IEEE Transactions on Nuclear Science, Vol. NS-34, No. 4, August 1987
DIII-D TIMING SYSTEM
GABRIEL BRAMSONGA Technologies Inc.
San Diego, California 92138, U.S.A.
Abstract
The DIII-D tokamak, an upgrade of Doublet III, a de-
vice to study magnetic confinement fusion at GA Technologies,became operational in February 1986. The new timing system
consists of two major subsystems. The trigger subsystem syn-chronizes control and data acquisition for the experiment. The
clock subsystem is a centralized source of pulses for data ac-
quisition equipment. The CAMAC hardware and the database
software for the DIII-D timing system is discussed.
Introduction
The DIII-D tokamak, an upgrade of Doublet III, a de-
vice to study magnetic confinement fusion at GA Technologies,became operational in February 1986.1 This paper discussesthe CAMAC hardware and the database software for the new
DIII-D timing system which consists of the trigger and clock
subsystems.
Section 1 is a brief description of the DIII-D experimentalenvironment and data acquisition system. Section 2 describes
the design criteria of the DIII-D timing system. Section 3 de-
scribes the trigger subsystem which synchronizes control and
data acquisition for the experiment. Section 4 describes the
clock subsystem which provides a centralized source of pulsesfor data acquisition equipment. Finally, Section 5 describes
the integration of the timing system with the experimentaldatabase, ending with a short summary.
1. Data Acquisition
In this paper, the DIII-D tokamak and its associated
power systems7 auxiliary heating systems, and diagnostics are
considered as a black box which requires control signals and
provides data signals. A tokamak discharge occurs when gas
is injected into the DIII-D vacuum vessel and is ionized, be-
coming a plasma. An experimental run consists of tokamak
discharges which last a few seconds and are repeated every 10
to 15 minutes. Before each discharge, control and data ac-
quiisition equipment is initialized. During each discharge, in
real-time, data is collected by CAMAC digitizers, scalers, and
other modules. Currently, DIII-D collects 12 Mbytes of data
per discharge.
Figure 1 is a block diagram of the DIII-D data acquisi-tion system. Machine operators and physics operators run the
experiment with the tokamak control computer, a MODCOMPClassic II/25. Power supplies for discharge initiation and con-
trol, gas injectors, and the timing trigger subsystem are among
the devices controlled.
FIG. 1. DIII-D data acquisition system hard-ware block diagram.
CAMAC data acquisition equipment including the timingclock subsystem is initialized by the data acquisition comput-ers. Following each discharge, in near real-time, data is fetched
from CAMAC modules by the data acquisition computers. Data
is funneled through the central data acquisition computer, a
MODCOMP 7870, and transferred to the tokamak experimentaldatabase residing on DEC VAX clusters where it is available for
analysis and display. Figure 2 is a display of several signal timehistories for a typical tokamak discharge.
This includes interpreting what may be complicated timinginformation for a user retrieving data from the experimentaldatabase.
Additionally, the DIII-D timing system can easily be ex-panded to handle more triggers and clocks as needs arise.
3. Trigger Subsystem
Hardware
Figure 3 is a block diagram of the trigger subsystem hard-ware. CAMAC delay generators (Jorway 220s and 226s) supplypulses to a "timing generator" which uniquely codes each pulseand sends the sequence of trigger codes (called a timing chain)to several CAMAC "timing receivers" distributed throughout thefacility. Each timing receiver channel may be selected to pulsein response to one of the trigger codes. Timing receiver out-puts provide trigger pulses for any piece of equipment includingclocks and digitizers. A tokamak discharge is initiated when thetokamak control computer starts a timing chain.
1750 2000
FIG. 2. Example plot of DIII-D signalsversus time.
2. Design Criteria
Limited development time for the new DIII-D timing sys-tem required the use of existing or off-the-shelf components.Software implementation was required to remain within thescope of the existing data acquisition and control database andcomputers.2 In addition, the following design criteria, whichhave been met, were established for the DIII-D timing system.
The trigger subsystem provides synchronization duringtokamak discharges for the DIII-D experiment subsystems in-cluding power systems, auxiliary heating systems, and diag-nostics. The clock subsystem provides centralized and flexibleclock waveforms for data acquisition equipment including dig-itizers, scalers, and counters. The capability of measuring andresponding to external asynchronous events is available.
Physics operators are able to inspect and change easilytiming parameters such as trigger times and clock frequenciesbetween discharges without compromising the consistency be-tween the hardware configurations and software descriptions.
Any and all timing parameter changes are automaticallyand transparently propagated throughout the data acquisitionand control database and the central experimental database.
FIG. 3. Trigger subsystem hardware blockdiagram.
In more detail, each CAMAC delay generator channel isprogrammed to countdown from a start input before outputtinga pulse to a timing generator input. A common 1 MHz masterclock (Jorway 217) drives the delay generators.
