Studies on RFQ accelerators and its applications

Development activities of Radio Frequency Quadrupole (RFQ) accelerators in China are presented.A 1 MeV O~+ RFQ and a 3.5 MeV ADS proton RFQ have been constructed.A novel separated function RFQ is under beam test, a 2 MeV D~+ RFQ is under construction and a CSNS RFQ is going to be constructed.The RFQ dynamics and the simultaneous dual beam acceleration with positive and negative ions were investigated and related codes were developed.The applications of RFQ will be further promoted in China.     

Study on shunt impedance and voltage distribution of 4-rod RFQ cavity

A high current RFQ (radio frequency quadrupole) is being studied at the Institute of Modern Physics,CAS for the direct plasma injection scheme. Shunt impedance is an important parameter when designing a 4-rod RFQ cavity,it reflects the RF efficiency of the cavity,and has a direct influence on the cost of the structure.Voltage distribution of a RFQ cavity has an effect on beam transmission,and particles would be lost if the actual voltage distribution is not as what it should be.The influence of cell length,stem thickness and height on shunt impedance and voltage distribution have been studied,in particular the effect of projecting electrodes has been investigated in detail.     

Design of 57.5 MHz CW RFQ structure for the Rare Isotope Accelarator Facility

The Rare Isotope Accelerator (RIA) facility includes a driver LINAC for production of 400 kW CW heavy-ion beams. The initial acceleration of heavy ions delivered from an ECR ion source can be effectively performed by a 57.5 MHz 4 m long RFQ. The principal specifications of the RFQ are: (1) formation of extremely low longitudinal emittance: (2) stable operation over a wide range of voltage for acceleration of various ion species needed for RIA operation; (3) simultaneous acceleration of two-charge states of uranium ions. CW operation of an accelerating structure leads to a number of requirements for the resonators such as high shunt impedance, efficient water cooling of all parts of the resonant cavity, mechanical stability together with precise alignment, reliable rf contacts, a stable operating mode and fine tuning of the resonant frequency during operation. To satisfy these requirements a new resonant structure has been developed. This paper discusses beam dynamics and electrodynamics design of the RFQ cavity, as well as, some aspects of the mechanical design of this low-frequency CW RFQ.     

Power test of the separated function RFQ accelerator

The progress of the Separated Function RFQ (SFRFQ) accelerator, which can raise the field gradient of acceleration while maintaining the transverse focusing power sufficient for high current beam, is presented. In order to demonstrate the feasibilities of the novel accelerator, a prototype cavity was designed and constructed. Correspondingly, a code SFRFQCODEV1.0 was developed specially for cavity design and beam dynamics simulation. The prototype cavity will be verified as a post-accelerator for ISR RFQ-1000 (Integral Split Ring RFQ) and accelerate O+ from 1 MeV to 1.6 MeV. To inject a higher current oxygen beam for the prototype cavity, the beam current of ISR RFQ-1000 was upgraded to 2 mA. The status of high power and beam test preparation for the prototype cavity are presented in this paper.     

Matrix method for the solution of RF field perturbations due to local frequency shifts

To tune the accelerating field to the design value in a periodical radio frequency accelerating structure, Slater's perturbation theorem is commonly used. This theorem solves a second-order differential equation to obtain the electrical field variation due to a local frequency shift. The solution becomes very difficult for a complex distribution of the local frequency shifts. Noticing the similarity between the field perturbation equation and the equation describing the transverse motion of a particle in a quadrupole channel, we propose in this paper a new method in which the transfer matrix method is applied to the field calculation instead of directly solving the differential equation. The advantage of the matrix method is illustrated in examples.     

高频H型放电离子源的场特性

 从Maxwell方程组出发,推导了高频H型放电离子源放电空间的场分布, 并采用Mafia软件进行了三维实体建模,计算了高频离子源放电击穿前和稳定工作后的电磁场分布,得到了高频离子源放电空间电磁场分布的直观图像。通过比较击穿前高频电场的轴向和环向分量,得出了轴向电场在高频离子源击穿中起主要作用的结论,并进而推导出了高频离子源的击穿判据,得出了气体击穿时离子源击穿电压和放电管内气压的关系,与实验结果符合较好。     

RFQ冷却聚束器

简要介绍了RFQ冷却聚束器的基本原理, 并对设计中的RFQ冷却聚束器的特点和利用SIMION程序进行模拟的初步结果进行了讨论.The principle of RFQ (Radiofrequency Quadrupole) cooler and buncher is briefly introduced. The characteristics of the being designed RFQ cooler and buncher and the preliminary results simulated by using the SIMION code are discussed.     

磁场中矩形空心阴极离子分布的计算

本文用解析方法研究了在均匀磁场中入射离子在矩形空心阴极中的分布。结果表明当空心阴极的深度和宽度接近的时候，入射离子在一个侧面上的分布不受另一个侧面的影响，同时受阴极深度影响的在底面上的离子分布及粒子数量都从阴极的中心到边缘而单调地降低。     

Analysis of the impacts of mechanical errors on the RF performance of a single spoke cavity

Mechanical errors can not be avoided in fabrication. They will cause geometry errors and have impacts on the cavity performance. This paper systematically analyzes the impacts of mechanical errors on the RF performance of Peking University single spoke cavity. The various kinds of shape and size errors are considered, the influences on the resonation frequency and field flatness are studied by numerical simulation and the theoretical models are analyzed. The results show that the single spoke cavity is robust with respect to the mechanical tolerance. It also indicates the most essential factors for fabrication.     

