Digital Signal Processing for Plasma Control at KT-1
J.S.Yoona, S.H.Jeonga, K.W.Leea, B.H.Oha, S.J.Hongb, H.J.Choic, D.C.Sonda Korea Atomic Energy Research Institute, Taejeon, South Korea
b Korea University, Seoul, South Korea
c Samsung Advanced Institute of Technology, Taejeon, South Korea
d Kyungpook National University, Taegu, South Korea

Abstract(or the horizontal field power supply) to turn back theplasma column to the equilibrium position.

We report on the status of a feedback control system
based on VME DSP, MIX DSP and ADC modules and areal-time operating system. A feedback control system isadded to an existing analog-based control system installedat KT-1, a small size Tokamak at the Korea Atomic Energy
Research Institute (KAERI). The feedback control system
controls the position and current of the plasma column for
optimum plasma operation at KT-1.
1 Introduction
A Digital Signal Processor (DSP) has various
applications, such as image and voice processing andautomatic feedback control. Recently, DSP has been
applied to plasma position control and verticalstability[1,2]. In this paper, we present the status of afeedback control system based on DSP to control theposition and current of the plasma column at KT-1. Section
2 briefly describes a relevant part of KT-1 to this work.
The existing analog-based feedback control system ispresented in Section 3. The DSP-based control system isunder way and described in Section 4. Finally, conclusionand future plan are given in Section 5.
2 KT-1 Tokamak
KT-1 is a small size Tokamak with a major radius of
0.27 m and minor radius of 0.05 m. It was completed in
1987. It produces circular plasma with 5 kA for 20 msec
under a toroidal magnetic field of 1 T. Fig. 1 shows themagnet system of KT-1. The vertical and horizontal fieldmagnets, providing equilibrium fields (EF), are used in thefeedback control of plasma position.
3 Analog-based feedback control system
The plasma position of KT-1 is controlled in such a waythat the last closed magnetic flux surface (LCFS) is
coincided with the first wall of a torus using two
orthogonal saddle loops, covering a poloidal angle of 180
degrees, and equilibrium field coils. The control scheme isas follows: whenever the plasma column departs outward(or upward) from its equilibrium position, the perturbed
poloidal field is immediately picked up at the saddle loop,
S0-180 (or S90-270). The pick-up signal is integrated andcompared to a preset value (usually zero) and then thecomparator output drives the vertical field power supply

Fig. 1 The Magnet system and saddle loops of KT-1.
The plasma current (Ip) control is carried out by
controlling the ohmic coil current (IOH). The Ip controller
compares the integrated Rogowski signal with the presetvalue and turns on or off the IOH.
Fig. 2 shows a schematic diagram of the plasma position
and the current feedback control system of KT-1. As
shown in Fig. 2, the pick-up signals on all poloidal
magnetic probes due to the toroidal field are subtractedusing an Adder and Analog Calculator before the signals
enter the Ip or EF controller.
The response time of the feedback control system isdetermined by several factors such as the integration
constant of the integrator, TR switching time, photo TR
on/off delay time and magnetic field delay time. The
response time of the KT-1 feedback control system is about50 µsec. Considering the plasma rising time of KT-1,
which is about 2 msec, the response time is sufficient.
A set of KT-1 operation data is presented in Fig. 3. It
shows that the plasma position and current are wellcontrolled.
4 DSP based feedback control system
The DSP based feedback control system consists of :-. Host computer : HP 745i-. Operating system for host computer : HP-UX 8.07

integrator

IPDIOH

CH.1
RC=10 -4 s

IP

CH.6
RC=10 -3 s

ITF

CH.4
RC=10 -3 s

S 0-180

CH.5
RC=10 -4 s

S 90-270

CH.2
RC=10 -4 sA

Adder
CH.1
bias=5.11

T(D- γ A)

Controller
preset=U

IOH out

0.5V/turnγ
D

AnalogCalculator
AB
AQ

S 0-180

EF P/SController

IVF Control

VFSignalpreset=P-2x( α A-B)

IVF

αS 90-270

α-2x( α A+Q)

EF P/S

RController
HF

IHF Control
Signal4.1 System requirements

preset=S

IHF

Fig. 2 The schematic diagram of plasma feedback control
system of KT-1.

10Vloop

VkA

V

505432100.4

Ip

S0.180

kA

2030

2030

2030

2030

0.20.01.5

Ivf

1.00.50.0
10software development environment for Pentek's MIX
family of digital signal processing and data acquisition
products. SwiftTools provides a) Source code creation andediting b) C/C++ compiling/assembling/linking c) Object
code downloading and d) Program debugging
Swiftnet provides an interrupt driven, inter-device
communication protocol for distributed digital signal
processing applications. An Ethernet link is used forhost-node controller communications.

40Time(msec)

Fig. 3 A set of data of KT-1. Vloop : loop voltage, Ip :plasma current, S0-180 : saddle loop signal, Ivf : verticalfield coil current.

