Overview of Control Systemfor Jefferson Lab's High Power Free Electron Laser
A. S. Hofler, A. C. Grippo, M. S. Keesee, and J. SongThomas Jefferson National Accelerator Facility,
MS85A, 12000 Jefferson Ave., Newport News, VA 23606
USAAbstract
In this paper the current plans for the control system for
Jefferson Lab's Infrared Free Electron Laser (FEL) arepresented. The goals for the FEL control system are
fourfold: 1) to use EPICS and EPICS compatible tools, 2)
to use VME and IndustryPack (IPs) interfaces for FEL
specific devices such as controls and diagnostics for thedrive laser, high power optics, photocathode gun, andelectron-beam diagnostics, 3) to migrate Continuous
Electron Beam Accelerator Facility (CEBAF) technologies
to VME when possible, and 4) to use CAMAC solutions
for systems that duplicate CEBAF technologies such as RFlinacs and DC magnets. This paper will describe thesoftware developed for FEL specific devices and provide
an overview of the FEL control system.
1 Introduction
Jefferson Lab is building an FEL based on superconducting radio frequency (SRF) technology developedfor CEBAF. The machine will produce 3-6 µm laser lightusing a 350 kV continuous wave electron beam from a
photocathode gun that is bunched into 0.5 psec rms
bunches and then accelerated to 10 MeV in the injector.
After passing through a 32 MeV linac the electron beam isaccelerated to 42 MeV. The 42 MeV electron beam passes
through a wiggler magnet to produce photons that laseusing a pair of mirror boxes located upstream anddownstream of the wiggler. The laser light is extracted andtransported to the user labs for experiments. After passing
through the wiggler, the 42 MeV electron beam isrecirculated through two arcs which deliver the beam to theentrance of the 32 MeV linac approximately 180 degrees
out of phase with the injected electron beam. The 32 MeV
linac extracts 75% of the energy from the recirculated
electron beam and reuses the energy to accelerate theinjected electron beam. The energy recovery scheme
reduces RF power requirements, waste heat, and radiation[1, 2].
This paper focuses on the software part of the control
system. First an overview of the control system layout isgiven followed by a brief description of control systemprocedures, and finally descriptions of FEL specificdevices will be presented. Although the control system
hardware is mentioned in this paper, another paper
presented in these proceedings provides a more in depth
discussion of the FEL control system hardware [3].
2 Control system overview

The FEL control system uses the Experimental Physics
and Industrial Control System (EPICS) [4, 5] which
follows the standard model for accelerator and storage ring
control systems. Initially, the FEL requires nine EPICS
input output controllers (iocs) in individual VME crates.
The operator interfaces (OPIs) will run on five Hewlett
Packard (HP) B class workstations and two HP Xterminals distributed across two control rooms in twobuildings, the Laser Control Center (LCC) in the FEL
building and the CEBAF control room in the Machine
Control Center (MCC). High level applications such as
slow feedback and emittance measurement packages will
also run on the workstations. There are an additional six
HP 715 workstations for the six user labs in the FEL. Two
HP D class file servers resident in the FEL service theworkstations, X-terminals, and iocs. The final operational
FEL configuration calls for monitor and control for all
systems from the MCC which will simultaneously monitor
and control CEBAF. During FEL commissioning and fordaily operations tasks related to high power optics control
and photon-beam delivery, control will be performed in theLCC. The FEL computers and iocs are isolated fromCEBAF systems on an FEL controls subnet.
The nine iocs are located outside the beam enclosure inthe FEL building. Eight of the iocs are dedicated to device
control. One ioc is dedicated to cryogenic controls for theliquid helium for the SRF cavities while the other seven
perform device control for FEL hardware such as thephotocathode gun drive laser, high power optics, linacs,magnets, beam diagnostics, vacuum, and timing system.
The ninth ioc is not directly used for device control. It will
be used to provide FEL system status information for
comfort screens for the FEL operators and users.
Four systems use CAMAC since they mirror CEBAF
machine systems. Two of these, cryogenics and magnets,
are exact duplicates requiring no new softwaredevelopment beyond replication for the FEL components.
The other two systems are the four channel beam position
monitors (BPMs) and RF. Because of FEL design
requirements such as wider vacuum chambers, affecting
BPM sensitivity calibrations, and increased energy gains
per RF cavity, affecting the RF system, these two systems
require some software development to support FELoperations.
Most of the devices for the FEL interface to EPICS
through VME modules or GreenSpring IPs through astand-alone operating system developed at Argonne
National Laboratory (ANL) called HiDEOS [6] running on
a slave CPU in the VME crate. Systems using VME
modules include vacuum, drive laser components such asthe drive laser attenuator, pulse controller, and electrooptical (EO) system, beam profile monitor, photocathodegun high voltage, video digitizer, machine protection, andswitched electrode electronics (SEE) BPMs.
