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RF Phase Control for BEPCII Linac Accelerator Wenchun Gao

CONTENTS PART 1 Requirement and Capability PART 2 Configuration and Technology PART 3 Implementation of Key Components

PART 1 Requirement and Capability

The Origin of RF Phase Control The particle energy spread is effected by phase error according the following equation,  E/E  1/2  (  /2+  n /N) 2 where  is angular spread of particle distribution,  n is the algebraic sum of the individual phasing errors of accelerating sections and N is the number of accelerating sections. RF phase control aims at keeping the phase error  n within its designated range.

Phase Requirement of BEPCII The microwave of 2856MHz is used in BEPCII injector to boost the energy of electrons and positrons. The limitation of maximum deviation of  2  from optimum phasing location is put on the RF phasing system by general design of BEPCII injector. |Phase error|≤ 2 

RF Phase decomposition Given a point on beam line, if the optimum drive phase of microwave may be acquired by adjust the corresponding phase shifter, then the drive phase at this point may be decomposed as  =  0 +  s (t)+  q (t) where  0 is the optimum drive phase,  s (t) is phase drift caused by all sorts of real physical devices and varies slowly,  q (t) is random item called phase noise also caused by physical devices and varies quickly.

RF Phase Control Capability RF phasing system can only work out phase drift item of  s (t) through measurement and dynamically eliminate the effect of  s (t). Estimation theory will be used in RF phasing control algorithms.

PART 2 Configuration and Technology

Framework of BEPCII Injector e- gun K1K1 Osc.PSK PBBA1A1 PS Att Amp Drive line Beam line PS A2A2 A3A3 A4A4 A5A5 e+ PS K2K2 ED PS KjKj

Topology Structure of RF Phasing System of BEPCII Injector Subsystem of computer analysis and control on the basis of EPICS (Phasing control, data collection and analysis in mid and long term, interface to other systems) phase reference line optic fiber communi- cation link servo control  shifter attenuator energy doubler PAD servo control  shifter attenuator PAD oscillator 2856MHz modulator amplifier Drive line e+ phase shifter K1K16 accelerating structure PAD e

Technology of RF Phase Control for BEPCII Injector To put up phase reference line along BEPCII injector gallery by means of optical technology to provide a benchmark for phase measurement at each individual klystron station. To make precise phase & amplitude measurement devices with technology of digital signal processing. Autonomous phase & amplitude adjustment equipment based on phase-locked loop fundamentals will be installed for each individual klystron station. To connect the distributed RF phase system with optic fiber and have its parts communicate with digital stream through these optic links in order to enhance the capability of the system to immunize against interference. To study the RF phasing system of BEPCII injector and design the control algorithm on the basis of probability, statistics and stochastics. To prepare to introduce EPICS into high level phase control.

Configuration of RF phasing system of BEPCII injector Next page

Four levels of RF Phasing System Configuration The top level includes the large capacity data recorder for mid- term and long-term data collection, and a global computer for data analysis, global task control and task interface to other exterior systems. The second level is a LAN connecting global level with local control level situated at each individual klystron station by optic fiber communication links. The third level is the local control level distributed with similar devices at each individual klystron station along the klystron gallery. Its main parts are PAD and VME units. The bottom level is optic fiber phase reference line established as a phase measurement benchmark for PAD at each klystron station.

PART 3 Implementation of Key Components

Autonomous phase adjuster for each individual klystron station. Input s i (t) Klystron stationPhase shifter Output s o (t) Phase measurement PAD Kalman filter controller  i (t)  o (t)  o,r (t)=  o (t)-  r (t) s r (t) Optic fiber phase reference line E[  s (t)]

Digital PAD Digital signal is not apt to be interfered. Digital PAD may reduce the difficulties of analog circuit design in PAD and a PAD insensitive to temperature is available. s o (t)RF Mixer Local Osc. Mixer s r (t) Phase reference line IF ADC Digital signal processing  o,r (t)=  o (t)-  r (t) Drive pulse detector Memory

optic phase reference line The figure above is only a simplified optic phase reference line. phase-locked technique is often needed in optic phase reference line. Osc. 2856MHz Modulator Single mode laser Optic fiberOptic phase reference line Optical receiver Optical transmitter 2856MHz reference signal PAD

SUMMARY RF phase control plays an important role in BEPCII Linac accelerator. A phase error of less than 2  is put on RF phase control by BEPCII Linac design. Many types of advanced technology, such as optical technique, digital signal processing and EPICS, etc, will be applied to ensure the RF phase control performance. The preliminary design is finished and detail design has begun.

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