T.L. Lavine

A multi-moded rf delay line distribution system for the next linear collider

The Delay Line Distribution System (DLDS) is an alternative to conventional pulse compression, which enhances the peak power of rf sources while matching the long pulse of those sources to the shorter filling time of accelerator structures. We present an implementation of this scheme that combines pairs of parallel delay lines of the system into single lines. The power of several sources is combined into a single waveguide delay line using a multi-mode launcher. The output mode of the launcher is determined by the phase coding of the input signals. The combined power is extracted from the delay line using mode-selective extractors, each of which extracts a single mode. Hence, the phase coding of the sources controls the output port of the combined power. The power is then fed to the local accelerator structures. We present a detailed design of such a system, including several implementation methods for the launchers, extractors, and ancillary high power rf components. The system is designed so that it can handle the 600 MW peak power required by the NLC design while maintaining high efficiency.

Beam-based alignment technique for the SLC linac

It is pointed out that misalignments of quadrupole magnets and beam position monitors (BPMs) in the linac of the SLAC (Stanford Linear Accelerator Center) Linear Collider (SLC) cause the electron and positron beams to be steered off-center in the disk-loaded waveguide accelerator structures. Off-center beams produce wakefields which limit the SLC performance at high beam intensities by causing emittance growth. A general method is presented for simultaneously determining quadrupole magnet and BPM offsets using beam trajectory measurements. Results from the application of the method to the SLC linac are described. The alignment precision achieved is approximately 100 mu m, which is significantly better than that obtained using optical surveying techniques.<<ETX>>

High-power RF pulse compression with SLED-II at SLAC

Increasing the peak RF power available from X-band microwave tubes by means of RF pulse compression is envisioned as a way of achieving the few-hundred-megawatt power levels needed to drive a next-generation linear collider with 50-100 MW klystrons. SLED-II is a method of pulse compression similar in principal to the SLED method currently in use on the SLC and the LEP injector linac. It utilizes low-loss resonant delay lines in place of the storage cavities of the latter. This produces the added benefit of a flat-topped output pulse. At SLAC, we have designed and constructed a prototype SLED-II pulse-compression system which operates in the circular TE/sub 01/ mode. It includes a circular guide 3-dB coupler and other novel components. Low-power and initial high-power tests have been made, yielding a peak power multiplication of 4.8 at an efficiency of 40%. The system will be used in providing power for structure tests in the ASTA (Accelerator Structures Test Area) bunker. An upgraded second prototype will have improved efficiency and will serve as a model for the pulse compression system of the NLCTA (Next Linear Collider Test Accelerator).<<ETX>>

Beam loading compensation in the NLCTA

In the design of the Next Linear Collider (NLC), multibunch operation is employed to improve efficiency at the cost of substantial beam loading. The RF pulse that powers the accelerator structures will be shaped to compensate for the effect of the transient loading along the bunch train. This scheme has been implemented in the Next Linear Collider Test Accelerator (NLCTA), a facility built to test the key accelerator technology of the NLC. In this paper we describe the compensation method, the techniques used to measure the energy variation along the bunch train, and results from tests with NLC-like beam currents.

Accelerator and RF system development for NLC

An experimental station for an X-band Next Linear Collider has been constructed at SLAC. This station consists of a klystron and modulator, a low-loss waveguide system for RF power distribution, a SLED II pulse-compression and peak-power multiplication system, acceleration sections and beam-line components (gun, pre-buncher, pre-accelerator, focussing elements and spectrometer). An extensive program of experiments to evaluate the performance of all components is underway. The station is described in detail in this paper, and results to date are presented.<<ETX>>

Two-klystron binary pulse compression at SLAC

The Binary Pulse Compression system installed at SLAC was tested using two klystrons, one with 10 MW and the other with 34 MW output. By compressing 560 ns klystron pulses into 70 ns, the measured BPC output was 175 MW, limited by the available power from the two klystrons. This output was used to provide 100-MW input to a 30-cell X-band structure in which a 100-MV/m gradient was obtained. This system, using the higher klystron outputs expected in the future has the potential to deliver the 350 MW needed to obtain 100 MV/m gradients in the 1.8-m NLC prototype structure. This note describes the timing, triggering, and phase coding used in the two-klystron experiment, and the expected and measured network response to three- or two-stage modulation.<<ETX>>

The Next Linear Collider test accelerator's RF pulse compression and transmission systems

The overmoded RF transmission and pulsed power compression system for SLACs Next Linear Collider (NLC) program requires a high degree of transmission efficiency and mode purity to be economically feasible. To this end, a number of new, high power components and systems have been developed at X-band, which transmit RF power in the low loss, circular TE01 mode with negligible mode conversion. In addition, a highly efficient SLED-II pulse compressor has been developed and successfully tested at high power. The system produced a 200 MW, 250 ns wide pulse with a near-perfect flat-top. In this paper we describe the design and test results of the high power pulse compression system using SLED-II.

High-power radio-frequency binary pulse-compression experiment at SLAC

A high-power X-band three-stage binary RF pulse compressor has been implemented and operated at the Stanford Linear Accelerator Center (SLAC). In each of three successive stages, the RF pulse length is compressed by half, and the peak power is approximately doubled. The experimental results presented have been obtained at power levels up to 25-MW input (from an X-band klystron) and up to 120-MW output (compressed to 60 ns). Peak power gains greater than 5.2 have been measured.<<ETX>>

Transient analysis of multicavity klystrons

A model for analytic analysis of transients in multicavity klystron output power and phase is described. Cavities are modeled as resonant circuits, while bunching of the beam is modeled using linear space-charge wave theory. The analysis has been implemented in a computer program used to design multicavity klystrons with stable output power and phase. The authors present as examples transient analyses of a relativistic klystron using a magnetic pulse compression modulator and of a conventional klystron designed to use phase-shifting techniques for RF pulse compression.<<ETX>>

SLC polarized beam source ultra-high-vacuum design

The authors describe the design of the ultra-high vacuum system for the beam-line from the 160 kV polarized electron gun to the linac injector in the Stanford Linear Collider (SLC). The polarized electron source is a GaAS photocathode, requiring 10/sup -11/ Torr-range pressure for adequate quantum efficiency and longevity. The photocathode is illuminated by 3 ns-long laser pulses. Photocathode maintenance and improvements require occasional substitution of guns with rapid restoration of UHV conditions. Differential pumping is crucial since the pressure in the injector is more than 10 times greater than the photocathode can tolerate and since electron-stimulated gas desorption from beam loss in excess of 0.1% of the 20 nC pulses may poison the photocathode. The design for the transport line contains a differential pumping region isolated by a pair of valves. Exchange of guns requires venting only this isolated region, which can be restored to UHV rapidly by baking. The differential pumping is performed by nonevaporable getters and an ion pump.<<ETX>>