DEVELOPMENT AND TESTING OF THE NIF PROTOTYPE MODULE*

Sandia National Laboratories' P.O. Box 5800, Albuquerque, NM 87185-1184

C. Hq'es, M. Wilson, G. Mower, and J. Adcock ECEIV 3Ut t 3

Abstract The NIF Power Conditioning System (PCS) is required to deliver - 68 kJ to each of the 3840 flashlamp pairs in the NIF laser in a current pulse with a peak of - 500 kA and rise time of - 150 ps. The PCS will consist of 192 modules each of whch drive 20 lamp-pairs. Each module will basically be a 6 mF capacitor bank with a nominal charge voltage of 23.5 kV which is switched by a single pressurized air gas switch to 20 RG-220 cables that are connected to individual lamp loads. In addition each

Table 1. Key NIF PCS module require Q t S T 3 1

module will have a number of subsystems including; a lamp pre-ionization system, power supplies, isolation circuits, trigger systems, safety dump systems, gas system, and an embedded control system. A module will also include components whose primary function is to limit fault currents and thus minimize collateral damage in faults. In the Prototype Development and Testing effort at Sandia National Laboratories all of these were integrated into a single system and proper functionality was demonstrated. Extensive testing was done at nominal operating levels into resistive dummy loads and some testing in fault modes was also done. A description of the system and a summary of testing is given in this paper.

I. INTRODUCTION The purpose of the National Ignition Facility (NIF) is

to provide an experimental facility capable of achieving fusion ignition [I]. In NE, the fusion target will be located at the center of a 10 m diameter spherical chamber and be driven by 192 laser beams. The lasers will be pumped by 7680 flash lamps which are excited by the capacitor bank modules in the Power Conditioning System (PCS). There will be 192 PCS modules each of which will have 20 lamp loads where a lamp load consists of a series pair of flash lamps. Ih Table I, the key design requirements for a PCS module are summarized. The module is required to deliver two independent pulses with adjustable timing between them. The first pulse is called the Pre-Ionization/Lamp Check (PILC) pulse. It serves two purposes; first it pre-ionizes the flash lamps so that reliability and efficiency is improved, and second (since it can be fired independent of the main pulse) allows the functionality of lamps to be checked with a low energy pulse that is unlikely to rupture a faulty lamp. The Average Gain Coefficient (AGC) is obtained using the GainCalc V1.0[2] code developed at Lawrence Livermore * The US DOE under Contract No. W-7405-ENG-48 supported h s work.

Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US DOE under Contract DE-AC04-94AL85000.

#

National Laboratory (LLNL). The code was provided by LLNL to be used as a metric for the PCS module design. It is based on data from laser glass experiments and uses the flash lamp electrical power pulse to the to derive a corresponding AGC pulse. The basic module circuit is shown in Fig. 1 along with a brief description of the key components and a block diagram of all the subsystems associated with the module. Detailed descriptions of all the subsystems and components can be found in other NIF PCS papers being presented at this conference. In this paper, the prototype development and testing effort whch lead to present NIF PCS module design will be described. The prototype effort began in mid 1996 and continued through mid 1998.

11. INITIAL PROTOTYPE Initial prototype tests only included a main power

supply, a power supply isolation circuit, a 6 mF capacitor bank, damping elements, bank bus work, a main switch, and a resistive dummy load. The bank configuration was similar to that which was purposed in the NIF Conceptual Design. The first 1000 shots were conducted without a trigger generator by charging the bank and then reducing the switch pressure until self-break occurred. The purpose of this test series has to gain experience with a 1.7 MJ bank and develop the techniques necessary to make it perform as required. The initial 1000 shot run is summarized in Fig. 2. Following this run, a prototype trigger generator shown in Fig. 3 was developed and successful operation was demonstrated on the prototype. The most important data obtained during these tests was;

components performed as expected, the output current pulse was stable, and agreed with circuit simulations, methods for making good high current density bus connections were applied to eliminate arcing, the need for insulating the capacitor cases from the rack to prevent arcing was confirmed, an optically triggered, essentially solid state trigger circuit concept was successfully demonstrated, and the bus fault demonstrated that shrapnel shielding would have to be part of the module design.

