Physics Research Facility Research Articles

Electron beam dynamics in the 3D magnetic field of alpha magnet at the PBP-CMU Electron Linac Laboratory

The electron injector system for a future mid-infrared free-electron laser (MIR FEL) and coherent terahertz (THz) radiation source at the Plasma and Beam Physics (PBP) Research Facility of Chiang Mai University (CMU) in Thailand will consists mainly of a thermionic cathode radio-frequency (RF) electron gun, an alpha magnet, a traveling-wave linear accelerator (linac) structure and two magnetic bunch compressor systems. The detailed study of all components in the injector system is very important for generation of high quality electron beam and radiation. This paper focuses on the alpha magnet, which is used as a magnetic bunch compressor and an energy filter before the beam being accelerated in a post-acceleration linac structure. We report its characteristics and the electron beam dynamics in its three-dimensional (3D) magnetic field distribution that was simulated with the program CST EM Studio 2016. The simulated alpha magnet’s fields are comparable with the measurement data. The beam dynamic simulations in 3D magnetic field were performed by using programs ASTRA and ELEGANT. The simulation results allow us to investigate the influence of the fringe field, the vertical focusing and the space charge effect in the alpha magnet. This is significantly important for electron beam manipulation in the injector system. The study outputs reveal that the proper injection angle in the practical beam operation to compensate the fringe field effect should be 42∘ that is 1.29∘ larger than the ideal injection angle. We also found that the ratio of the vertical focal length to the total path length in the alpha magnet is 0.544, which is independent on the magnetic field gradient. Moreover, the space charge force in the electron bunch leads to the rotation of transverse phase spaces while the beam traverses in the alpha magnet’s field. This effect has considerably small influence on the longitudinal phase space, especially for the beam with a bunch charge not exceeding 0.2 nC.

Development of a compact electromagnetic undulator for linac-based coherent THz radiation source in Thailand

A linac-based accelerator system at the Plasma and Beam Physics Research Facility of Chiang Mai University in Thailand can produce relativistic femtosecond electron bunches for generating coherent THz radiation. Originally, this accelerator system was built to produce high peak power THz/FIR radiation via transition radiation technique. To extend the system’s capability in production of higher average power and tunable coherent radiation, a project to add an undulator magnet as a source of coherent THz radiation was started. Construction of a compact 40-period electromagnetic undulator with short period length is in progress. Low electricity consumption, inexpensive materials and uncomplicated fabrication process are aimed. An undulator prototype was designed and built to investigate the magnetic field properties and to estimate the possibility on manufacturing of a future undulator magnet. A computer program RADIA was used to model a three-dimensional undulator prototype based on the fabricated magnet material and dimensions. This prototype has 4.5 periods consisting of 8 main-poles and 2 end-poles for magnetic field correction. The simulated peak magnetic field was 0.1858 T or equivalents to an undulator parameter of 0.98. This led to a calculated undulator radiation wavelength at the first harmonic of 109 μm for electron beam with energy of 10 MeV. This radiation is in the forward direction. After manufacturing, the magnetic field of the undulator prototype was measured and analyzed. The measurement result at an optimal excitation current of 5 A showed that an average period length of 54.8 mm and an average peak magnetic field of 0.1855 T were achieved. The measured undulator parameter was 0.97, which resulted in the undulator radiation wavelength of about 107 μm. A future electromagnetic undulator with 40 periods was designed and will be built based on the study results of this undulator prototype. The estimated angular flux density of the radiation emitted from 40 period undulator magnet is dominated at the first harmonic radiation wavelength of 109 μm, which is equivalent to the radiation frequency of 2.8 THz.