.5 I i -7
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c0-4
C:)--4
730
The timing generator is the heart of the trigger subsys-
tem. A pulse at any timing generator input generates a unique
sequence of ten high or low voltage levels comprising a 10,us
wide trigger frame. A trigger frame is represented by a 2-digit
hexadecimal code corresponding to the data bits in the trigger
frame. Each trigger frame is directed in parallel to an unlim-
ited number of outputs. Thus, as pulses appear at the timing
generator inputs, a sequence of trigger frames (or hex trigger
codes) called a timing chain is distributed from each timing
generator output to several timing receivers. A priority scheme
in the timing generator hardware mediates among simultaneous
input pulses. Figure 4 is a typical DIII-D trigger time line.
FIG. 4. Trigger time line.
Software
The trigger subsystem CAMAC delay generators are con-
trolled by the tokamak control computer. For each delay gen-
erator channel, an entry in the control computer database as-
sociates the channel with a name, a CAMAC address, and a
CAMAC function. Another database structure, the trigger ta-
ble, associates each CAMAC delay generator channel name withthe following parameters.
* The hex trigger code corresponding to the timing genera-
tor input to which the delay generator channel is attached.
* The delay countdown start source, usuially another delay
generator channel.
A time relative to time zero at which to pulse. Time zero
is defined by initiation of plasma current.
* The trigger type, which is described below.
As part of discharge start-up procedures, the tokamak
control computer uses the database information to initialize
the CAMAC delay generators.
Trigger Types
During discharge procedures, tokamak control computercommands act as "manual" triggers to initiate separately "pre-trigger" and "synchronous" trigger timing chains. Other triggertypes include "calculated" and "asynchronous" triggers.
Pretriggers occur several seconds to minutes before a dis-
charge. They are used by various DIII-D systems to auto-
matically perform set-up tasks which require some time but do
not require synchronization with time zero. Examples include
ramping up high voltage supplies and positioning mirrors.
Synchronous triggers are well-defined relative to timezero. The first synchronous trigger is the master synchroniza-tion trigger typically occurring at -19 sec. Other synchronoustriggers are used to inject gas into the tokamak and start digi-tizer data collection. Typically these triggers occur before time
zero. Synchronous triggers typically occurring after time zero
include the firing times for the Thomson profile diagnostic laser
and the electron cyclotron auxiliary heating system gyrotrons.
Calculated triggers are also synchronous to time zero. Un-
like pretrigger and synchronous trigger times which may be set
by the physics operators, calculated trigger times are deter-
mined automatically by the tokamak control computer based
on desired discharge parameters such as the plasma current
level.
Asynchronous triggers are generated in response to ex-
ternal events which may occur at any time relative to time
zero. Examples include plasma current disruption and electron
density thresholds. The major use of asynchronous triggers is
digitizer importance sampling. The time relative to time zero
of an asynchronous trigger is measured by CAMAC elapsed time
counters as described later.
4. Clock Subsystem
Hardware
Figure 5 is a block diagram of the clock subsystem hard-
ware. The main components of the clock subsystem are CAMAC
programmable clocks (Jorway 224s) which are loaded with the
PRETRIGGERShex P P P P P p ' pcode: .60 61 62 63 64 65 66 67
approx. -1:25 -1:10 -1 min -50 s -45 s -40 s -30 s -20 s
tures offer a wide range of data sampling options making the
system very flexible. Figure 6 shows a typical clock waveform.
TRIGGER PULSE OUT TRIGGER PULSE OUT
OR gate
xs ternaltrigger in PROGRAMMABLE
CLOCKclc outI
CLOCK WAVEFORM
xte:irnalclock in
DIGITIZER
stop01 trigger in
FI,G. 5. Clock subsystem hardware blockdiagram.
desired clock waveforms before each tokamak discharge by the
data acquisition computer. Clock waveforms consist of one to
sixteen programmable domains. Each domain has an associatedclock frequency which is output for a preset number of counts
or continuously. Furthermore, a domain may be programmedto wait for an external trigger, and/or output pulses until an
external trigger, and/or inhibit clock pulse output. Addition-ally, each domain may recycle up to 16 times and the entire
sequence of domains may repeat up to 16 times. A common
1 MHz master clock (Jorway 217) drives the Jorway 224s.
One of the Jorway 224 clock inputs is for an external trig-ger. If any clock domains are programmed to respond to a trig-ger, then a sufficient number of trigger pulses must be OR'edfor the clock to produce the entire clock waveform. In general,timing receiver outputs which correspond to hex trigger codesare used as clock trigger inputs.
The Jorway 224 output waveforms are used as clock in-
puts for data acquisition equipment including digitizers, scalers,and counters. A programmed clu k waveform must provideenough pulses to fill its associated :igitizers' memory. Further-
more, one of the triggers to the clock must be used to start
digitizer "post-trigger" sampling. Importance sampling is ac-
complished by using the same asynchronous trigger to start
digitizer post-trigger sampling and shift the clock into the next
(usually higher frequency) domain. Programming a domain to
inhibit clock pulse output will prevent a digitizer from samplingfor the duration of the domain. These programmable clock fea-
FIG. 6. Example clock waveforms.
