Analytic RF design of a linear accelerator with a SLED-Ⅰ type RF pulse compressor

This study presents the RF design of a linear accelerator(linac)operated in single-bunch mode.The accelerator is powered by a compressed RF pulse produced from a SLED-I type RF pulse compressor.The compressed RF pulse has an unflattened shape with a gradient distribution which varies over the structure cells.An analytical study to optimize the accelerating structure together with the RF pulse compressor is performed.The optimization aims to maximize the efficiency by minimizing the required RF power from the generator for a given average accelerating gradient.The study shows that,owing to the compressed RF pulse shape,the constant-impedance structure has a similar efficiency to the optimal structure using varying iris apertures.The constant-impedance structure is easily fabricated and is favorable for the design of a linac with a pulse compressor.We utilize these findings to optimize the RF design of a X-band linac using the constant-impedance accelerating structure for the Tsinghua Thomson X-ray source facility.

Investigation of NO removal using a pulseassisted RF discharge

In this paper,removal of nitrogen oxide（NO） is investigated in capacitive atmospheric pressure discharges driven by both radio-frequency（RF） and trapezoidal pulsed power with a onedimensional self-consistent fluid model.The results show that the number density of NO could be reduced significantly once a short pulse of low duty ratio is additionally applied to the RF power.It is found that the process of NO removal by the pulse-modulated RF discharge could be divided into three stages：the quick reaction stage,the NO removal stage,and the sustaining stage.Furthermore,the temporal evolution of particle densities is analyzed,and the key reactions in each stage are discovered.Finally,the influence on the removal efficiency of the voltage amplitude of the pulse and the RF voltage amplitude is investigated.

RF power design optimization in MRI system

Magnetic resonance image quality and patient safety have been the focus of engineering and research ever since the invention of equipment in the early 1970s.In high field(or ultrahigh field)MRI systems,the emerging techniques induced by B1 field challenges have promoted various potential solutions.This paper describes the relationship between RF power and B1þfield performance,and the overall requirements considered in RF subsystem design.The design of the RF in the MR system is systematically summarized,including the entire transmission chain,sequence algorithm and RF pulse design,and the probabilities for improvement and optimization in the system design are indicated.At the same time,the radio frequency related issues of the human whole-body 7 T MR and animal MR systems are discussed,especially the promising future showed by the technologies such as radio frequency parallel transmission technology in the ultrahigh field.

Characteristics of a Collisional Sheath Biased by a Dual Frequency Source

A hybrid model is used to simulate the characteristics of a collisional sheath in a capacitively coupled plasma （CCP） driven by a dual frequency source including a RF and a pulsed current source applied to the same electrode. The hybrid model includes a fluid model used to simulate the characteristics of the collisional sheath, and a Monte-Carlo （MC） method to obtain both ion energy and ion angular distributions （IEDs and IADs） impinging on the substrate. The effects of the low frequency of the pulsed source and the gas pressure on the characteristics of the sheath, as well as the IEDs and IADs, are studied. The results show that the ratio of pulse/RF frequency and the gas pressure are crucial for the characteristics of the sheath and the IEDs. The IADs are significantly more sensitive to the gas pressure.

Error and jitter effect studies on the SLED for the BEPC Ⅱ-linac

An RF pulse compressor is a device used to convert a long RF pulse to a short one with a much higher peak RF magnitude. SLED can be regarded as the earliest RF pulse compressor to be used in large-scale linear accelerators. It has been widely studied around the world and applied in the BEPC and BEPCⅡ linac for many years. During routine operation, error and jitter effects will deteriorate the performance of SLED, either on the output electromagnetic wave amplitude or phase. The error effects mainly include the frequency drift induced by cooling water temperature variation and the frequency/Q 0 /β unbalances between the two energy storage cavities caused by mechanical fabrication or microwave tuning. The jitter effects refer to the PSK switching phase and time jitters. In this paper, we re-derive the generalized formulae for the conventional SLED used in the BEPCⅡ linac, and the error and jitter effects on SLED performance are also investigated.

Switching speed effect of phase shift keying in SLED for generating high power microwaves

SLAC energy doubler （SLED） type radio-frequency pulse compressors are widely used in large-scale particle accelerators for converting long-duration moderate-power input pulses into short-duration high-power output pulses. Phase shift keying （PSK） is one of the key components in SLED pulse compression systems. Performance of the PSK will influence the output characteristics of the SLED, such as the rise-time of the output pulse, maximal peak power gain, and energy efficiency. In this paper, a high power microwave source based on power combining and pulse compression of conventional klystrons is introduced. The effects of nonideal PSK with slow switching speed and PSK without power output during the switching process are investigated, and the experimental results with nonideal PSK agree well with the analytical results.