RF deflecting cavity design for bunch length measurement of photoinjector at Tsinghua University

RF deflecting cavity can be used for bunch length measurement and is designed to diagnose the beam produced by the photocathode electron gun which was built at Tsinghua University for the Thomson scattering experiment. Detailed discussion and calculation for measuring the 3.5 MeV bunch and another with further acceleration to 50 MeV, which is under development, are presented. A standing-wave deflecting cavity working at 2856 MHz is designed and the power feeding system has been planned.

Bunch length measurement at Tsinghua Thomson scattering X-ray source

The length of electron beam from a photocathode RF gun is determined by a spectrometer, according to the relative energy spread induced by the bunch length during the acceleration in a linac. For a photocathode RF gun, different laser injected phase and beam charge are studied. The compression is changed for the different laser phases, as from 10 ° to 30 ° , and the bunch length is lengthened due to the strong longitudinal space charge force, caused by the increased charge.

Streak camera calibration for bunch length measurement at BEPC II

Purpose Streak camera is an important device for bunch length measurement.The errors introduced by streak camera should be considered in measurement.In order to accurately measure bunch length,some streak camera calibrations are studied at BEPC II.Methods In this context,the point spread function,caused by the optical system,is measured at focus mode with both deflec-tions disabled.In the same conditions,the influence of camera gain on measurement is studied.For the dual sweep mode of streak camera,a calibration method using electrical delays is applied to calibrate the beam size result.Results Firstly,a point spread function calibration 1.78 ps of streak camera optical system is obtained with focus mode.Secondly,studying the effect of camera gain on measurement,an optimal camera gain range from 506 to 662 is selected,in which the bunch length measurement error is within 0.5 ps.Thirdly,a dual sweep calibration factor 1.02 is obtained.Conclusions For BEPCII bunch length,there is a little difference before and after the streak camera calibrations.However,for the accurate measurement of ultra-short bunch length,the calibrations are of great significance.

RF deffecting cavity for bunch length measurement in Tsinghua Thomson scattering X-ray source

An RF deffecting cavity used for bunch length measurement has been designed and fabricated at Tsinghua University for the Thomson Scattering X-Ray Source. The cavity is a 2856 MHz, π-mode, 3-cell standing-wave cavity, to diagnose the 3.5 MeV beam produced by photocathode electron gun. With a larger power source, the same cavity will again be used to measure the accelerated beam with energy of 50 MeV before colliding with the laser pulse. The RF design using MAFIA for both the cavity shape and the power coupler is reviewed, followed by presenting the fabrication procedure and bench measurement results of two cavities.

Correlation between Endocervical Length in the First Trimester and Spontaneous Preterm Delivery

Introduction: Prematurity is a public health problem in Brazil, with 12% of deliveries occurring before 37 weeks of gestation. The measurement of the cervix in the second trimester is already established as a method of screening for prematurity and some studies point out advantages to start this screening in the first trimester. Objective: To define the correlation between the length of the endocervix by transvaginal ultrasound in the first trimester (11 to 14 weeks) with spontaneous early deliveries. Method: A prospective and observational study realized in a suplementar and private ultrasound clinic and hospital of Porto Alegre, Brazil between 2019-2020. Ultrasound screening of cervix was performed in singleton pregnancies in first and second trimester of pregnancy and correlated with age of delivery. Results: 142 pregnant women were studied, 80% were in the first pregnancy. The average age was 33.8 years. The rate of prematurity was 18% before 37 weeks and 4% before 34 weeks. The average of cervix measured in the first and second trimesters in deliveries before 34 weeks was 32.7 mm and 29.3 mm, respectively. In term deliveries the median cervical length was 38.8 mm and 37.8 mm, respectively. When analyzing the measurements of the cervix in the second trimester, the cervix was smaller (p = 0.008) among deliveries below 34 weeks (29 mm) than deliveries after 37 weeks. No statistically related differences were found between preterm birth and first trimester cervix measurements. Conclusions: In this study, we did not observe a statistically significant relationship between first trimester cervix measurement and prematurity. More studies are needed to evaluate this finding. However, the measurement of the cervix in the second trimester is different from that found in the literature. This suggests a possible new cut-off point that increases the sensitivity of transvaginal ultrasound as a method of preventing prematurity.

Is the Two-Color Method Superior to Empirical Equations in Refractive Index Compensation?

The Edlén empirical equations and the two-color method are the commonly used approaches to converting a length measured in air to the corresponding length in vacuum to eliminate the influence of the refractive index of air. However, it is not well known whether the two-color method is superior to empirical equations in refractive index compensation. We investigated the uncertainties of these approaches via numerical calculations of their sensitivity coefficients of environmental parameters. On the basis of a comparison of their uncertainties, we found that in a 0% humidity environment, the two-color method had potential to provide greater measurement accuracy than the empirical equations.