HiDEOS is used heavily in the FEL. Four HiDEOS CPUs
are scheduled for use for interfacing to several IPs. All
GPIB devices in the FEL interface to EPICS using IP-488s
with the EPICS GPIB common device support and the IP488 HiDEOS support [7]. Serial communications through
HiDEOS use IP-Octal support from ANL. At Jefferson
Lab, HiDEOS support for IP-Digital 48, IP-Relay, and IP16ADC has been developed and interfaced into EPICS for
beam diagnostic and laser control and readback. TheHiDEOS support developed for the FEL was tested andfound to be very reliable during early commissioning FEL
photocathode tests.
The control system file structure on the FEL servers andworkstations mirrors on a smaller scale that of the CEBAF
machine. With a resident duplicate version of the filesystem as opposed to accessing the CEBAF machine filesystem via NFS, the FEL is guaranteed quick response
times and independent operations. It is possible to run theFEL without interruption while the CEBAF machine filesystem is down for maintenance activities and vice versa.
The file system consists of three disks. One disk is usedfor storing FEL archive data. A second disk is for FEL
experimenter user accounts, OPI screens, tcl tools foroperations, high level applications, high level application
support packages, and BURT (an EPICS parameter saveand restore utility) files for ioc hourly saves and FEL
machine configurations. The third disk is for UNIX
executables for EPICS tools such as MEDM (an EPICSOPI), data archiver, and archive data viewer; ioc databases,
object code, and boot files; and the electronic logbook.
In addition to carrying over the file system, procedures
associated with modifying the CEBAF control system areapplied to the FEL. These procedures include versioning
software, documenting rollback positions, documentingchanges in operations logs, and scheduling test andinstallation time in coordination with operations personnel.
These procedures lead to a more stable operating
environment because reliable operations is the goal at
every step [8].
3 FEL devices
By now it is clear that the FEL uses many CEBAF
standard devices. In addition to these devices, the FEL has
unique needs for its laser systems, photocathode gun, andbeam diagnostics. Some of these systems are discussed inthis section.
3.1 Optical devices for drive laser and high-power optics
The drive laser is used to generate electrons on thephotocathode of the gun to produce the FEL electron beam.
This laser is located upstairs in the FEL building in an
isolated clean room. The pulsed laser light is transported
from the clean room to the gun downstairs in the beam
enclosure using a series of adjustable mirrors. The drive
laser consists principally of the drive laser, the drive laser

pulser, and modulator.
The drive laser itself is controlled via RS-232 using an IPOctal under HiDEOS. The drive laser software provides
control for the laser, laser shutter, and laser power. Laser
status is updated at a 1Hz rate. If the laser is not working
correctly, warning messages are displayed on the laser
screen. The RS-232 link between HiDEOS and the laser ismonitored. If the communication is lost, a sequence
program tries to restore normal communication.
Pockel cells are used for the drive laser EO system. Three
EO modulators and an RF amplifier are used to control thepolarized beam pulses obtained from the FEL drive laser.
Together, they allow a user to modulate the beam power
output and to select out beam pulses. An EPICS
application provides single point control for this equipment
by controlling the power supplies to the modulators; theRF amplifier is controlled through a pin diode which
provides 0-60 dB of signal attenuation. Communication
with these devices is through a DATEL DVME 628 D/Aboard, with voltage readback from a VME Microsystems
International Corporation VMIVME-3122 A/D board.
The high power optics are used in the lasing process andfor transporting the FEL laser beam from the beam
enclosure to the user labs in the FEL building. The highpower optics consists of a series of adjustable mirrors.
The optics used in the lasing process require alignment ofgimbal mount mirrors in the optical cavity. Two 6" opticalmirrors are used to focus the laser beam in the wiggler
cavity. Each of the mirrors may be moved in yaw and pitchby controlling high precision stepper motors, one for eachaxis of motion. In addition, a fifth stepper motor is used tocontrol the distance between the motors. The mirrors may
be moved in steps of approximately 10 microradians ofangle, or several microns of linear distance. Because it isnecessary to obtain absolute mirror positions, each of themirrors is tracked using linear variable differentialtransformers (LVDTs) capable of accuracies on the orderof several microns. The stepper motors are controlled using
an Oregon Microsystems card in conjunction with Epics
steppermotor records, and the LVDT positions are readfrom a Highland Technology Model V550 VME card. In
addition to implementing motion commands, the software
also uses the mirror positions to calculate the cavity modes
at the wiggler center; conversely, given a set of cavity
modes, the software will automatically position the mirrors
to achieve these modes. Future upgrades will implement an
autolock feature which will use a PID loop to keep themirrors positioned without operator intervention.
Capability exists to save mirror positions on a periodic
basis and when indicated by the operator. Thus, in theevent of system crashes, the mirror alignments arerestorable. A related application for these mirrors allows
the operator to insert pellicles or viewers into the beam,and also to control lamps for these viewers. This capability
allows the user to immediately see the effects ofrepositioning the gimbal mount mirrors.
