Control of Total Voltage in the Large Distributed RF System of LEP
E. Ciapala and L. Arnaudon
SL Division, CERN, 1211 Geneva 23, Switzerland
Abstract
The LEP RF system is made up of a large number of independent RF units situated around the ring near theinteraction points. These have different available RF voltages depending on their type and they may be inactive or unable
to provide full voltage for certain periods. The original RF voltage control system was based on local RF unit voltage
function generators pre-loaded with individual tables for energy ramping. This was replaced this year by a more flexible
global RF voltage control system. A central controller in the main control room has direct access to the units over the
LEP TDM system via multiplexers and local serial links. It continuously checks the state of all the units and adjusts their
voltages to maintain the desired total voltage under all conditions. This voltage is distributed among the individual units
to reduce the adverse effects of RF voltage asymmetry around the machine as far as possible.
The central controller is a VME system with a 68040 CPU and real-time multitasking operating system. Event driven
communication handlers allow fast reliable concurrent data communication with the remote units. The RF unit low level
RF G64 equipment controllers use a VME 68030 CPU to achieve the necessary response time and reliability.

I. INTRODUCTION
IP5

IP4

IP6
RF Units

IP3

IP7

Underground
FibresPrevessin(PCR )

IP2
IP8

IP1

Sector 23

Sector 27

IP 2
231

232

1
2
233

1

2
273

Sector 43

272

271

Sector 47

IP 4
1
2
432

2
1
433

1

2
473

Sector 63

1
2
472

Sector 67

IP 6
631

632

1
2
633

1

2
673

Sector 83

672

671

Sector 87

IP 8
1
2
832

Installed SC
Sub-Unit

1
2
833

1

SC Sub-Unit
to be Installed

2
873

1
2
872

Copper Cavity
Unit

Figure 1. Present Distribution of the RF System in LEP

For the first phase of LEP and operation around 45.5 GeV,
128 room-temperature coupled-cavity assemblies were
installed as eight separate RF units distributed around theInteraction Points (IPs) 2 and 6. Each unit consists of 16
cavities, two 1 MW klystron power sources operating at
slightly different frequencies, high voltage power supply,
auxiliary equipment, low level RF and controls. For the LEP2
upgrade to energies approaching 90 GeV 12 additional RFunits based on super-conducting (SC) cavities, 192 in total,
are at present being installed and commissioned. These new
RF units make use of a similar infrastructure as far as
possible. However both klystrons operate at the same
frequency and each powers its own group of eight cavities.
The RF on the two groups (sub-units) is therefore controlled
independently. At the time of writing seven SC cavity subunits are operational at IPs 2 and 6 together with the original
copper cavity units. In the coming year a remaining sub-unit
will become operational at IP 2 and four more SC cavity units
will be commissioned at IPs 4 and 8. The total number of RF
sources to be controlled will then be 32. The distribution of
the RF system around the circumference of LEP is illustrated
in Figure 1.
Operation of LEP requires that total RF voltage seen by thebeam be maintained at a value which gives the required
synchrotron tune Qs at all times i.e. during injection,
accumulation, energy ramping and physics running. At low
energy, variation in Qs produces harmful effects on the beam
due to the crossing of synchro-betatron resonances. At highenergy a value providing sufficient beam quantum lifetime
must be maintained. At high energies it also becomes
important to maintain symmetrical distribution of the RF

voltage around the machine. The RF system is not evenly distributed around the machine and the various types of units
and sub-units have different available minimum and maximum RF voltages. Furthermore particular RF units may besubject to field limitations or be in fault condition for certain periods. During running a unit may trip at any time due toany one of a large number of causes.
The method originally used for control of the total RF voltage was based simply on individually setting the voltage ofeach unit to get the required total. For energy ramping, when the total RF voltage must be increased to maintain Qs, local
RF voltage function generators were used. These were pre-loaded with calculated values prior to the ramp and triggered
simultaneously by ramp events transmitted over the LEP general machine timing (GMT) system. This worked
satisfactorily provided that all units continued to contribute the expected voltages during the ramp. It did not cope with
unexpected changes in the state of individual RF units due to faults or interlock trips. In addition the calculation andloading of the individual ramp tables (300 values) for each new configuration was time consuming.
A single overall RF voltage control system is necessary. The problems in implementing this with classical analog
techniques lie in the increased complexity of the local interface and the large distances involved. A computer based
system with access to the RF units, monitoring their states and controlling individual voltages as required, can be made toprovide sufficient performance with minimum RF hardware modification and can provide a high degree of flexibility.
Initially a simple software system, running on a central workstation using the existing Ethernet and local GPIB
connections to the low-level RF controllers of each RF unit was used to successfully maintain fixed voltage at injection orat top energy. However this was much too slow to follow the ramp with the necessary precision. Fixed bounds on the
response of the system could not be estimated since both Ethernet and GPIB bus transaction times were dependent onother traffic and activities.
PCR Rack