Interval analysis of transient temperature field with uncertain-but-bounded parameters

Based on the traditional finite volume method, a new numerical technique is presented for the transient temperature field prediction with interval uncertainties in both the physical parameters and initial/boundary conditions. New stability theory applicable to interval discrete schemes is developed. Interval ranges of the uncertain temperature field can be approximately yielded by two kinds of parameter perturbation methods. Different order Neumann series are adopted to approximate the interval matrix inverse. By comparing the results with traditional Monte Carlo simulation, a numerical example is given to demonstrate the feasibility and effectiveness of the proposed model and methods.     

同向电磁场中氢原子低能级简并的解除

利用同向电磁场的作用使氢原子低能级简并完全解除.     

Stray field measurements of flow displacement distributions without pulsed field gradients

The probability distribution P(zeta) of diffusive and advective molecular displacements is determined using a fixed field gradient (FFG) pulse sequence, on fluid flow through a Bentheimer sandstone, in the grossly inhomogeneous stray field of a super-conducting magnet. Two decades of q-space are scanned with stimulated echoes, using the gradient of the stray field and variable encoding times delta. The strength of the gradient permits the use of short encoding times, which is desirable for limiting the distorting effects produced by flow displacements through susceptibility induced field inhomogeneities. CPMG and CP echo trains are used to refocus separately the real and imaginary parts of the stimulated echo, for experimental efficiency.     

游船空调降温过程舱内温度场及速度场的动态模拟

以工程中广泛采用的k-ε方程湍流模式为基础,运用计算流体力学的有限差分法和SIMPLE法对船舱的三维温度场进行动态数值模拟,为舱内空调的优化设计和空调设备节能研究提供一定的理论依据。     

A field theory approach to cosmological density perturbations

Adiabatic perturbations propagate in the expanding universe like scalar massless fields in some effective Robertson–Walker space–time.     

Transport of the ion bounced by the plug potential caused by radial electric field in the tandem mirror

In the tandem mirror, plug potentials are created in the plug/barrier cells by microwave injection, and also anchor cells with a minimum B magnetic field configuration are installed in order to reduce the MHD instability. Existence of the ion bounced by the plug potential (PP-bounced ion) is essential for the ion confinement in the tandem mirror. We study the transport of the PP-bounced ion on the assumption that the cross-sectional shape of the magnetic flux tube is slightly different from the cross-sectional shape of the equi-potential surface at the mirror throats of the anchor cell as small deviation from the equilibrium state. The radial-potential profile of the core plasma is adjusted by controlling the electrostatic potentials of the coaxially separated end plates. We find that the spread type of radial-potential profile is effective for the confinement of the plug-potential-bounced ion.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

An effective potential method for the calculation of hydrogen molecule ion energy in a strong magnetic field

Using the adiabatic approximation and adiabatic variational approximation with an effective potential, the total energies and the equilibrium internuclear separations of H₂⁺ ion in states σ_g, π_u, δ_g, φ_u, γ_g, η_u in strong magnetic fields have been calculated. Our results reproduced those obtained by Vincke and Baye (Vincke M and Baye D 1985 J. Phys. B: At. Mol. Phys. 18 167), which are considered the most reliable. We found a basic function with better convergence than the Landau functions.     

RF electric field penetration and power deposition into nonequilibrium planar-type inductively coupled plasmas

The RF electric field penetration and the power deposition into planar-type inductively coupled plasmas in low-pressure discharges have been studied by means of a self-consistent model which consists of Maxwell equations combined with the kinetic equation of electrons. The Maxwell equations are solved based on the expansion of the Fourier--Bessel series for determining the RF electric field. Numerical results show the influence of a non-Maxwellian electron energy distribution on the RF electric field penetration and the power deposition for different coil currents. Moreover, the two-dimensional spatial profiles of RF electric field and power density are also shown for different numbers of RF coil turns.     

Electric field measurements and computational modeling at ultrahigh-field MRI

While magnetic resonance images essentially contain a map of the both circularly polarized components of the RF transverse magnetic fields (B(1) field), the thermal heat and electromagnetic power deposition is generated by the associated electric fields. Measurement of electric field distributions/intensities across a sample yields an indirect indication of possible cause of heating within the sample and potentially enables the detection of "hot spots," which can be present within inhomogeneous radiofrequency (RF) fields, such as the case with magnetic resonance imaging at high field strength. As a result, establishing a valid technique for direct measurements of the electric field and its correlation, obtained using computational electromagnetics, is essential in assessing (1) the safety of the RF coil designs and (2) the validity of the calculations. In this work, a probe was built and used to measure the transverse electric field (E(1) field) distributions within an empty 8 T (tuned to 340 MHz) RF head coil and within a saline water phantom loaded in the same coil. The measured E(1) field distributions were favorably compared to the distributions obtained utilizing a finite difference time domain in-house package.