J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 460463

Simultaneous Measurement of Coarse and Unscheduled
Rapidly Changing Plasma Parameters during
Long Pulse Discharges
PUJARA Harshad Dayaram and KUMAR Amit Srivastava
Institute For Plasma Research, Gandhinag,ar - 382 428 India(Received: 18 January 2000 / Accepted: 14 June 2000)

Abstract
In a long pulse discharge plasma parameters changes rapidly because of unpredictable unknown
events. These unpredictable rapid changes in plasma parameters should be examined in detail in order tounderstand the properties of plasma. Normally the unknown events causes a fast changes in plasma
parameters and to capture these rapid changes one need to change the sampling rate from the normal
sampling rate. This requires to digitize the plasma parameters at two different sampling rate, one at slow
sampling rate during the low activity period and another at fast sampling rate to capture the changes
occurring at the time of unknown events. This demands for simultaneous coarse and unscheduled rapidchange measurement of plasma parameters at different sampling rates. To fulfill above requirement aCAMAC digitizer module has been designed to acquire the data for the whole shot at one sampling rate
and to capture the unscheduled rapid changes at higher sampling rate simultaneously. The module iscapable of detecting unknown event and changes the sampling rate by ten times for rapid changes
measurements. It can store 16 K samples for fast events and 32 K samples for whole shot duration.
Besides, the module has some other important features, different modes of operation, a monitoring mode,a single shot mode (Pre/Post trigger acquisition) with selectable sampling rate upto I MHz and a losslesscontinuous acquisition mode. The module was tested for I kHz nominal sampling rate for 3.2 seconds
data and l0 kHz at the time of event occurrence for 160 msec. data. with 8 K pre trigger samples.

Keywords:
CAMAC, data acquisition, steady state, digitizer

1. Requirements Involved in long Pulse
Digitizers
Nowadays, because of the advent of continuousresearch.

Specifically the data management due to large
amount of data generated by experiment will be very
crucial. The devices with long pulses or continuous
running device requires, a fundamentally different typeof digitizers. The important difference between pulsed

measuring, multi-channel diagnostics and elongation ofpulse duration, the data volume has grown excessively
high. Acquisition and storage of data at high rate isneither practical for technical reasons, nor necessary
because much of data does not contribute to better
insight into the observed phenomena. Data acquisition
design should therefore develop methods to acquire or

and near steady state operation of acquisition system
will be in the acquisition technique of signals. In thecase of pulsed operation, the most common way is toacquire data and store it in a local storage. But the same

keep only the data that are required for scientific@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research460

Pujara H.D. et a/., Simultaneous Measurement of Coarse and Unscheduled Rapidly Changing Plasma Parametersconcept cannot be adopted for steady state devices sincemeasurement can be selected by CAMAC commands.

it would lead to a very large size of dual ported data
storage elements on the digitizer. A more suitable

The available sampling rate is from 0.1 Hz to I
MHz. A sampling clock that has been selected forapproach would be event-based acquisition. In such andetailed measurements is divided further and used as asampling clock for coarse measurements. The analogue
signal is digitized at a fast sampling rate and digitizeddata is first places in the FIFO buffer. FIFO is read out
at same rate after taking the pre trigger data.
The data read out from FIFO are stored in theRAM memory at selected coarse sampling rate. e.g. if
the sampling rate for 100 kHz is selected for detailmeasurements and l0 kHz has been selected for coarse
measurements, then in FIFO every sample at 100 kHz isstored and every tenth read out from the FIFO is stored
into the RAM buffer for long term storage. Saving data
into the RAM buffer is started after taking the preapproach most of the time data are acquired at a

relatively lower sampling rate during low activity
period, but when a given event occurs, acquisitionsampling rate is increased to a level fast enough toextract the maximum information for a limited period.
The demand placed on instrumentation is considerable
in case of event driven sampling based data acquisition.
It will require a system that can deal with continuous
flow of events as well as event detection anddistribution systems.
For not losing any physics information during shot,
all digitizers have to be ready to sample data at required

trigger data. Once the acquisition is started with start
trigger, the acquisition can be stopped by stop trigger

highest rate, to detect the event and to change sampling
rate when event takes place. This ensures no loss of any
physics information. The digitizer system should
automatically recognise when it is necessary to begin
taking data, so that data can be continuously taken andpulse.

The stop trigger is generated on detection of fastevent. On receiving the stop trigger the FIFO storage
contains the data at 100 kHz for last (10 micro sec x 16K) 160 milli sec. And RAM contains coarse data for(100 micro sec x 32 K) 3.2 seconds.

archived until the pulse ends. The local front-end
storage facilities should accommodate and manage thisvoluminous acquired data. Further, the embedded
intelligence should determine whether to increase or

decrease the sampling rate depending upon theexperimental status to take care of any schedule or

2.1 Block diagram
The block diagram Fig. I shows how various

unscheduled events.

elements are internally connected to various buses for

Most of the available digitizers in CAMAC do notsupport for simultaneous reading and writing of data.
Either the module is in the digitization mode or in acomputer read out mode. Hence it can not support
lossless continuos acquisition. Moreover it does nothave any event detection hardware on board. It has avery limited local storage and hence cannot support highachieving the described features.
r CAMAC access to program various registerso Transfers of data from ADC to FIFO and RAM
memory.

r Transfers from Memory to CAMAC data way
r Clock Generation.

sampling rate for a long time. Limited storage compels
one to restrict the sampling rate to value low enough toaccommodate the entire shot. Hence one loses the highfrequency component which may originate fromunscheduled events. The scheme described below
address these issues.