II. THE GLOBAL VOLTAGE CONTROL SYSTEM
An independent dedicated global RF voltage control(GVC) system based on a central controller with direct
links to the equipment was first tested in 1993 [1]. This
overcame the limitations described above and could
maintain the RF voltage at its required value at all times,
including the ramp. An improved version having better
communications handling and increased functionality
was made operational at the beginning of this year. This
new system is also capable of adjusting the distributionof the RF voltage around the machine to avoid or at least
minimize the effects of RF asymmetries on machine
performance. Individual operating levels for the various
units are now determined by a fixed strategy rather thanby operator choice and this helps to provide betteroverall machine reproducibility.
General Layout

LEP Timing
Ethernet

VME Crate
Master CPU 040
System Controller

LEP Timing Receiverand Counter Interface

VME BUS18 Lines
RS232
Slave CPU 010
up toCommunication Controller 1
19.2Kb18 Lines
Slave CPU 010 RS232
up to2
Communication Controller
19.2Kb

TDM MUX . RF P2 TDM MUX . RF P4 TDM MUX .RF P6

TDM MUX . RF P8

4 TDM Link 2Mb G703 / HDB3 one for each point
TDM Link 2Mb G703 / HDB3LEP RF Point

TDM MUX . RF P4

The general layout of the present system is shown in
Figure 2. The central controller is situated in thePrevessin control room (PCR). TDM channels at 2 Mbit/s
have been allocated for each of the points of LEP where
RF is already installed (IPs 2 and 6) or will be installed(IPs 4 and 8). Serial line multiplexers at the end of each
TDM link provide individual connection between thecentral controller and the RF units at each point. In the
PCR connection is by sets of RS232 lines, one for each
unit. In the underground klystron gallery where thedistances are up to 500 metres the connection to theremote RF units is by RS422 differential transmission.

8 Lines
RS232

RS232 to RS422
8 Lines
RS422
To all other LL1 Eq. Controllers
Low Level
Low Level
Eq.Controller Eq.Controller
Unit 231
Unit 232

Low Level
Eq.Controller
Unit 271

Figure 2. The Global Voltage Control System

GVC Controller
The central controller is a VME crate containing a 68040-based CPU module. Two slave 68010 CPU modules carry
the multiple serial IO controllers for communication with the RF units. The OS-9 operating system is used. The global
voltage control function is carried out by a C program which continuously monitors the state and RF level of each of the
RF units or sub-units and applies corrections as required. Information on current unit states, operating levels (minimum
and maximum) required voltage and symmetry method are stored in a local data module. Concurrent data transfer to thedifferent units is implemented by having multiple communications handlers, one for each unit or sub-unit. These are in asleep state until triggered simultaneously by operating system events sent by the main program when data is to betransferred. All data, previously received or waiting to be sent, is stored in the main data module. Other control and statusacquisition programs read or set data in this module.

Data Transmission and Communication
START

The LEP time domain multiplexing (TDM) system is used
for the dedicated connections needed to guarantee fixedmaximum access time. This system uses fiber optic point-topoint links between the PCR and each of the IPs of LEP. It is
used to carry machine timing systems and for other dedicatedloop control applications. The lowest level provides 2 Mbits/s
data channels with HDB3 protocol, the physical connection
being to the CCITT G703 standard. Commercial equipment isused to further multiplex serial data channels within thisbandwidth, 31 channels of 64 kbit/sec data being available. For
each IP eight bi-directional serial channels presently operating at
9,600 Baud are used to connect the RF units and sub-units.
Low Level Equipment Interface
The serial RS422 line is connected to the low level
Equipment Controller (EC) of the RF unit. This G64 hardware
based EC contains the interfaces which allow the setting of RF
voltage and phase, the reading of the RF sum of all cavity
voltages and the state of the voltage control loop. The Low
Level ECs in all units are fitted with a VME 68030 typeprocessor module and VME to G64 converter [2]. Interrupt
driven multi-tasking software is used. The global voltage control
system can therefore set values and read information in the same
way as for normal control via GPIB but independently of allother internal processes and with maximum priority.
System Operation
The main control program operates in a loop, continuously
monitoring the state and voltage levels of the active RF units.
This is shown in detail in Figure 3. If a new total voltage
reference is required the voltages on the individual units are
incremented or decremented accordingly. If a change in state ofan active unit is found the voltages are redistributed in order tomaintain the required total. The system is not a closed loop in
the normal sense, since the actual voltage read by the cavity
voltage detectors on the RF cavities is not used, but instead thecurrent setting is used, together with a status bit indicatingcorrect operation of the unit. This was done to avoid problems ofN
Reboot = TRUEY
Set default

