Edmond Ciapala

Ultimate performance of the LEP RF system

The LEP Superconducting RF system reached its maximum configuration of 288 four-cell cavities powered by 36 klystrons in 1999. In 2000, this system, together with 56 cavities of the original copper RF system, routinely provided more than 3630 MV, allowing the beam energy to be raised up to 104.5 GeV. This not only required operating the cavities more than 15% above their design gradient, but has also demanded a very high operational reliability from the entire system. This paper will describe the operation of the LEP RF system during 2000, including new features, operational procedures and limitations.

The LEP Radio-Frequency Low Power System

A brief description of the low power and controls electronics for running the 128 cavity assemblies in the initial phase of LEP is given. Virtually identical hardware will be used for the future extension of LEP with superconducting niobium cavities. One complete RF unit consisting of 16 cavities and all associated drive and controls electronics has been successfully tested.1

Phase Displacement Acceleration of High Intensity Stacks in the CERN ISR

In the ISR high intensity stacks of more than 25 A are accelerated by phase displacement from 26.6 to 31.4 GeV/c. Phase displacement is the only known means of accelerating stacks of such large momentum spread (¿p/p = 3%) with the existing low power RF system. Acceleration in this way may produce loss of intensity due to RF and power supply magnet noise, momentum blowup of the stack, closed orbit and working line variations, and changes in the RF bucket size while traversing the stack. The existing instrumentation allows close control of all relevant parameters during acceleration and has resulted in reducing the intensity losses to as little as 10%. In this way, luminosities significantly in excess of the ISR design luminosity are achieved in an operational way, making 31.4 GeV/c one of the standard ISR momenta for physics data taking and giving an equivalent nomentum of greater than 2,000 GeV/c when related to stationary target machines.

A dedicated multi-process controller for a LEP RF unit

The operation of each RF unit for the LEP (Large Electron Positron collider) requires that several control processes be run simultaneously. Local and remote access is required to around 2500 individual parameters and status indications inside the unit. Normal operation of the unit involves complex procedures requiring a considerable amount of sequential access equipment within the unit. Optimum performance and overall maintainability are achieved by calling up locally resident procedures. The needed surveillance and alarm monitoring can run locally, making a summary status available for the control center. The local process controller hardware is realized as a hybrid VME/G64 crate with a 68020 VME module as the main processor. Two 68000 VME slave processors having G64 ports handle communication with equipment buses. VME high-resolution graphics interfaces provide interactive local control via a touch screen and separate data display. A VME MIL-1553 interface provides the connection to the control system. Local control, remote control, and surveillance run concurrently on the main processor under the OS-9/68 K multitasking operating system.<<ETX>>

The variation of γ t with Δp/p in the CERN ISR

The variation of the transition energy γt m_oc² across the momentum aperture in the ISR has been determined by measuring the non-linear change of the revolution frequency as a function of the radial displacement and the momentum deviation and by measuring the phase oscillation frequency variation across the aperture while operating close to transition energy. The ratio of the relative slopes of γ_t and γ was found to be dγ_ttc ≈ -0.75. This value could be changed to nearly zero by strongly exciting the sextupole magnets. All measurements are in good agreement with each other and with computations carried out with a modified version of the program AGS. The change of γ_t across the aperture causes an area change of the empty buckets while traversing the beam during phase displacement acceleration. This leads to a momentum blow-up.