2. Hardware Description
The designed CAMAC digitizer consists of two
data storage elements for storing the data at two
different sampling rates. The first one uses a FIFO (firstin first out memory), stores the data at faster samplingrate. Another, which is a RAM memory, stores data atslower sampling rate for coarse measurements.
Sampling rate for detailed measurements and coarse

Fig. 1 Block Diagram461

Pujara H.D. er al., Simultaneous Measurement of Coarse and Unscheduled Rapidly Changing Plasma Parameters. CAMAC function decoding.
. Control Logic Generation

2.1.5 Stop trigger generataon
To detect the fast event a comparator and a DAC

are used. The anolog signal is compared with athreshold generated by DAC and generates the stop2.1.1 Clock generation
The master oscillator is running continuously and isgenerated using a 10 MHz crystal. For sampling clocktrigger. During the initialization of module, a required
number is loaded to DAC latch for generating required

this basic clock is further divided by various stages ofcounter. Multiplexer selects the required fast samplingclock via CAMAC commands. The same clock isfurther divided by ten for slow sampling rate.

DC threshold.

2.1.6 Control logic and timing
The required logic for controlling various elements

2.1.2 ADC

is generated using gates, flip-flops and sequential

The selected ADC is flash type 8 bit ADC. It has
built-in reference voltage source and sample & holdcircuits. The selected sample rate clock is applied tocircuits.

start conversion for digitizing the analogue signal. The
digitized data is latched and made available to CAMAC
data bus for on fly read out as well as to FIFO for data

A user friendly software has been developed using
LabAVindows CVI which provides a graphical panel tothe user for different modes of operation. The module
can be utilized in various operating modes. With thehelp of control panel user can select various modes of

3. Application Softwarestorage.

2.1.3 FIFO buffer
The FIFO (first in first out) type of memory
supports simultaneous reading and writing to it. The

operation, configure the module, plot the data points andcan make the hard copies of plots. Figure 2 shows dataacquired in a single shot mode and control panel forvarious modes of operation. Figure 3 shows dataacquisition with event. The first plot shows the complete
32 K data points shored in RAM memory which is taken
at I kHz sampling rate. Second plot shows the selected
4 K data points of shot. While third plot shows the datafrom the FIFO around the event occurrence with moredetails which has been taken at 100 Hz sampling rate.

continuous lossless acquisition is made possible becauseof this feature. Initially the FIFO is reset by CAMAC
init. command F9. Once the digitization is started, by

F28 CAMAC command, digitized data is written toFIFO on every sample clock. Once the Half-Full (HF)
flag is set, the same clock is applied to read out theFIFO and FIFO enters into a dummy read out mode.
The HF is used for LAM generation in continuos
acquisition mode. This is continued till the start trigger
is applied externally or by software F25 command. On

receipt of start trigger, the data transfer to RAM
memory starts. This goes on till the stop trigger isreceived. On receipt of stop trigger the FIFO stops
reading out and stores the data till full flag is generated.

2.1.4 RAM buffer
Data for the entire shot is stored in 8 Bit by 32
Kbytes read/write memory. With initialization on F9command this modtrle goes into the write mode and alladdress generation counters are set to zero pointing at

first location of memory. The data read out from FIFO
after the receipt of the start trigger is stored into thememory. The address generation logic increments
address on every sampling clock selected for coarse
sampling measurements. On receipt of stop trigger it
continues storing data till the full flag of FIFO stops theclock.

Fig. 2 Data in single shot mode462

Pujara H.D. et c/., Simultaneous Measurement of Coarse and Unscheduled Rapidly Changing Plasma Parameters

3.1 Operating modes
The module can be operated in three different
modes. Single shot with dual sampling, capturing theevent. Monitoring mode for long term monitoring byreading the current ADC data value. Continuous
acquisition mode where transfer of data from FIFO
buffer to host takes on half-full flag. Figure 4 shows theflow diagram and operation sequence for deferent modeof operation.

4. Summary
The digitizer was tested with various different
kinds of signals and operated in all deferent modes. The
events were generated to by varying the amplitude byprogramming the arbitrary waveform generator. Theplots of acquired signals clearly shows finer details of
waveform obtained in FIFO buffer whereas the plots of

RAM buffer which takes at ten times slower sampling
rate to accommodate entire shot. One of the mostsignificant advantage of the module is that it captures
entire shot at slower sampling rate, ten times fasterFig. 3 Event capturing in single shot mode

sapling rate around event for detailed data, on board
event detection with polarity and magnitude, everythingin single module.

Acknowledgement
The author express their sincere thanks to all themembers of data acquisition group for their timely help.

Special thanks to Mr. Sharma for his well-plannedwiring.

References

tll Eriko JOTAKI and Sataoshi Itoh, A Data Acquisition Method against Unpredictable Events during
Long-time Discharges and its Application toTRIAM- lM Tokamak Experiment. FURKU Re-Fig. 4 Sequence of operations in different modes ofoPeration

port 97-05(46).

I2l Eriko JOTAKI and Sataoshi Itoh, Continuous
Monitoring and Data Acquisition System for
Steady-State Tokamak Operation. Fusion T ech. 27March 1995.
t3l Paul C. Van Haren and Noud A. Oomens, Data Acquisition System for Fusion Device. Fusion Tech.
24 1993.

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