CAMAC elapsed time counters (BiRa 2413s) measure thetime relative to time zero of asynchronous triggers. The elapsedtime counters are driven by the same 1 MHz master clock as thetrigger subsystem CAMAC delay generators. Each elapsed timecounter channel counts down from a start input and stores theelapsed count until a stop input. Start inputs are synchronoustriggers. All asynchronous triggers are associated with a stop
input. The time of other events can also be measured in thismanner.
Software
Several database tables exist on the data acquisition com-puter to control the CAMAC clocks and associated data acquisi-tion equipment. Additionally, the trigger table is copied fromthe tokamak control computer each discharge by the data ac-
quisition computer.
A table for each clock has entries for each domain con-
taining the CAMAC commands to produce the desired waveform.For domains with an associated trigger, the hex trigger code is
included in the clock table. An interactive program on the data
acquisition computer reads and updates the trigger table and
clock tables as appropriate to allow the physics operators to in-
spect and/or change trigger times or clock waveforms between
discharges.
STANDARD BASELINE CLOCK WAVEFORMcalculated synchronous trigger 6Ctrigger 70 (could use an asynchronous
4v\ / trigger)
DOMAIN I DOM 2 DOMAIN 3 DOMAIN 4
advance 40 pulses at delay clock pulses continuouslyon 2 ms period until at 2 ms periodtrigger for baseline trigger filling digitizer
samples with doao samples
ANOTHER CLOCK WAVEFORMsynchronoustrigger
Hl DOM 2 H3 DOM 4 H5 DOM 6 H7
The first domain waits for a trigger and then outputsa burst of pulses (eg. 1024 pulses, 2 microsec period).Domain 2 inhibits clock output for a preset amount of time.The pattern repeats for a preset number of bursts.
A table for each elapsed time counter channel includes thehex trigger codes which start and stop its countdown. A tablefor each digitizer includes the name of its associated clock withdigitizer configuration information. The next section describeshow these tables are used during data collection and analysis.
5. Timing System Integration withthe Experimental Database
For every tokamak discharge, the data acquisition com-
puter creates a "pointname" for each raw data channel. Apointname contains the raw data and a header with the infor-mation necessary to interpret the raw data. The trigger, clock,digitizer, and elapsed time counter tables are used to providetime information with each pointname as described below anddiagrammed in Fig. 7.
FIG. 7. Data acquisition database timinigsystem tables.
As a discharge progresses data is collected by the CAMAC
equipment in real-time. When the discharge terminates, the
data acquisition computer performs the following tasks related
to the timing system.
* Copy the trigger table from the tokamak control com-
puter.
* Read the number of counts collected in each elapsed time
counter channel and use the elapsed time counter table to
calculate the time relative to time zero of the associated
hex trigger code (usually an asynchronous trigger).
* For each such hex trigger code, enter this time in the
trigger table.
* From the clock tables, use the clock waveform informationand the hex trigger code entries (if any exist) as pointers
into the trigger table to calculate start and end times
relative to time zero for each clock domain.
* From the digitizer tables, use the digitizer configurationinformation and the clock name as a pointer to the appro-
priate clock table to build a pointname for each digitizer
channel.
Two additional pointnames are also created. One incor-
porates the trigger table and the elapsed time counter tables to
summarize the trigger subsystem. The other uses the trigger
table and the clock tables to summarize the clock waveforms.
Finally, the data acquisition computer transfers the point-names to DEC VAX clusters where they are incorporated intothe tokamak experimental database. Each pointname in the ex-
perimental database includes enough information to construct
the time history of the signal independently of other databaseinformation. A general purpose FORTRAN interface routine isused on the VAXes to retrieve raw data. It can return a time ar-
ray whose elements correspond one-to-one with each raw dataarray element.
In summary, the new timing system for GA Technologies'DIII-D fusion tokamak experiment provides a high degree offlexibility without compromising the integrity of the data. Atone end physics operators may easily modify timing parame-
ters. The tokamak control and data acquisition computer sys-
tems ensure the consistency of the hardware and software. Atthe other end, a user retrieving data for display and analysis isprovided with the necessary timing information transparent to
the intricacies of the timing system equipment and database.
Acknowledgements
This is a report of work sponsored by the U.S. Departmentof Energy under Contract No. DE-AC03-84ER51044.
References
'J. Luxon et al., in Plasma Physics and Controlled Nuclear
Fusion, 1986 (International Atomic Energy Agency, Vienna,1987) (to be published).
2B.B. McHarg, Jr., Hardware and Software of the Doublet III
Diagnostic Data Acquisition Computer System, May 1983,GA Technologies Report GA-A17031.