Measurement of ocular axial length using full-range spectral-domain low-coherence interferometry

We demonstrate a system for measuring the ocular axial length （AL） with high sensitivity and high speed using spectral-domain low-coherence interferometry （SD-LCI）. To address the limit in measuring such a large range by using SD-LCI, we propose a full-range method to recognize the positive and negative depths. The reference arm length is changed synchronously with the shift of the focal point of the probing beam. The system provides a composite depth range that is sufficient to cover the whole eye. We demonstrate the performance of the presented system by measuring the ALs of five volunteers. This system can provide the A-scan ocular biometric assessment of the corneal thickness and AL in 0.1 s.

Electro-optical sampling non-synchronous delay scanning measurement of electron beam bunch length at BFEL

The Electro-optical sampling delay scanning technique can be used for electron beam bunch length measurement. A novel non-synchronous delay scanning technique based on the electro-optical sampling measurements is presented. Based on Beijing Free Electron Laser （BFEL）, the electron beam bunch length was measured with the electro-optical sampling technique for the first time in China. The result shows that the electron beam bunch length at BFEL is about 5.6±1.2 ps.

Generation and measurement of sub-picosecond electron bunch in photocathode rf gun

We consider a scheme to generate a sub-picosecond electron bunch in the photocathode rf gun by im-proving the acceleration gradient in the gun, suitably tuning the bunch charge, the laser spot size and the acceleration phase, and reducing the growth of transverse emittance by laser shaping. A nondestructive technique is also reported to measure the electron bunch length, by measuring the high-frequency spectrum of wakefield radiation which is caused by the passage of a relativistic electron bunch through a channel surrounded by a dielectric.

Cervical Length at 28-32 Weeks of Gestation Predicts Preterm Birth

Objective:To evaluate the ability of cervical length(CL)at 28-32 weeks of gestation to predict spontaneous preterm delivery and preterm premature rupture of membranes(PPROM).Methods:It was a retrospective cohort study that vaginal ultrasonography at approximately 28-32 weeks of gestation was performed in 14,953 women between 17-49 years old with singleton pregnancies who delivered after 28 weeks of gestation at the Peking University First Hospital from June 2008 to December 2012.The pregnancy outcomes were followed and the relationship between the CL and preterm delivery or PPROM was assessed.The relative risk was calculated to assess group differences in the likelihood of an event occurring.Results:The overall prevalence of preterm delivery was 5.7%(858/14,953);the incidence for therapeutic preterm delivery was 2.1%(318/14,953),for spontaneous preterm delivery was 0.9%(133/14,953),and for PPROM was 2.7%(407/14,953).Excluding the 318 women who had therapeutic preterm delivery,the relative risk of preterm delivery for women with a CL between 25 mm and<30 mm,between 15 mm and<25 mm,and<15 mm was 3.7,9.3,and 30.2,respectively.The sensitivity,specificity,and positive and negative predictive values of CL<25 mm at 28-32 weeks of gestation as the cut-off value for predicting preterm delivery were 19.8%,97.5%,23.4%,and 96.9%,respectively.For preterm delivery before 35 weeks of gestation,the sensitivity,specificity,and positive and negative predictive values of a CL<25 mm at 28-32 weeks of gestation as the cut-off value were 30.0%,97.2%,11.8%,and 99.1%,respectively.In addition,women with PPROM(n=407)had significantly shorter CL(31.9±7.4)mm at 28-32 weeks of gestation compared that of women without PPROM(34.0±8.3)mm.The incidence of PPROM in women with a CL£25 mm at 28-32 weeks of gestation(19.4%,79/407)was significantly lower than that for women with a CL>25 mm(80.6%,328/407,P<0.05).Conclusion:CL at 28-32 weeks of gestation can predict spontaneous preterm delivery,and is valuable for predicting PPROM.

Pinpointing the sources and measuring the lengths of the principal rivers of the world

Cultures throughout the world are associating with the rivers.People depend upon the rivers and their tributaries for food,water,transport,and many other aspects of their daily lives.Unfortunately,human beings have not calculated the accurate lengths for the great rivers even today.The lengths of the rivers are very different in popular textbooks,magazines,atlases and encyclopedias,etc.To accurately determine the lengths of the principal rivers of the world,the combination of satellite image analysis and field investigations to the source regions is proposed in this paper.The lengths of the Nile,Amazon,Yangtze,Mississippi,Yellow,Ob,Yenisey,Amur,Congo and Mekong,with lengths over or close to 5000 km,were calculated using the proposed method.The results may represent the most reliable and accurate lengths of the principal rivers of the world that are currently achievable.

First experimental research of the bunch compressor at CAEP

Magnetic bunch compressor is one of the key technologies on the path to next generation accelerator driven facilities. In this paper we report the design principles and the first experimental research of the bunch compressor developed at Chinese Academy of Engineering Physics （CAEP）. The length of the bunch after compressor is found to be about 0.7 ps （rms） and the peak current exceeds 500 A when operated in the optimized condition. The sensitivity of the bunch length on the phase of the acceleration field and magnetic field of the bunch compressor was also measured and analyzed.