As described in Section 3, the plasma control system
uses a Rogowski, a toroidal and two saddle loop signals
with a feedback loop time requirement of about 50 µsec.
We expects a conversion time of 4 channels to be less than
5 µsec, since the total maximum conversion time of 32
ADC channels is less than 10 µsec. Mathematicaloperations to be done with TMS320C40 are simple
multiplication, addition and comparison, each of whichtakes less than 1 µsec. The TTL pulse outputs to control
the power supplies are provided by Pentek 4284 DSP
modules with negligible time delay.
The toroidal, poloidal filed and saddle loop coils pick thesignals only if the plasma condition in Tokamak changes(i.e., differentiated signals). Four signals to be used forthe feedback control are differentiated signals or integratedsignals. The integrated signals are obtained by analog
integration circuits which integrates the four differentiatedsignals. If the integrated signals sampled with 20 kHz areto be used for the feedback control, the MIX DSP is notneeded. If we start with the differentiated signals whichhave to be sampled about 200 kHz, VME and MIX DSPs
are needed for parallel processing described more inSection 4.3. In any case, the plasma feedback control
system is expected to be adequate for our purpose.
4.2 System configuration-. Target : MVME 167 (Motorola MC 68040)-. Target operating system : Tornado 1.01-. VME DSP module : Pentek 4284-. MIX DSP module : Pentek 4254-. MIX ADC module : Pentek 4248-. DSP related software : Texas Instrument software andPentek SwiftTools and SwiftNet
The VME DSP module is based on the Texas
Instruments TMS320C40 Floating Point Digital Signal
Processor with 4 MB DRAM. The VME DSP acts as aMIX bus master. It serves as a 40 Mflop DSP, as well as
a complete DMA controller. The MIX DSP is also basedon the Texas Instruments TMS320C40 processor. The MIX
ADC module is a data acquisition expansion module for
the Pentek series of VME/MIX Baseboards. It provides up
to 32 input channels of A/D conversion to a 12-bit
accuracy. The inputs are connected to 32 sample-and-hold
amplifiers for simultaneous sampling. The sampling rate issoftware programmable to 124 kHz with 32 channels andto 588 kHz with 1 channel. The Texas Instruments DSP
software includes a C-complier, assembler, archiver, linker
and hex conversion utility. SwiftTools is an integrated

Fig. 4 shows the hardware configuration of the VME
data acquisition system at KT-1.
The entire dataacquisition system consists of VME, VXI and CAMAC
modules. The MIX ADC in the top VME crate digitizes theplasma current(Ip), the toroidal coil current(ITF) and twosaddle loop signals(S0-180, S90-270) used for feedback
control, with sampling rates of 20 kHz for integrated
signals or of 200 kHz for differentiated signals. The ADC
modules in the bottom VME crate digitize other signals
such as the vertical and horizontal coil current(Ivf, IHF),
ohmic heating coil current(IOH) and loop voltage(VP).
4.3 Methods
All signals used for feedback control are originallydifferentiated which have to be integrated by analog
integration circuits (Method 1) or by DSP (Method 2). ThePentek 4248 ADC digitizes the differentiated signals with asampling rate of 200 kHz or the integrated signals with asampling rate of 20 kHz. Digitized data occupy a memory
space at DSP and are later transferred to a hard disk forfurther analysis.

ADC

Initiation

Ethernet
VME Crate
M
VM
E
1
67VM
EM
IX

M
IX

DS
P

DSPA
DC

ADC Readout(200 kH z)

M athem aticalO p e ration
20kH z

N sam ple + + ;No

RS-232> preset
S ignalIntegration

VME Crate

RS-232Workstation

M
VM
E
1
67

path1
path2

No TTL
Pulse O ut

YesVM
EVMEVMEADCADCADCNoIF(N sam p le= 10)

Yes
N sam ple= 0

TTL Pulse O ut

Fig.4 The hardware configuration of the VME dataacquisition system at KT-1.
Fig. 5 shows the flow diagram when the Pentek 4248
ADC digitizes the differentiated signals (Method 1). In Fig.
5, paths À and Á have to be done in parallel. We plan touse the VME DSP for the path À and MIX DSP for thepath Á.
Mathematical operations for plasma position and current
control by DSP can be summarized as follows:
l Vertical field control : position control-. Integration of the toroidal coil current ITF (A)-. Integration of the saddle loop signal S0-180 (B)-. B - αA = C-. Comparison of C with a preset value Pl Horizontal field control : position control-. Integration of the toroidal coil current ITF (A)-. Integration of the saddle loop signal S90-270 (Q)-. Q + βA = - R-. Comparison of R with a preset value S
l Current control-. Integration of the toroidal coil current ITF (A)-. Integration of the plasma current Ip (D)-. D - γA = T-. Comparison of T with a preset value U
Here, α, β and γ are to be experimentally determined. IfC (or R,T) is bigger than P (or S,U), a TTL high pulse fromPentek 4284 is provided to power supplies. Otherwise, aTTL low is provided.
One of the reasons that we try to start with thedifferentiated signals is sensitive variations of analog
integration circuits due to the change in temperature.
In the Method 2, the integrated signals obtained from theanalog integration circuits are sampled by the Pentek 4248
ADC with 20 kHz. The Pentek 4284 DSP does the samemathematical operations as in the Method 1, except for thesignal integration.
4.4 Status

Fig. 5 Feedback flow diagram (Method 1).
The entire system except for the MIX DSP module hasbeen
installed and tested. We have simulated theMethod 2 with apulse generator. We find that the DSP based feedback
control system in the Method 2 gives a feedback loop timeabout 30 µsec. Considering that theanalog-based feedback system results in a feedback loop
time of 50 µsec, the DSP based system is adequate forKT-1.
5 Conclusion
We have installed the DSP based feedback control
system and tested all the hardware and software except for
the MIX DSP board. Initial test shows the DSP based
system is adequate for KT-1. We are testing the DSP based
system (Methods À and Á) in parallel with the analogbased system. We also plan to integrate the existing VME,VXI and CAMAC system for plasma heating facilities andproton accelerator projects at KAERI.
Acknowledgement
We would like to thank the technical staff at the Nuclear
Fusion Laboratory in KAERI and Bill Marcotte at Pentekfor their excellent technical support. Two of us (S.J. Hongand D. Son) would like to thank KAERI for providing a
grant for this work.
References[1] T. Kimura et al., "DSP application to fastparallel processing in JT-60U plasma control," Fusion
Technology (1994).
[2] F. Sartori et al., "DSP control of the fusion plasma
configuration of JET," JET-P(95)/29 (1995)