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Abstract
The Delay Line Distribution System (DLDS) invented by KEK has been considered for the compression and distribution of the RF power from klystrons to accelerator structures in the proposed projects of linear colliders, such as the Japan Linear Collider (JLC) and Next Linear Collider (NLC). In DLDS, the long pulse of combined klystron output is subdivided into a train of shorter pulses. Each sub-pulse is delivered to the accelerating structures through a delay line distribution system. This system utilizes the delay of the electron beam in the accelerator structure of the linear collider to reduce the length of the waveguide assembly. A conceptual improvement is proposed by SLAC to further reduce the length of the waveguide system by multiplexing several low-loss RF modes in a same waveguide. Thus, the sub-pulse in the distribution waveguide is carried by different waveguide modes so that they can be extracted at designated locations according to their mode patterns.
In chapter two, the historical background for the RF pulse compression system and its developments have been presented. The SLAC group proposed a clever configuration of a DLDS which could save the delay line length by the use of multi-mode loading of the lines. This multi-mode DLDS scheme works as follows, instead of the several delay lines, one delay line is set to have several mode extractors that are inserted on the one delay line to give the delay time as required by the DLDS operation. Each mode extractor is designed as to extract only a specific mode and at the same time to transp0l1 the other modes completely. At one end of the delay line where RF sources are located. the mode launcher is attached. Primary RF pulse from the klystron is divided into the short RF bins and each RF bin is converted into the different mode through the mode launcher and lead into the delay line. Thus, each RF bin is extracted, in other words, distributed to the necessary location just as an ordinary DLDS. This new version of DLDS is currently considered as the best candidate for the next linear collider that is under study by joint effort between Stanford Linear Accelerator (SLAC) and The High Energy Accelerator Organization (KEK). This multi-mode DLDS requires several new RF devices such as a mode launcher, mode extractor, mode converter, special waveguides and other devices.
In chapter three, the characteristics of multimode x-band wraparound mode converter and extractor for RF pulse compression systems have been analyzed using a full 3D electromagnetic field inside a structure based on the finite element method (FEM). The converter performance is sensitive to the number of guide feeding and the feed of the WR90 rectangular waveguide while the extractor at two and seven guide feeding is sensitive to the feed of the WR90 rectangular waveguide and the circular output waveguide radius, which allow small reflection near 0.2 occurring at the input. The two output ports have transmission equal to unity at most x-band frequencies for the useful circular TEo, mode. For the specific circular mode extractor, the transmission is large or is equal to 0.5 at most x-band frequencies. The converter and extractor characteristics at the first five modes TEll, TE21, TEo” TEI2 and TEn such as reflection coefficient, transmission coefficient, the magnitude of electric and magnetic fields are obtained using numerical simulations. Also, it was found that the narrow feeding with two guides feeding and the wide feeding with seven guides feeding are the best choice for the first five modes.
In chapter four , DLDS and its application are based on the fact that the RF pulses are applied on the beam in the different linac positions to achieve the multiple acceleration by
each RF bins that are cut from the longer original RF pulse. Therefore, the FEM has been used to study and analyze the effect of multimode delay line on the x-band pulse compression systems. The multimode delay line performance is sensitive to the output port and the delay line length whose value is equal to a fractional wavelength. It allows transmission equal to unity and different compression ratios with various delay line lengths for the useful circular TEoI mode of the two and seven guide feeding wraparound mode converter. The transmission, relative input and output power with delay line length effect at the first five modes are obtained using numerical simulations, which satisfies transmission equals to unity and compression ratio equals to 2 up to 47.6 for the x-band linear colIiders.
From the work introduced in this thesis, it was found that the DLDS has several advantages above other conventional RF pulse compression systems; however DLDS can be applied only to a long linac such as a linear coIIider. At X-band, in comparison between an original DLDS and a multimode DLDS, a multimode DLDS is expected to be less cost due to the saving of delay length by its multimode loading of the delay lines and considered as the top ranked candidate as an RF pulse compression scheme for linear colIiders. As result of the advantages mentioned, the multimode x-band wraparound mode converter can be designed with multimode delay line instead of several delay lines, which set to have several mode extractors that are inserted on the one delay line to give the delay time as required by the DLDS operation for RF pulse compression systems.