Coherent transition radiation from femtosecond electron bunches at the accelerator-based THz light source in Thailand

A THz radiation source based on electron linear accelerator (linac) has been built and commissioned at the Plasma and Beam Physics (PBP) Research Facility, Chiang Mai University (CMU) in Thailand since 2005. The accelerator system consists of a thermionic cathode RF electron gun, a magnetic bunch compressor in a form of an alpha-magnet, an S-band travelling-wave linac structure, beam steering and focusing magnets, beam and radiation diagnostic instruments, control system and other support components. Electron bunches with an average kinetic energy of about 8 MeV, a bunch charge of 100 pC, and a bunch length of 300 fs can be produced from the PBP-CMU electron linac system. Coherent THz radiation generated via transition radiation technique is used as a source of the THz spectroscopy and THz imaging applications. The measured power spectrum indicates that the radiation obtained from short electron bunches covers the frequency range of 0.15–1.80 THz or the wavenumber range of 5–60 cm−1. Experimental results reveal that a peak radiation power as high as 0.2 MW per micropulse and an average radiation power of 2.5 mW can be obtained. Generation and characterization of the coherent THz transition radiation as well as some examples of experiments to achieve electron bunch length and radiation power spectrum are reported and discussed in this paper.

Development of Condensing Mesh Method for Corner Domain at Numerical Simulation Magnetic System

A magnetostatic problem arises in searching for the distribution of the magnetic field generated by magnet systems of many physics research facilities, e.g., accelerators. The domain in which the boundaryvalue problem is solved often has a piecewise smooth boundary. In this case, numerical calculations of the problem require the consideration of the solution behavior in the corner domain. In this work we obtained the upper estimation of the magnetic field growth and propose a method of condensing the differential grid near the corner domain of vacuum in case of 3-dimensional space based on this estimation. An example of calculating a real model problem for SDP NICA in the domain containing a corner point is given.

Estimation of Magnetic Field Growth and Construction of Adaptive Mesh in Corner Domain for the Magnetostatic Problem in Three-Dimensional Space

A magnetostatics problem arises when searching for the distribution of the magnetic field generated by magnet systems of many physics research facilities, e.g., accelerators. The domain in which the boundary-value problem is solved often has a piecewise smooth boundary. In this case, numerical calculations of the problem require consideration of the solution behavior in the corner domain. In this work we obtained an upper estimation of the magnetic field growth using integral formulation of the magnetostatic problem and propose a method for condensing the differential mesh near the corner domain of the vacuum in the three-dimensional space based on this estimation.

Design and Development of Emittance Measurement Device by Using the Pepper-pot Technique

Transverse emittance of a charged particle beam is one of the most important properties that reveals the quality of the beam. It is related to charge density, transvers size and angular displacement of the beam in transverse phase space. There are several techniques to measure the transverse emittance value. One of practical methods is the pepper-pot technique, which can measure both horizontal and vertical emittance value in a single measurement. This research concentrates on development of a pepper-pot device to measure the transverse emittance of electron beam produced from an accelerator injector system, which consists of a thermionic cathode RF electron gun and an alpha magnet, at the Plasma and Beam Physics Research Facility, Chiang Mai University. Simulation of beam dynamics was conducted with programs PARMELA, ELEGANT and self-developed codes using C and MATLAB. The geometry, dimensions and location of the pepper-pot as well as its corresponding screen station position were included in the simulation. The result from this study will be used to design and develop a practical pepper-pot experimental station.

