PHOTON FACTORY LINAC RF PHASING COMPUTER CONTROL SYSTEM

Roger Bissonnette
KEK National Laboratory for High Energy Physics
Oho-machi, Tsukuba-gun, Ibaraki-ken, 300-32 JapanIntroduction
The phasing of the 40 high power S-band klystrons of the PF injector will be done under computer
control. This is neccessary to optimize the accelerationJ Various indirect comparison "Beam Induced"
23
methods for long linacs have been adopted by SLAC and ORSAY . In the Beam Induced method a CW refer­ence (Eref) is first matched in phase with the pulsed RF signal (E^) induced by the beam in an otherwise
empty cavity or waveguide section.

Then the klystron is matched to the reference phase.

Computercontrol has been done at SLAC'
In the PF linac the low power phase shifters for each klystron are contained in Ic()A units (Isolation. Phasing, Attenuation) located in each modulator.

Since the I(|)A is controlled by a microprocessor

FOCUS was investigated for implementing the entire phasing function there.
Outline of Functional Operation
It turns put phase detection can't be done at the I0A level because of no inputs, the monitor RFsignals from the 8 klystron in a sector will be routed directly to the sector control level. The finalform that the phase detection will take is still under active investigation, as described in the paper
by OSAWA et al. in this conference, and the overall features and design concepts of the linac computer
control system are discussed in the paper by NAKAHARA et al. Therefore F i g . 1 shows only the control
units involved in phasing.

The actual phase comparisons are made by the Phase Detection Control Unit (PDCU) at the command of
the sector control MELC0M-70, and the phase of each klystron is set by its associated

The CAMAC

Crate and Klystron Modulator Controller boxes serve merely to carry messages.
Phase Detection Control Unit and I^A
A function diagram of the PDCU is shown Fig. 2.

In general this unit is designed to work as

independently as possible.

Given a "start" command from the MELC0M-70 it first sets up the RF switches

and sample timing delays.

On the reference adjustment cycle it will control its local phasing unit witha stepper motor-encoder.

On the klystron adjustment cycle it will make up command messages for theklystron IcfA unit.

JLldJlExcept for a different communication protocall the I(|)A is just a copy of the part of

the PDCU that controls
reference phasing.

堂"PU”6800

Fig.

Fig,

Appendix - Logarithmic Number Systems for Microcomputers…

8-bit microprocessors have become cheap enough that they are fincjing many new applications, but
the normal signed binary representation of numbers allows quantization only to the 0.7% level9 forphasing this would be 1.4°.

For better resolution 16-bit microcomputers are becomming available but

slowly and at greater cost, multiple precision arithmetic can be used at slowed execution time andincreased storage requirements.

A third approach would be a more efficient encoding of the data andthis is often done via logarithmic coding.
The primary advantage of all log systems is the great increase in the number of states near theorigin, at a proportionate rougher quantization for extreme values. The FOCUS System

5,6represents

numbers by;
Y = Sign (x)*{ 8 + log? 卜丨}|x| < 0.004

The outstanding virtue of FOCUS and the system of SWARTZLANDER & ALEXOROULOS7 is that the usual 4arithmetic operations can be defined and implemented efficiently.

However the input/output codectransformations are not easily done.
Another important class is that developed by Bell Labs for codecs using for example, the ijf255 Law:
Sign ( X ) 1糊

^IxNl

This transform i s quite robust when the data i s more randomly d is t rib u t e d

and I C ’ s implementing i teither as a stand alone D/A (DAC-/6 from PMI) or as a complete telecommunications system
available.

9are readily

However9 in the transform space it is impossible to define simple operations to implement

arithmetic.
Any log system can be effective in relaying digitally a servo loop error signal. FOCUS allows easy
implementaion of digital filters at the cost of codec difficulties,

y-しaw solves the codec problems at

the sacrifice of any possibility of digital processing.
Such systems were rejected for the PDCU-Ic()A processors for the following r e a s o n s : 1 ) the output ofthe phase detector is a randomly distributed variable, only the difference between 2 measurements iszero-centered.

But the conversion operation is best done by the microprocessor.

2) the conversion fromlogarithmic representation to the number of drive pulses for stepper motors is too complex to warrantconsideration.

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LEVELFig. AlRFFERENCES
1 . H. Herminghaus, "The Effect of Simultaneous Phase Errors and Phase Jitter on the Energy Resolutionof Multi section Electron Linacs", in Particle Accelerators, vol 2, pp. 181-184, 1971.
2.

H. Hogg, ed. "The Phasing System", pp. 383-409 in "The Standford Two-Mile Accelerator", R. Neal ed ，W.A» Benjamin, New York， 1968.

3.

P. Brunet et a l , " L 1Accel^rateur Lin^aire et sa r^cente extension", in L'Onde Electrique, vol 49 ，pp 758-761 , 1969.

4.

S. Howry and A. Wilmunder, "A Microprocessor Controller for Phasing the Accel era tor'1, in IEEE Trans
on Nuclear Science, vol NS-24, pp. 1804-1806， June 1977.

5. S . しee and A. Edgar, "FOCUS Microcomputer Number System", in "Microcomputer Design and Applications"S. Lee ed. Academic Press, New York, 1977.
6. A, Edgar and S. Lee ， "FOCUS Microcomputer Number System"9 in Comm of the ACM, vol 22 ，pp 166-177,
1979.
7.

E. Swartzlander and A, Alexopoulos， "The Sign/Logarithm Number System in IEEE Trans on Computers,
vol C-24, pp 1238-1242, 1975.

8.

A. Gersho, "Principles of Quantization", in IEEE Trans on Circuits and Systems9 vol CAS-25，
pp 427-436,21978.

9. H. Hindin，"Technologies, Architecture Compete for Huge Codec Market", in Electronics, vol 51 ，
pp 105-108, S e p . 14,1978.