CHARGING CONTROL SYSTEM OF THE ENERGY CAPACITOR STORAGE OF THE LASERDEVICE ISKRA-5
A.N. Gascheev, I.V. Galachov, V.C. Bezuglov, V.M. Murugov
The Scientific Research Institute of Experimental Physics
Arsamas, Russian Federation.
I.V. Mozin, V.P. Gerasimov, A.M. Gusev, V.D. Fedorov
D.V. Efremov Scientific Research Institute of Electrophysical Apparatus
St. Petersburg 189631, Russian Federation

ABSTRACTThe ISKRA-5 pulsed laser device is designed for research in nuclear fusion. It has 12 channels
working synchronously against a common load to support different experiments on thermonuclear target
heating. For the main power supply it uses stored capacitor energy. The power used (67 MJ) is sufficient
to cause fatal damage, so safety must be guaranteed. Therefore, the ensuring of power supply reliability is
one of the main functions of the control system. This requirement can be satisfied by improving power
supply viability, increasing the stability of capacitor bank charge and emergency energy discharge when
one of the capacitor sections fails.

1. INTRODUCTION
The physical device ISKRA-5 was designed for the study of the interaction of high power laser
radiation with matter in the fusion programme [1,2]. The main part of the device is an Iodine Laser inwhich a light pulse, formed by a master oscillator (MO) is multiplied and amplified in 12 similar
channels. Each channel has 5 cascades of amplification. The total laser output energy is 30 kJ in a0.25 ns.
The power supply for the source of light, producing excitation of the master oscillator and laser
amplifiers, is a pulsed power system based on a capacitor bank. Thr total stored energy is 67.2 MJ and theoperating voltage is 50 kV. For the combination of values of stored energy and pulse length, 10-35 µs, thr
capacitor bank of ISKRA-5 has no equivalent in the world. Closest to it in parameters is that of the device
NOVA (USA) which has a stored energy 60 MJ and pulse length of 600 µs. Such a level of stored energy
can cause fatal damage to the power supply system and the device as a whole, so safety should beguaranteed. Ensuring the reliability of the power supply system is one of main functions of the controlsystem.

2. FUNCTIONAL REQUIREMENTS
The structure of the ISKRA-5 laser and pulsed power system is shown in Fig.1. Each of the amplifiercascade A1-A4 has its own capacitor bank (CB-A1)-(CB-A4), to ensure that the independent operation ofa cascade MO, with its capacitor bank CB-MO, is common to all channels. For laser work optimization, it
is necessary that the capacitor banks of identical modules are charged to the same level of voltage, which
can be changed. The charge system controls the energy storage and power line and contains some types ofindustrially available units. The system allows the control of charge and level of voltage on the storage.
The main charge parameters of various capacitor bank modules are shown in Table 1, the total
number of such modules is 738, including those of the synchronization system.

Table 1
module typeamplifierspecial generator
synchronization system

operating voltage, kV
505070charge error, %222charge time, ms
9060
20The power units are assembled as the usual circuit with step-up transformers and uncontrolled
rectifiers. Feed control for the power units is provided by thyristor regulators.