Init Data

Y

Read
Units
Data
Y

Unit = OFF

GVC = ONN

Read
Units Data

Set Unit
Mv = 0.0N

Calculate
Total Mv

Get Ramp
Table Ref Ramp Ref
Const Ref

Get Ref Mv

Total Mv

Get Ref

Calculate
Sector Mv
Reference

Ref > maxNY

Tot = ref(+/- 4Mv)

Y
No action

No Action

N

ASYM

Y

Calc.
Units Ref
ASym

N
Y
IPSYM

Calc
Units Ref
IpSym

SYM

Calc
Units Ref
Sym

Figure 3. GVC Program Flow

Send
Units
Data

instability resulting from noise and field oscillations on the RF units. The two modes of operation are defined in terms of
the source of the voltage reference :1) CONSTANT modeThis mode is applicable to injection and coast. The reference is a value of total RF voltage in MV. The operator
selects the Qs value required. Sloppysoft application software calculates the corresponding voltage reference for the GVC
system taking the relevant machine parameters and settings into account.
2) RAMP mode
At the beginning of the fill the total RF voltages required at each 0.125 GeV step in energy during the ramp are loaded
into a table in the central controller. The reference voltage during the ramp is derived from this table using a timer started
and stopped by the ramp events sent over the GMT. At the end of the ramp CONSTANT mode is selected and Qs trims
are made if required.

Operations Interface
The system is set up and controlled fornormal operation by the standard LEP
operations software package, 'Sloppysoft'.
Information about which units are to read,which units may be acted on, their available
levels and other relevant information, is stored
in an ORACLE database. This data, known as
the RF Current Data Set (CDS), is used by all
applications programs which deal with the RF
system. The CDS is transferred to the GVC
system when the running configuration ischanged and during the setting up procedure
prior to each LEP fill.
Figure 4. Display of Voltage in RF Sectors.
An X-window-based application, using acommercial graphics package, has been written
to monitor the operation of the system and to display the current RF voltage and its distribution around the machine. An
example of the data display is shown in Figure 4. This shows symmetrical distribution of RF voltage at each side of IPs 2
and 6.

III. CONTROL OF TOTAL RF VOLTAGE AND
RF ASYMMETRY
10000
Lifetime (mn)The total voltage must be maintained at avalue which avoids synchrotron resonances(important at injection) and provides sufficientquantum lifetime at top energy. Quantum lifetime
decreases rapidly once the RF voltage decreases
below a certain value as shown in Figure 5 and theRF system must be run with sufficient reserve (80MV at least) in order that the GVC system canrestore the effect of a unit trip before other trips
occur.

Lifetime vs. RF Voltage
8000
6000
4000
1985 MV2000

15 hrs lifetime

0
1600

1700

1800

1900

2000

Rf Voltage (MV)

Figure 5. Quantum Lifetime vs. RF Voltage at 88 GeV.
The effects of IP RF asymmetry on machine
operation and performance are mainly related todifferences produced between electron and positron energy variations around the machine. Synchrotron radiation losses
will be enormous in LEP2, around 1730 MV per turn at 88 GeV and this represents a significant fraction of the total beam

energy. There is a gradual, almost linear, decrease in energy as the particles traverse the arcs. Energy is restored by the
RF units around the RF equipped IPs, by an amount VRF sinØs where VRF is the RF voltage amplitude and Øs is thesynchronous phase angle. This effect is known as the energy sawtooth. The shape is directly dependent on energy and on
the RF distribution around the machine.
Examples of different energy sawtooth variation for different RF distributions are shown in Figure 6 for the simple
case of RF installed at IPs 2 and 6 only. With completely symmetric RF distribution electrons and positrons have equaland opposite energy (and position) variations from the center. With IP 6 providing more RF voltage the amplitudes are
IP 2
IP 4
IP 6
IP 8
IP 2
1) Equal Distribution IP2/6
increased, there are energy differences between
electrons and positrons at IPs 4 and 8 but the center- E
e+of-mass collision energy is the same. Similarly if RF• Even energy sawtooth
voltage is not equal on both sides of a given IP thesame effect occurs. The IPs are equipped with low
beta insertions which have strongly focusing super
econducting quadrupoles. The insertion can only becorrectly matched for one particular energy and errors
produce modulation of the beta functions, phase
IP 2
IP 4
IP 6
IP 8
IP 2
2) Asymmetry IP2/6
advance differences in the cells for either or both
• Uneven energy sawtoothE
particle types and a vertical tune difference (Q split).
+• Increased diff. orbit e+ ee
These effects are difficult to handle in operations and• Different E e + e- Pts 4 & 8
result in lost luminosity and background problems.
E diff = V 2 - V 6
The RF voltages around the machine must therefore(Same E cm)
ebe set to minimize these effects as far as possible
under all conditions.
The GVC system can be operated with various
conditions on symmetrical RF distribution. These canbe specified by the operator and the system will
maintain these as long as the available voltage ateach point or individual unit permits. After limiting
values for symmetry have been reached the RF
voltage at each sector or on each unit is simply
increased by an amount proportional to that which
remains. The following conditions are allowed for :