Electron linear accelerator system for natural rubber vulcanization

Development of an electron accelerator system, beam diagnostic instruments, an irradiation apparatus and electron beam processing methodology for natural rubber vulcanization is underway at the Plasma and Beam Physics Research Facility, Chiang Mai University, Thailand. The project is carried out with the aims to improve the qualities of natural rubber products. The system consists of a DC thermionic electron gun, 5-cell standing-wave radio-frequency (RF) linear accelerator (linac) with side-coupling cavities and an electron beam irradiation apparatus. This system is used to produce electron beams with an adjustable energy between 0.5 and 4MeV and a pulse current of 10–100mA at a pulse repetition rate of 20–400Hz. An average absorbed dose between 160 and 640Gy is expected to be archived for 4MeV electron beam when the accelerator is operated at 400Hz. The research activities focus firstly on assembling of the accelerator system, study on accelerator properties and electron beam dynamic simulations. The resonant frequency of the RF linac in π/2 operating mode is 2996.82MHz for the operating temperature of 35°C. The beam dynamic simulations were conducted by using the code ASTRA. Simulation results suggest that electron beams with an average energy of 4.002MeV can be obtained when the linac accelerating gradient is 41.7MV/m. The rms transverse beam size and normalized rms transverse emittance at the linac exit are 0.91mm and 10.48πmm·mrad, respectively. This information can then be used as the input data for Monte Carlo simulations to estimate the electron beam penetration depth and dose distribution in the natural rubber latex. The study results from this research will be used to define optimal conditions for natural rubber vulcanization with different electron beam energies and doses. This is very useful for development of future practical industrial accelerator units.

Estimation of magnetic field growth and constraction of adaptive mesh in corner domain for magnetostatic problem

A magnetostatic problem arises in searching for the distribution of the magnetic field generated by magnet systems of many physics research facilities, e.g., accelerators. The domain in which the boundaryvalue problem is solved often has a piecewise smooth boundary. In this case, numerical calculations of the problem require the consideration of the solution behavior in the corner domain. In this paper an upper estimate is obtained for the maximum possible growth of the magnetic field in the corner domain of vacuum. Based on this estimate, we propose a method of condensing the differential grid near the corner domain of vacuum. An example of the modeling problem calculation in the corner domain is given.

Design and Low Power Test of Pulse Forming Network for Klystron Modulator at Chiang Mai University

A new pulse forming network (PFN) for klystron modulator has been designed and tested at Plasma and Beam Physics Research Facility (PBP), Chiang Mai University. The PFN consists of six capacitor and inductor sections. The output waveform of the PFN has pulse width of 3.2 μs and a flat-top of 2.0 μs with ripple less than ± 2.0%. In this paper discusses the design, simulation and low power test results of the PFN.

RF Power Commissioning and an Electron Beam Diagnostic Station for a New RF-gun at Chiang Mai University

This paper presents results of the RF power commissioning for a new thermionic RF electron gun of the linear accelerator system at the Plasma and Beam Physics Research Facility, Chiang Mai University. The research focuses on generation and measurements of a radio-frequency wave for using as a power source for electron acceleration. The low-level RF power measurements were conducted to study the performance of the system. Then, the commissioning of the RF-gun with the high-level RF power obtained from a new 7-MW klystron tube was performed to study the proper condition for the RF-gun operation. The gun temperature was optimized and the proper temperature was about 33oC-34oC. In nominal operation, the RF system produces the RF pulses with the repetition rate of 10Hz for the proper RF pulse length of 3-4μs and the maximum peak power of ∼3.9 MW. The absorbed RF peak power is 2.7 MW, which leads to the average power of 81W. In order to investigate the influence of the asymmetric electromagnetic field distribution inside the RF-gun, an integrated electron beam diagnostic station has been developed and is installed in the accelerator beam transport line. The results of the RF commissioning and development of the diagnostic system for measurements of the electron pulse current, the transverse beam emittance and the beam energy are reported and discussed.

Microwave remote plasma enhanced-atomic layer deposition system with multicusp confinement chamber.

A microwave remote Plasma Enhanced-Atomic Layer Deposition system with multicusp confinement chamber is established at the Plasma and Beam Physics research facilities, Chiang Mai, Thailand. The system produces highly-reactive plasma species in order to enhance the deposition process of thin films. The addition of the multicusp magnetic fields further improves the plasma density and uniformity in the reaction chamber. Thus, the system is more favorable to temperature-sensitive substrates when heating becomes unwanted. Furthermore, the remote-plasma feature, which is generated via microwave power source, offers tunability of the plasma properties separately from the process. As a result, the system provides high flexibility in choice of materials and design experiments, particularly for low-temperature applications. Performance evaluations of the system were carried on coating experiments of Al2O3 layers onto a silicon wafer. The plasma characteristics in the chamber will be described. The resulted Al2O3 films-analyzed by Rutherford Backscattering Spectrometry in channeling mode and by X-ray Photoelectron Spectroscopy techniques-will be discussed.