Fig.1. Structure of the ISKRA-5 laser and pulsed power system
The power supply circuit provides the following modes of capacitor bank charging:- capacitor bank charging with fixed value of charge current,- capacitor bank charging with fixed rate of voltage increase,- combined charging mode.
When using the mode with a fixed value of current, the current is adjusted, as a rule, to the maximum
value the power unit permits. The advantage of this mode is a stable load on the power unit, which is notchanged even in the case of a short circuit. The shortcoming is that with a reduction of the bankcapacitance, which is necessary for some experiments, the charging current is maintained. However all
the elements of a capacitor bank module, to allow for the consequences of emergencies, should bedesigned on a complete charge current basis.
When charging the capacitor bank with a fixed rate of voltage increase, the charging current ischanged depending on the capacitor load. This eliminates the shortcoming of the mode with fixed current.
However, in case of an emergency short circuit, the charging current sharply increases and it is necessary
to stop the operation.
As a rule, such a switching-off occurs as consequence of the operation disaster protection.
Unfortunately we couldn’t manage to obtain a good enough thyristor to maintain an output load with thelevels of protection required.
Therefore a combined mode in the control of capacitor bank charging is preferable, in which a
restriction of the sharp increase of charging current in an emergency situation is introduced
simultaneously with a fixed rate of voltage increase. This avoids current overload and the necessity tostop charging. This mode is used with the incomplete composition of the capacitor bank. The control
mode with a fixed value of charging current is chosen for experiments with the complete composition.
The control system should provide, together with conventional tasks:- flexibility of storage charge control,- remote and local control of storage charge using central and technological control panels,- automatic charging,- high-speed electronic protection,- diagnostics of conditions of all technological systems, forecasting emergency conditions and theformation of controlcommands in extreme situations,- localization and switching-off of the emergency part of the power supply while preserving thefunctioning of the device as a whole.
3. IMPLEMENTATION
The thyristor regulator of the input voltage for the power unit is designed using conventional circuit
phase-pulse control. A high quality of auto-control is provided by a correction system. Some kinds ofprotection are provided, including high-speed protection against the maximum output voltage of thepower unit and maximum charging current.
For elimination of the influence of emergencies in cells of a capacitor storage bank on the workingsof the charging system, the mode with a fixed value of charging current is used. This permits thestabilization of a value of charging current with changes of load.
Particular attention was given to noise immunity of the control system. The general policy was to use
electromagnetic relays not only as switches but also as logic elements. As it turned out, considerable
distortion of the voltage waveform on the feeder cable occurs due to the large current drawn by thecharging system. These distortions resulted in failure of the control system. Complete removal of thisinterference was obtained only by the transfer of the control system supply to another substation.
4. MAIN RESULTS OF OPERATION OF CHARGE CONTROL SYSTEM
Experience of operation with the energy storage has shown that it is possible to ensure reliable
realization of experiments with a capacitor storage of 67.2 MJ. As was expected, in a number ofexperiments there were failures of some modules. Therefore the design of charging system provides acontinuous capacitor bank charge despite failures occurring, which is stopped only by a command of theoperator from a central control panel.
The technical decisions made in the design of the charging control system have allowed therealization of these aims. On failures of different kinds, including momentary short circuit of the power
supply, there is no source switching-off and only a limiting of the charging current takes place. Afterswitching-off a broken module with the help of a special high-speed safety fuse, the charging of the restof the capacitor bank goes on in a given mode. The transients arising at failures are smoothed sufficiently
and do not influence the charging process.
5. CONCLUSIONSThe operation of capacitor banks with a total stored energy of 67.2 MJ at an operating voltage of 50
kV allows us to make the following conclusions about the practicability of energy storage designs ofsimilar and larger sizes.
1. With an increase of the value of storage energy a bank becomes more multielement andaccordingly the probability of failure of some element is increased during bank charging or discharge.
Thus, the failure of capacitor elements becomes a planned event, and appropriate measures for thereduction of such failures’ influence on the efficiency of the bank and the device as a whole should beadopted.
2. The reliability of experiment realization is largely defined by the charge of the capacitor bank.
Therefore it is very important to fulfill the following conditions during the charging stage:- the tolarance of the charging to emergency operation,- the localization of an emergency in any part of the charging system and its switching-off,- the emergency discharge of the energy stored in the capacitor bank into a ballast load,- the minimum time between the attainment of full voltage on the capacitor bank the issue of a firingcommand.
The design methodology for a high-energy capacitor bank and its control system which have beendeveloped allows the creation of a capacitor bank of order of 100 MJ with the reliability required forexperiments.
REFERENCES
1. G.A. Kirillov, V.M. Murugov, V.T. Punin, V.L. Shemyakin "High Power Laser System ISKRA-5"
International Conference SPIE, OE/LASE'90, Los Angeles, CA, USA, January 1990.
2. V.G. Bezuglov, I.V. Galakhov, A.S. Gasheev et al. "The 67MJ, 50 kV Capacitor Bank for theHigh-Power Iodine Laser of the Facility ISKRA-5: Description and Experience" 10 th IEEE International
Pulsed Power Conference, Albuquerque, USA, July 1995.