D I SCLAlM ER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER

Portions of this document may be illegible in electronic image produced from the document.

products. Images are best available original

Capaciton

Damplng Elements

Dump Cinults

Main Switch

Ballast Inductors

Output Cables

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Zoea 3oOpF 24kV >ZE4shotlife metailmd fllm for reliability and graceful dearadation

Zoea S f l 25nul stainless steel coil In a fiberglass envebpe to limit current in a capacltor fault, designed to remain substantially intact for the wont case 390 kA fault

2 ea 1.7 kR ceramic disk reslstor 120 Vac actuated relay redundant circuits to enhance safety

1 ea graph. elearodes press. air

20 ea 13-19 pH depends on cable length so total induct. (ballast+cable) - 26 pH copper coli in a flberghss envelope to force load current sharing and limit current in load fault, designed to remain substantially intact for the worst case 190 kAfault

Zoea RG-220 20-55m 2-3 braids depending on length

>I800 shots between rebulMs

Figure 1. Basic NJT PCS module circuit, block diagram of subsystems, and component descriptions.

Bade Circuit I

Oihu h m p Asrrmbly (on oppodte side) - HVR AW112B172L

Maiu P. s. (t&in!l ract) UZXWCU CCSl0030Pla)(LH 30 kV. IO W l v c

Figure 2. Summary of initial prototype tests from 10/96 to 3/97.

Prototype Trigger Generator Circuit Photo (transformer, +-

... ... Optical

SUMMARY OF TESTS

> 1Dw. B.5 kV shots

Currem waveform very d m e to NIF requirwnm.

NO sultdl @ires observed

N e w ground bus workinstailed on stwr 4 2 l to eliminate aning at capacitDr mnnedam.

Capacitnrs imuWW fmmrack on shot 420 to elminae case to rack amng.

One full w w e bus faunobswed on shot 431 dueto fwnessemkal sat* dran A pmjeclile with est. penelmima poww of a 45 caliber butlet was waunched d u m the faun Mhimal bus damage. repairable in a feu hours.

of Oil Filled High Voltage Section 1.7 & 10 nF caps, and peaking switch)

1 : 41

Summary of Tests

Capable of Peak Output Voltage > 100 kV Rise Time - 40 ns do

do

T- (n)

Figure 3. Summary of initial prototype trigger generator tests.

111. FOLDED GEOMETRY PROTOTYPE The initial prototype was very similar to the design presented in the NIF conceptual design because the Sandia team was relatively new to the NLF project when it was designed. The folded geometry (FG) design was based on experience gained on the initial prototype and on the switch test stand. Electrically it is essentially the same has the conceptual design with the primary difference being in the configuration of the capacitor bank and the high voltage bus. This configuration was considered to offer the following advantages; 1) simpler and smaller high voltage bus, 2) simpler and more accessible main switch connection

(this is important the ST-300A will have to removed

the trigger generator fiom the initial prototype, but, it was soon replaced with a commercial unit of similar circuit topology fiom Physics International. In addition to demonstrating the proper fimctionality and stability of the system with this trigger generator, an extensive test series, was conducted to define the triggering range of the main switch. The trigger generator was capable of an in situ maximum peak trigger of - 60 kV. With this, the data fiom the test series indicated; 1) good triggerability was possible down to 40 % Vsb

(for minimum switch pressure and a charge voltage of 23.5 kV, 40% Vsb corresponds to a gap of - 0.9"), below 40% Vsb delay and jitter increase rapidly and

Table 11. Summary of FG prototype tests TASK PURPOSE

and refurbished 10-20 times during a module's life), 3) force bracing of the H V bus is simpler, 4) shrapnel containment simpler to implement, and 5) lower cost because these things are simpler to do. In Fig. 4, photos of the 2x2~5 FG prototype are shown along with descriptions of the components that were used during testing. The primary purpose for building and testing this module was to demonstrate that this design concept fully integrated with all subsystems could meet the NIF requirements. A summary of the tests that were done is given in Table 11. The prototype PILC system that was designed and constructed for these tests is summarized in Fig. 5. The proper functionality and stability of the system with this type of PILC circuit was demonstrated. Testing on the FG prototype began with

Ballast inductor assembly

Switch blocking cores

,Trigger fuse

Trigger blocking Eapr

Main mggn unit

Capacitor

Ballast inductor assembly

West capacitor 'rack

20 output cabla (1 braid RG220)

Conuolrack I

Dummy load assembly

Independent. ' floatingload

Figure 4. Photographs of the folded geometry prototype module.

ntrolln

hU.,

Figure 5. S+?f the prototype PILC system.