RF study and 3-D simulations of a side-coupling thermionic RF-gun

A thermionic RF-gun for generating ultra-short electron bunches was optimized, developed and used as a source at a linac-based THz radiation research laboratory of the Plasma and Beam Physics Research Facility, Chiang Mai University, Thailand. The RF-gun is a π/2-mode standing wave structure, which consists of two S-band accelerating cells and a side-coupling cavity. The 2856MHz RF wave is supplied from an S-band klystron to the gun through the waveguide input-port at the cylindrical wall of the second cell. A fraction of the RF power is coupled from the second cell to the first one via a side-coupling cavity. Both the waveguide input-port and the side-coupling cavity lead to an asymmetric geometry of the gun. RF properties and electromagnetic field distributions inside the RF-gun were studied and numerically simulated by using computer codes SUPERFISH 7.19 and CST Microwave Studio 2012©. RF characterizations and tunings of the RF-gun were performed to ensure the reliability of the gun operation. The results from 3D simulations and measurements are compared and discussed in this paper. The influence of asymmetric field distributions inside the RF-gun on the electron beam properties was investigated via 3D beam dynamics simulations. A change in the coupling-plane of the side-coupling cavity is suggested to improve the gun performance.

Optimization of Longitudinal Electron Beam Properties for Linac-based Infrared Free-electron Laser at Chiang Mai University

A study to upgrade the linear accelerator system at the Plasma and Beam Physics (PBP) Research Facility at Chiang Mai University (CMU) to be an injector system for an infrared free-electron laser (IR FEL) is underway. The current PBP linac system consists of an S-band thermionic cathode RF-gun, a bunch compressor in a form of alpha-magnet and a SLAC-type linear accelerating structure. Since characteristics of the emitted FEL light strongly depend on electron beam properties, a dedicated work to develop and optimize the injector system to drive the FEL is particularly important. In this paper, the results of numerical study to optimize the longitudinal electron beam properties as well as some preliminary results of FEL simulations are presented and discussed.

Teacher Preparation: One Key to Unlocking the Gate to STEM Literacy

Abstracts of NSF awards may be found by searching the NSF web-site (http://nsf.gov/awardsearch/) using the project number, whichbegins with the last two digits of the year of the award. Francisco Exploratorium to provide experiences for preser-vice and in-service teachers and their students at this state-of-the-art physics research facility (Meyinsee, 0917543). Andmany directorates support teachers conducting laboratory orfield research with NSF-funded scientists in Research Experi-ences for Teachers. Table 1 summarizes the results of a searchoftheNSFdatabasesforprojectsinvolvingteachereducation.Clearly, such projects are funded throughout the foundation,notonlyinEHR.InterestedreadersmayfindsupportfortheirideastostrengthenteacherpreparationinSTEMfieldswithina number of programs at NSF.With increased attention to teacher preparation and in-sistence on strong content mastery, we predict that studentlearning will improve and the pathway for students to STEMliteracy and participation in the STEM workforce will becleared. The combination of deep understanding of scien-tific content, high expectations for student performance, andengaging pedagogy, as epitomized by graduates of NoyceScholar programs, is the gold standard for teacher prepara-tion. The need is great, however, so widespread efforts on allfronts are needed to achieve the desired results. The NSF iseager to partner with you in this endeavor.

