Z. D. Farkas

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.

Recent Progress on Sled, The SLAC Energy Doubler

From the following description, it will be seen that SLED raises the accelerator peak energy by 40% if the present 2.7 1s RF pulse length is used. The energy increase ” is 80% if the pulse length is extended to 5 ps. TO do this, however, changes have to be made in the modulators and trigger system, and the maximum repetition rate has to be halved to maintain the present average power level. In addition, more extensive switchyard modifications are required for~handling the higher energy beams. For these reasons, the SLED system will be initially installed and run at the present 2.7 ns pulse length. Performance at both pulse lengths is discussed in this paper.

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>>

RF pulse compression experiment at SLAC

Using RF pulse compression it is possible to boost a 50-100 MW output, expected from high-power microwave tubes operating in the 10-20 GHz frequency range, to the 300-600 MW level required by the next generation of high gradient linear colliders. Experiments have been performed at Stanford Linear Accelerator Center to test, at low power, a two-stage binary energy compressor (BEC) operating at 11.424 GHz. Using over-moded delay lines and 3 dB hybrid couplers, a 312-ns pulse was compressed to 78 ns, giving a power multiplication ratio of approximately 3.2 and a power efficiency of 81%. Individual component insertion losses were measured to be in the range of 0.6% to 8.5%. Overall efficiency calculated using these values agreed with measured values to approximately 1.4%.<<ETX>>

Effects of temperature variation on the SLC linac RF system

The RF system of the Stanford Linear Collider in California is subjected to daily temperature cycles of up to 15/spl deg/C. This can result in phase variations of 15/spl deg/ at 3 GHz over the 3 km length of the main drive line system. Subsystems show local changes of the order of 3/spl deg/ over 100 meters. When operating with flat beams and normalized emittances of 0.3*10/sup -5/ m-rad in the vertical plane, changes as small as 0.5/spl deg/ perturb the wakefield tail compensation and make continuous tuning necessary. Different approaches to stabilization of the RF phases and amplitudes are discussed.

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>>

A New Formulation for Linear Accelerator Design

We define three basic, calculable and measurable parameters. With these parameters we derive expressions for the radio frequency induced, beam induced, and loaded section voltages and average section gradients. Unlike the present well known expressions these alternate expressions are valid for continuous wave, pulsed, and single bunch beams, for both lossy and lossless sections and for both standing wave and travelling wave sections. We use these alternate expressions to maximize the efficiency and the gradient for a given peak power.

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>>

Dynamics of an electron in an RF gap

An attempt is made to understand the limitation on the energy transfer efficiency of an electron beam to the RF output cavity of a klystron or a lasertron. An output cavity with drift tubes is modeled by a region of constant amplitude RF field with exponentially decreasing entrance and exit fringing fields. The exit velocity of an electron traversing such a gap is examined as a function of entrance phase for various values of the ratio of the peak RF cavity voltage to the electron entrance voltage. Depending on the ratio, the dynamics of the electron motion can become quite complex. For a gap with fringe fields it is found that, even if the gap voltage and phase are optimized, the maximum energy that can be extracted from a short bunch is always significantly less than 100%. The case in which the electron is created with zero velocity in the gap, and subsequently leaves the gap having extracted energy from the RF field, is also treated.<<ETX>>