Global voltage control for the LEP RF system

LEP RF system is installed as independent 16 cavity units. In addition to the eight copper cavity units originally installed 12 units with super-conducting cavities are being added for the LEP200 energy upgrade. The total RF voltage determines the synchrotron tune (Q/sub s/) and must be controlled precisely during energy ramping. Local function generators in each of the RF units are pre-loaded such that when triggered simultaneously by ramp timing events transmitted over the general timing system the total voltage varies to give the Q/sub s/ function required. A disadvantage is that loss of RF in a unit at any time after the loading process cannot be corrected. As the number of RF units increases automatic control of the total RF voltage and its distribution around LEP becomes desirable. A global voltage control system, based on a central VME controller, has recently been installed. It has direct and rapid access to the RF units over the LEP time division multiplexing system. Initial tests on operation and performance at fixed energy and during energy ramping are described, as well as the implementation of a Q/sub s/ loop in which Q/sub s/ can be set directly using on-line synchrotron frequency measurements.<<ETX>>

Longitudinal feedback in LEP

Dipole coupled bunch oscillations were observed at an early stage of LEP commissioning for currents above about 150 /spl mu/A per bunch. An improvised feedback system, acting on the phase of some of the accelerating cavities was developed and has been in operation for about three years. However, due to the small bandwidth of the RF cavities this system can only be used with four bunches or less per beam. With plans for eight bunch operation (the Pretzel scheme) the construction of a dedicated longitudinal feedback system was approved in 1991. The system operates at 999.95 MHz with phase modulation of a 200 kW klystron feeding four seven-cell cavities. The necessary bandwidth of 260 kHz is obtained by heavy over-coupling. With a total cavity voltage of 1.9 MV a damping rate of about 450 s/sup -1/ is obtained with phase excursions of one radian. The system has been in routine operation since July 1992 with a feedback cavity voltage of 1.2 MV and a damping rate of about 100 s/sup -1/. Longitudinal feedback eases operation and usually increases the maximum currents which can be accumulated.<<ETX>>

Digital control of the LEP RF system

The RF system for the initial phase of LEP (Large Electron Positron collider) consists of eight identical units of 16 cavity assemblies with associated high-power and control equipment. Digital control is achieved by assigning a G64-based equipment controller crate to each major element of the unit. The crates are linked by a bus to a VME-based data manager with overall control and connection to the control network. The manager executes locally or remotely initiated complex control and surveillance procedures to simplify operation. The different classes of parameter and data are summarized. Interfacing and control within the equipment controllers and communication with the data manager are outlined. The equipment access functions and the development of complex control procedures are discussed. The implementation of corresponding functions at the control console level is examined. Experience with the six RF units presently installed is described. The modular approach to the controls of the LEP RF system, together with an interactive local control facility, has been found vital in commissioning and testing the RF system. It also allows additional units with superconducting cavities to be integrated gradually into the controls system with minimal disturbance to the operation of the accelerator.<<ETX>>

The CERN ISR Control Scheme for Acceleration by Phase Displacement

In the CERN ISR high-intensity coasting beams are accelerated routinely from 26.6 to 31.4 GeV/c using the technique of phase displacement. The constraints imposed by this technique dictate a rather unusual control scheme. The total acceleration is performed in around two hundred steps; each incremental step in momentum is produced by sweeping empty RF buckets through the beam. The RF parameters are calculated and set by the control computer for each sweep. During each sweep the computer controls all magnetic elements synchronously so as to compensate for normal variations in the working line, the closed orbits and the beam position. This control is based on linear interpolation on the values held in a number of intermediate files between 26.6 and 31.4 GeV/c. The differences in the power supply settings contained in these files produce the magnetic changes required to maintain the correct space charge tune shift and to compensate for magnetic saturation of the poles of the main bending magnets. The intermediate files are generated from the results of measurements made during the acceleration of a bunched beam using a different mode of the acceleration program.

Use of Ethernet and TCP/IP socket communications library routines for data acquisition and control in the LEP RF system

The implementation of Ethernet and TCP/IP socket communications routines for RF data acquisition and control is described. The adaptation of almost all of the existing software for RF system control, data acquisition, and diagnostics to make use of this means of communication has proved straightforward. Furthermore, the transparent transfer of data in the form of C structures from the data managers to the control center workstations and other computers has considerably simplified the software required for remote surveillance and data logging with a corresponding increase in speed and reliability.<<ETX>>