Picomotors are used in the laser beam transport system.
The picomotor application allows single point control of all
FEL picomotors, currently 9 total. These motors are usedprimarily to make small positional changes in various

mirrors used for beam transport. One motor controls themotors are two-axis units, but there are also single andtriple axis motors. The software controls each of themotors through a New Focus 8732 driver, capable ofhandling up to 24 single-axis, or 8 three-axis motors in thepresent configuration; additional slots can hold connectors
for up to 48 additional axes. The 8732 driver has a GPIB
interface. Individual motors are addressed by setting aslot/connector/channel in GPIB; all subsequent commands
are taken to apply to that particular motor/axis, until theaddress is changed.
3.2 Photocathode gun high voltage controls
This software controls a 500 kV power supply used forthe photocathode gun. Communication with the power
supply is through a DATEL DVME628 D/A card and an
IP-Digital 48, with status and voltage and current
readbacks coming through a VMEVMI-3122. The software
calculates a ramp to safely bring the voltage up to the level
desired by the operator without electrical arcing. It does
this by using a ramp which increases the voltage
proportionally less as the setpoint voltage is approached,and by reading back the current and pausing in the ramp
whenever the current exceeds a threshold value.
3.3 Beam diagnostics
Examples of beam diagnostic devices in the FEL are
optical transition radiation beam viewers, stripline andbutton beam position monitors, multislit beam emittancemeasuring devices [9], coherent radiation bunch length
monitors, and synchrotron light cameras for measuring thebeam profile at high average power. Most devices haveupdate rates faster than one second.
From the controls viewpoint, the beam viewers are
invoked when a particular camera in the FEL is chosen forviewing. An associated viewer foil is inserted in the beamand the video output from the camera is automaticallyswitched to a TV monitor in the control room. Normally,
the video output is not analyzed further. However, bydigitizing the video image with a Datacube MaxVideo
image processing VME card, many beam analysis
functions (e.g. finding beam centroids and profiles) may be
performed at a rapid rate. Such functions are also used toanalyze data from the multislit beam emittance measuring
devices and the synchrotron light cameras.
The video connection system allows video or other typesof signals to be connected to TV monitors, oscilloscopes
and similar display devices. The basis of the system is an
Analog Devices AD8116 crosspoint connection card which
can connect up to 16 inputs to any of 16 outputs. A custom
chassis was developed which stacks 4 of these cards, sothat any one of 64 inputs can be connected to any of 16
output monitors; additional chassis can be added to thelength of the drive laser optical cavity. The majority of thesystem, so that a very large number (up to 4096) of inputs
can be connected to the basic 16 monitors. Also, a single
input can be connected to more than one monitor. This
flexibility is achieved by making the AD8116 cards
individually addressable; connection commands are thensent to all cards, but only the card being addressed will act
on the command. All the chassis are addressed by a single
custom digital I/O card, which is controlled by an Epics
video connection application.
4 Conclusion
Jefferson Lab's FEL is nearly complete andcommissioning activities have begun. The EPICS control
system is supporting commissioning activities. All nineiocs, file servers, and some of the workstations are on-linewith the balance becoming operational in the near future.
Software discussed here is in use at the FEL with revisions
related to operations experience to follow. The softwareschedule closely follows the commissioning activities with
FEL first light in the spring and energy recovery in thesummer of 1998.
References[1] C. L. Bohn, et. al., "Status Report on Jefferson Lab's
High-Power Infrared Free-Electron Laser", FEL
Conference Proceedings, August, 1997 (in press).
[2] G. A. Krafft, et. al., "Electron-Beam Diagnostics forJefferson Lab's High Power Free Electron Laser", PAC
Proceedings, May, 1997 (in press).
[3] Kevin Jordan, et. al., "Control System for a HighAverage Power Free Electron Laser", ICALEPCS theseProceedings.
[4] Leo R. Dalesio, et. al., "The Experimental Physics andIndustrial Control System Architecture: Past, Present,and Future", ICALEPCS, Oct., 1993.
[5] Karen S. White, et. al., "The Migration of the CEBAF
Accelerator Control System from TACL to EPICS",CEBAF Control System Review, May, 1994.
[6] J. B. Kowalkowski, "A Cost-Effective Way to Operate
Instrumentation Using the Motorola MVME162
Industry Pack Bus and HiDEOS", ICALEPCS, Oct.,
1995.
[7] J.
Winans,"GPIB
DeviceSupport",http://www.aps.anl.gov/asd/controls/epics/EpicsDocumentation/HardwareManuals/GPIB/gpib.960325.html,March, 1996.
[8] Karen S. White, et. al., "Control System Reliability at
Jefferson Lab", ICALEPCS these Proceedings.
[9] K. Jordan, et. al., "A Multi-Slit Transverse-Emittance
Diagnostic for Space-Charge-Dominated Electron
Beams", ICALEPCS these Proceedings.