside the collector hole because the depth of the

hole is chosen at mm, which is twice as large as its diameter. With these chosen dimensions, the

first-order estimate of the RCFFC geometry parameters has been calculated using the analytical

formula (equation 1) and (equation 3), and are shown in figure 2(a).

geometry 1 geometry 2

Figure 2. Left side of the RCFFC geometry and its design parameters (unit: mm) for characteristic

impedance of 50W, (a) calculated using analytical formulas and (b) optimized using the CST-

MWS.

For the chosen dimensions and the medium properties, as shown in figure 2, the cut-off frequencies

of the N-type connector and RCFFC head are GHz and GHz, respectively. In order to

obtain the return loss () and insertion loss (), a two-port analysis [12] was carried out by

assigning two waveguide ports at the left and right N-type connectors, respectively. In addition, a

Time Domain Reflection (TDR) analysis [13] was also performed to compute the distributed

characteristic impedances along the coaxial and conical lines of the RCFFC. Using the parameters

as described in figure 2(a). The EM wave simulation was performed by considering only TEM

mode at the waveguide ports with a frequency range up to GHz to evaluate the performance of

the RCFFC. Figure 3. shows the simulated return loss at the input port, insertion loss between two

ports and the characteristic impedance  along the conical and coaxial lines for the non

optimized geometry as shown in figure 2 (a). It can be observed that the return loss of the RCFFC

depicts two resonance peaks below the cut-off frequency GHz which are due to impedance

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

10 玖币 0人已下载

立即下载

摘要：

StudyandimprovementsofaradiallycoupledcoaxialFastFaradaycupdesigntowardslowerintensitybeamsK.Mal1,S.Kumar1,G.Rodrigues1,R.Singh21InterUniversityAcceleratorCentre,NewDelhi110067,India2GSIHelmholtzzentrumfürSchwerionenforschungGmbH,Planckstrae1,,Darmstadt,GermanyAbstractAradially-coupledcoaxialfastFa...

展开>> 收起<<

Study and improvements of a radially coupled coaxial Fast Faraday cup design towards lower intensity beams.pdf

共38页,预览5页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

Study and improvements of a radially coupled coaxial Fast Faraday cup design towards lower intensity beams

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: