R. Akre

A solid state Marx type modulator for driving a TWT

This paper describes a solid state Marx type modulator design delivering an 11 kilovolt, 2-4 /spl mu/sec pulse to the cathode of an X-band driver TWT. Insulated gate bipolar transistors (IGBTs) are used as on/off switches to operate the Marx circuit in the energy storage capacitor partial discharge mode. With the aid of a passive compensation circuit, a very flat TWT cathode driver pulse is obtained. The 2 /spl mu/sec, 11 kV pulse amplitude is flat to 0.06%.

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.

Initial commissioning experience with the LCLS injector

The linac coherent light source (LCLS) is a SASE X- ray free-electron laser (FEL) project presently under construction at SLAC [1]. The injector section, from drive-laser and RF photocathode gun through first bunch compressor chicane, was installed in fall 2006. Initial system commissioning with an electron beam is taking place during the spring and summer of 2007. The second phase of construction, including second bunch compressor and full linac, will begin later, in the fall of 2007. We report here on experience gained during the first phase of machine commissioning, including RF photocathode gun, linac booster section, S-band and X-band RF systems, first bunch compressor, and the various beam diagnostics.

LCLS LLRF Upgrades to the SLAC linac

The Linac Coherent Light Source (LCLS) at SLAC will be the brightest X-ray laser in the world when it comes on line. In order to achieve the brightness a 200fS length electron bunch is passed through an undulator. To create the 200fS, 3kA bunch, a 10pS electron bunch, created from a photo cathode in an RF gun, is run off crest on the RF to set up a position to energy correlation. The bunch is then compressed by chicanes. The stability of the RF system is critical in setting up the position to energy correlation. Specifications derived from simulations require the RF system to be stable to below 200fS in several critical injector stations and the last kilometer of linac. The SLAC linac RF system is being upgraded to meet these requirements.

Measurements on SLAC linac RF system for LCLS operation

The Linac Coherent Light Source (LCLS) project at SLAC uses a dense 15 GeV electron beam passing through a long undulator to generate extremely bright x-rays at 1.5 angstroms. The project requires electron bunches with a nominal peak current of 3.5 kA and bunch lengths of 0.020 mm (70 fs). The RF stability required by the bunch compressors is tighter than what is currently required to run experiments. Measurements to determine how well the existing linac will meet the new requirements are ongoing. Presented is an update on the measurements and how they pertain to LCLS.

RF frequency shift during beam storage in the SLC damping rings

A method to reduce the horizontal damping time and equilibrium emittance of the SLC damping rings required changing the RF frequency during the beam storage time. The changed frequency causes the beam to pass off center through the quadrupoles effectively stretching the ring. The timing and phasing of the ring is required to be locked to the accelerator for both injection and extraction. The requirement was to change the RF frequency by up to 100 kHz after injection transients were damped. Before extraction from the ring, the bunch had to be in the correct bucket and phase locked to the linac. It was necessary that the frequency shift not interfere with the operation of several feedback loops and that any stimulated bunch oscillations be damped to less than 0.1/spl deg/ at 714 MHz before extraction, less than 200 /spl mu/s after returning to the nominal frequency of 714.000 MHz. Several methods were evaluated to perform the task. The modifications made to the rings RF system and operating parameters to accomplish the intrastore frequency shift are described.

SLC interferometer system and phase distribution upgrades

Many of the components used in the Stanford Linear Collider (SLC) phasing system date back 30 years to the construction of SLAC. At the start of SLC the phase reference system was upgraded with many of the original components remaining. The RF drive system phase stability requirements became tighter with operation and optimization of the SLC. This paper describes analysis done on the RF drive system and interferometer system during the 1996 run and down time, modifications to the systems during the 1996-97 down time, and the improvements in stability from the modifications.

Operating experience with high beam currents and transient beam loading in the SLC damping rings

During the 1994 SLC run the nominal operating intensity in the damping rings was raised from 3.5/spl times/10/sup 10/ to greater than 4/spl times/10/sup 10/ particles per bunch (ppb). Stricter regulation of rf system parameters was required to maintain stability of the rf system and particle beam. Improvements were made in the feedback loops which control the cavity amplitude and loading angles. Compensation for beam loading was also required to prevent klystron saturation during repetition rate changes. To minimize the effects of transient loading on the rf system, the gain of the direct rf feedback loop and the loading angles were optimized.

Low-level RF signal processing for the Next Linear Collider Test Accelerator

In the X-band accelerator system for the Next Linear Collider Test Accelerator (NLCTA), the low level RF (LLRF) drive system must be very phase stable, but concurrently, be very phase agile. Phase agility is needed to make the Stanford Linear Energy Doubler (SLED) power multiplier systems work and to shape the RF waveforms to compensate beam loading in the accelerator sections. Similarly, precision fast phase and amplitude monitors are required to view, track, and feed back on RF signals at various locations throughout the system. The LLRF is composed of several subsystems: the RF Reference System generates and distributes a reference 11.424 GHz signal to all of the RF stations, the Signal Processing Chassis creates the RF waveforms with the appropriate phase modulation, and the Phase Detector Assembly measures the amplitude and phase of monitored RF signals. The LLRF is run via VXI instrumentation. These instruments are controlled using HP VEE graphical programming software. Programs have been developed to shape the RF waveform, calibrate the phase modulators and demodulators, and display the measured waveforms. This paper describes these and other components of the LLRF system.

LCLS RF gun feedback control

The LCLC RF gun requires a water based thermal system to tune the resonance frequency of the cavity to 2856.03 MHz. The RF system operates in pulsed mode with bursts of 2 - 3 musec. duration at a repetition rate of 30 - 120 Hz. The thermal system operates in combination with the low-level RF system (LLRF) to set the operation point of the cavity. The LLRF system controls the amplitude and phase of the cavity voltage and defines the necessary slow signals to the thermal system. The thermal system operates by pre-heating / pre-cooling the water and mixing both channels to achieve the temperature to control the cavity resonant frequency. The tune control of the RF gun includes two systems with different dynamics. The dynamics of the thermal system is slow while the RF system is fast. Additionally, different actuators in the system present limits that introduce non-linearities to be taking into account during the start up process. Combining these characteristics, a controller is designed for the resulting hybrid system that allows convergence in large for all the operation conditions and achieve the performance in the magnitude and phase of the cavity voltage required around the operation point.