Fabrication of a negative PMMA master mold for soft-lithography by MeV ion beam lithography

In this study, poly(methyl methacrylate) (PMMA) was investigated as a negative resist by irradiation with a high-fluence 2 MeV proton beam. The beam from a 1.7 MV Tandetron accelerator at the Plasma and Beam Physics Research Facility (PBP) of Chiang Mai University is shaped by a pair of computer-controlled L-shaped apertures which are used to expose rectangular pattern elements with 1–1000 μm side length. Repeated exposure of rectangular pattern elements allows a complex pattern to be built up. After subsequent development, the negative PMMA microstructure was used as a master mold for casting poly(dimethylsiloxane) (PDMS) following a standard soft-lithography process. The PDMS chip fabricated by this technique was demonstrated to be a microfluidic device.

Bunch compression efficiency of the femtosecond electron source at Chiang Mai University

A femtosecond electron source has been developed at the Plasma and Beam Physics Research Facility (PBP), Chiang Mai University (CMU), Thailand. Ultra-short electron bunches can be produced with a bunch compression system consisting of a thermionic cathode RF-gun, an alpha-magnet as a magnetic bunch compressor, and a linear accelerator as a post acceleration section. To obtain effective bunch compression, it is crucial to provide a proper longitudinal phase-space distribution at the gun exit matched to the subsequent beam transport system. Via beam dynamics calculations and experiments, we investigate the bunch compression efficiency for various RF-gun fields. The particle distribution at the RF-gun exit will be tracked numerically through the alpha-magnet and beam transport. Details of the study and results leading to an optimum condition for our system will be presented.

Review of observations of ground diffusion in space and in time and fractal model of ground motion

We present evidences of the diffusive motion of the ground and tunnels and show that if systematic movements are excluded then the remaining uncorrelated component of the motion obeys a characteristic fractal law with the displacement variance dY^2 scaling with time- and spatial intervals T and L as dY^2 \propto T^(Alpha)L^(Gamma) with both exponents close to 1. We briefly describe experimental methods of the mesa- and microscopic ground motion detection used in the measurements at the physics research facilities sensitive to the motion, particularly, large high energy elementary particle accelerators. A simple mathematical model of the fractal motion demonstrating the observed scaling law is also presented and discussed.

Writing Material in Chemical Physics Research: The Laboratory Notebook as Locus of Technical and Textual Integration

This article, drawing on ethnographic study in a chemical physics research facility, explores how notebooks are used and produced in the conduct of laboratory science. Data include written field notes of laboratory activity; visual documentation of in situ writing processes; analysis of inscriptions, texts, and material artifacts produced in the laboratory and assembled in notebooks; and an in-depth interview with an expert chemist whose research and writing formed the basis of this investigation. Findings from this study suggest that the notebook occupies a negotiated space between the scientist’s contingent response to exigency in the laboratory and the genre-specific strategies that he or she deploys to communicate his or her work outside the laboratory. This text, the author argues, might therefore be understood as a locus in the sense that it facilitates a reflexive process whereby inscriptions are used both to interpret a-perceptual chemical phenomena in time, and, through their inclusion and integration in the notebook, to discipline that interpretation over time. Tracing the way inscriptions move between material synthesis, on the one hand, and text production, on the other, this article ultimately offers a methodical approach for investigating how the material, technical, and symbolic dimensions of writing and text converge in a modern scientific workplace.

Construction and performance of the magnetic bunch compressor for the THz facility at Chiang Mai University

The Plasma and Beam Physics Research Facility at Chiang Mai University has established a THz facility to focus on the study of ultra-short electron pulses. Short electron bunches can be generated from a system that consists of a radio-frequency (RF) gun with a thermionic cathode, an alpha magnet as a magnetic bunch compressor, and a linear accelerator as a post-acceleration section. The alpha magnet is a conventional and simple instrument for low-energy electron bunch compression. With the alpha magnet constructed in-house, several hundred femtosecond electron bunches for THz radiation production can be generated from the thermionic RF gun. The construction and performance of the alpha magnet, as well as some experimental results, are presented in this paper.

Simon Peter Rosen

Simon Peter Rosen