IP 2

IP 4

IP 6

E

IP 8

IP 2

3) As 2) but asym. IP6 L/R• Difference orbit as for1)• Different E e + e- L/R Pt6
E diff = V L - V R(Same E cm)• Tune split e+ e - from QS0s

Figure 6. Energy Sawtoothing and RF Asymmetry.

• Asymmetrical - All units maintained at the same fraction of their individual maximum voltage. This allows ramping
to maximum voltage with all units reaching maximum at the same time, without symmetry considerations• IP symmetry - Equal voltages from RF on either side of the IP, all units at each side having the same fraction of theirmaximum.
• Symmetrical - Equal voltages at opposite interaction points, This can be with or without IP symmetry.
During 1995 the RF system was run with IP symmetry, to minimize Q split between electrons and positrons in theinteraction points equipped with RF.

IV. SYSTEM PERFORMANCEThe effect of switching an active unit off is shown in Figure 7. The detector sum of all units was logged at 2 second
intervals using a commercial instrument control package independently of the GVC system. Unit 231 tripped just before
the start of the ramp. The system increased unit 232 to maximum, decreased sector 27 to maintain IP symmetry andincreased all units at IP 6 to maintain the reference total voltage. This was done within one logging interval. The time of
response to a change in RF reference is dependent mainly on the time taken to change the RF units to the new values.
The normal ramp rate for the RF unit is 10% of maximum per second, but this can be increased to 40 % per second. The
complete cycle of data acquisition, RF unit voltage recalculation and initiation of the voltage changes required in the RFunits and sub-units is of the order of 200 ms. If an RF unit trips and Qs is not too close to a harmful resonance or near thelimit for very low quantum lifetime thevoltage will be restored before critical beamloss. The overall response is also sufficiently
fast to allow the voltage ramp function to befollowed with an undetectable error in Qs.

.

Figure 7. Response to Unit Trip at Start of Ramp

V. IMPROVEMENTS FOR HIGHER ENERGIESFor running with SC cavities it may be necessary to de-tune certain RF cavities and perhaps adjust RF unit phases
during acceleration. This can be done by the GVC system which can issue direct commands directly to the low level
equipment or over the general control network to other equipment where time is less critical. To improve performanceand to simplify operation the SC cavity units will be equipped with a fast RF feedback loop operating on klystron drive
level instead of modulating anode voltage. This will allow a much faster ramp rate and the limitation in overall response
will then be that of the GVC system. The speed of the system will therefore be increased during the shutdown by
increasing the baud rate from 9600 to 19200. Individual commands sent to the RF unit will also be grouped to cut down
the number of transactions.
A further general improvement would be the replacement of the RS232/TDM based communication system withdirect links. The available options are being evaluated.

VI. CONCLUSIONS
A global RF voltage system has completely replaced the original local function generator based system. It was madeoperational for the 1995 startup and used routinely throughout the running period. It has successfully prevented beamlosses due to RF unit trips. It has maintained symmetric RF distributions and helped to provide improved and more
reproducible beam conditions. The addition of new RF units for LEP2 is straightforward and the system is able to handle
all future requirements which can be envisaged at present. Software improvements will continue to be implemented andhigh speed data links evaluated for the replacement of the multiplexed TDM system.

VII. ACKNOWLEDGMENTS
The TDM channels were provided by the SL-CO group and we are grateful to the specialists concerned for theirassistance. The addition of GVC control in LEP operations software was made by Paul Collier.

VIII. REFERENCES[1] E. Ciapala, A. Butterworth, E. Peschardt, "Global Voltage Control for the LEP RF System," Proceedings of the 1993 Particle
Accelerator Conference, Vol. 3, p1903 (1993)[2] E. Ciapala, L. Arnaudon, "Use of VME CPU Modules for the Performance Upgrade of G64 Equipment Controllers for the LEP
RF System," Proceedings of the Fourth European Particle Accelerator Conference, Vol. 3, p1833 (1994)