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at 35 ‘YO Vsb no fires were observed, the spark plug which serves as a uv source in the ST- 300 was found to be important in keeping the delay and jitter of the switch low, and a ferrite core cross sectional area of -0.06 m’ was found to be necessary to achieve optimal output voltage from the trigger generator. _ _ _

The proto-type embedded control system shown in Fig. 4 was also in place for most of the FG testing. This unit proved to robust, reliable, and stable in performing the control and data acquisition functions for the tests. Considerable effort was also devoted to diagnostics development. A summary of the diagnostic effort is given in Fig. 6. The NIF PCS module is required to provide the current signals listed in the figure. Low cost Rogowski coils with accuracy better than 1 % were developed to serve as the current monitors. In addition, a transformer isolated load voltage monitor with accuracy better than 2% was developed for monitoring lamp load voltage. This diagnostic is not required to be in place on all NIF modules, but, it is needed to make a measurement on a module to determine if the NIF AGC requirement is being meet by the module. Since only resistive dummy loads were used on the prototype, it was never possible to make such a measurement on the prototype. In stead, an accurate PSICE circuit model was developed and validated with the resistive load data. The model was then used with a simple lamp load model (Vlamp=l7.6*sqrt(I)) obtained from LLNL to simulate how the module would perform into lamp loads and calculate the expected AGC. In Fig. 8, the total (main pulse only) output current pulse from the module on a typical 23.5 kV charge shot is overlayed with the PSPICE simulated pulse. The good agreement indicates that the model is accurate. With a lamp load, the model predicts that the NIF AGC requirement would be satisfied by the prototype module. The shot to shot stability of the module is demonstrated

Calibration Waveforms I - cab* -cDb-k i

SNL &si& Ragowski ail PC bopd tah. low cas (- S 5 W u )

I - -

I Tim (SI

by the data plotted in Fig. 9. On 50 consecutive shots the variation in peak current from the module is < +/- 1 % and the variation that is evident can for the most part be attributed to a decrease in the resistance of the dummy load as it heats up. In order to demonstrate that an acceptable lamp trigger pulse would be generated by the PILC or the main bank, all of the load cables were disconnected, a 3 WZ voltage monitor was attached to the end of one of the cables, and the banks were fred separately (the main bank at a reduced charge voltage). In both cases ringing gains of - 1.8 were observed which is very close to what the PSPICE model predicts. Fig. 10 is the waveform recorded on the PILC shot. Upon completion of the module characterization tests, prototype operations were concluded with a series of fault tests. A summary of these tests is given in Table 111. In general, the results of these tests were encouraging. In all cases, the damage and the peak fault current was less than had been expected. It seemed feasible to recover from any of the faults within a few hours provided spare parts and necessary tools were available.

IV. SUMMARY In the prototype testing effort, a NIF PCS module design was developed and demonstrated which can satisfy all NIF requirements. Testing indicates that the design can satisfy ail NIF requirements. An accurate PSICE model of the module was also developed and validated which will serve.as a useful design tool in the development of the final NIF PCS module design.

V. REFERENCES [ l ] M. L. Kiefer and M. M. Widner, “SCREAMER - A

Pulsed Power Design Tool, User’s Guide,” in Digest of Tech. Papers, 5Ih IEEE Pulsed Power Conj, eds. M. F. Rose and P. J. Turch~, 1985, p. 685.

[2] Private communications with Ken Jancaitis, LLNL

Figure 6 . Summary of diagnostics development. Figure 7. Module waveform data.