The researchers have designed an economical device to measure the power time history in the National Ignition Facility`s (NIF) 192 beam laser. The heart of the system is a commercial, high-speed, four-channel digitizer with a 15,000 point record length. Samples of several beams are taken with fiberoptic pickoffs, separated in time with appropriate fiberoptic delays and presented to high-speed vacuum photodiodes, which convert the samples to electrical signals for the digitizer. Amplitude and time multiplexing are used to cover the required dynamic range and to record 12 samples on the digitizer, making the cost per sample competitive with alternative approaches. Forty-eight digitizers can record the required three samples from each of the 192 beams. An additional similar but lower bandwidth system is used to record the backreflected light from the main laser amplifiers and elsewhere. The recording electronics are discussed in detail

Boyd, B.

Davis, D.T.

Hall, B.

Thomas, S.

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ucRLJG124s62PREPRINTTemporal Multiplexing for Economical Measurement ofPower Versus Time on NIFS. Thomas, B. BoydD. T. Davis, B. HallThis paper was prepared for submittal to the2nd Annual International Conference onSolid-State Lasers for Application to ICFParis, FranceOctober 22-25,1996October 22,1996DISCLAIMERThis document was prepared as an account of work sponsored by an agency ofthe United States Government.  Neither the United States Government nor theUniversity of California nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California.  The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernment or the University of California, and shall not be used for advertisingor product endorsement purposes.Temporal multiplexing for economical measurement of power versus time on NIFStan Thomas, Bob Boyd, Don T. Davis and Bob HallLawrence Livermore National LaboratoryBox 808, L-495, Livermore, CA. 94551ABSTRACTWe have designed an economical device to measure the power time history in the National Ignition Facility’s (NIF)192 beam laser. The heart of the system is a commercial, high-speed, four-channel digitizer with a 15,000 point recordlength. Samples of several beams are taken with fiberoptic pickoffs, separated in time with appropriate fiberoptic delays andpresented to high-speed vacuum photodiodes, which convert the samples to electrical signals for the digitizer. Amplitude andtime multiplexing are used to cover the required dynamic range and to record 12 samples on the digitizer, making the cost persample competitive with alternative approaches. Forty-eight digitizers can record the required three samples from each of the192 beams. An additional similar but lower bandwidth system is used to record the backreflected light from the main laseramplifiers and elsewhere. The recording electronics are discussed in detail.Keywords: NIF laser power diagnostic transient digitizer backreflection2. BACKGROUNDThe system design of the National Ignition Facility’s (NIF) 192 beam laser incorporates a requirement to measure thepower balance of all beams to insure that it is within tolerance. In order to do this, the time history of the complex-shaped22 ns-long, power pulse must be measured on each beam line. The results are used to correct the input to each beam line toachieve the desired balance.3. SYSTEM DESCRIPTIONThe measurements are to be accurate to within two percent (averaged over any two-ns interval) covering a dynamic range of5000:1 and with a rise-time capability of 250 ps or less. Additionally, back-reflected light from several of the opticalcomponents in each beam line path between the preamplifier stages and the target chamber will be measured to insure thatdamage thresholds in the preamplifier sections will not be exceeded. This large number of measurements (768) will berecorded using commercial, high-speed digitizers with long record lengths and a variety of optical delays.3.1 Power samplingFigure 1 shows the beam paths and sampling points. The preamplifier (PAM) output at 1.053 pm ( 1co)is injected intothe transport spatial filter (TSF). The PAM output is sampled by the input sensor, which passes a sample to the power sensorby a fiberoptic bundle. The injected beam is next passed through the main amplifier, a four-pass amplifier, back through themain amplifier and then returned to the TSF where a second sample, still at 1.053 ~, is taken. The sample is guided to theoutput sensor by an air path and then to the power sensor by a fiberoptic bundle. The time delay between the first and secondsample, the time-of-flight through the main and four-pass amplifiers, is about 900 ns.The beam continues to the target chamber where it is converted to351 nm (3(0) light by a KD*P crystal array in thefinal optics assembly (FOA). A reflection from the crystal output surface is guided back through the optics to the TSF wherea third sample is taken and guided to the power sensor, first by an air path (to the output sensor) and then by a fiberoptic-cable bundle. The 351 nm sample arrives at the power sensor about 1.4 ps after the sample from the preamplifier.‘PAMOUTFOA. Target45 M 67M175MDD area223ns1251 nsDDx2 ‘FOAInjectionamp PointInputsensorFigure 1. Power sampling● Samples taken hy summstion of all four channels of one digitizer.beam 1 beam 2 beam 3 beam 4 beam 1 beam 2 beam 3 beam 4PfDUfo 30 60 90 ns 900 930 960 990 nsPreampsample,41 wbeams Amplifier snmple,4 lwbeamsbeam 1 beam 2 beam 3 beam 4 peakJLLJLJL/JL%f \1400 1430 1460 1490 nsA3000 nsTsrget sample, 4 3W ch 1foot}ch2● Four digitizem (16 beams) pereacb of12 power sensorchassis tbr a total of576 samples. \}ch3}ch4Figure2. Temporal multiplexing ofpowerssmples3.2Temporal MultiplexingFigure 2 shows that the result of the above sample collection scheme is three power samples from one beam line@aml), separated intimeby900 nsand500ns, respectively. Similar samples fiomthreemore bcamlines(lreams 2,3and4)wecomtined withthe first bydelaying eachofthese byanadditional 30,60and 90ns, respectively. (Sirrce thestabilityof the digitizer(100 ppm) meets our timing requirement accuracy of 100 ps only for a few hundred nanoseconds, a fiducialtimemark (FIDU)is added through anelec&ical input to channel 1 atthebeginning ofeachgroup of four samples. Thismark is generated from the timing generator, which controls timing for the entire laser system and is extremely accurate andstable. Along with appropriate calibration ofother system pammetem such asexact fiber path lengths, the FIDUaIlowsdetemination of thedegree ofsynchronization ofall192beams, animpotint factor inoverall laser pefiomance.) Thebeam samples are then collected by a fast photodicde and converted to an electrical input for the digitizer, as shown in Figure3. Inthisway, four beams aresampled atthree points, foratoml of12samples perdigitizercard, Forthe576 powermeasurements,48 digitizer systems are required. Additional units arerequired forthebackreflection memurement.3.3 Photodiade~eHamamatsu, 60-psrisetime photodode weusehas an S-lspec~l response photmathode. Since weuserhe samedetector system to rake measurements at Iw and 303 it is clear that our only choice is the S- I photocathode, which requiresabout 4KWat lroand 122 Wat3coin order toprovide therequired 20 Volts tothe2:l tmmsforrner atthepeak power of thepulse.Four TVS 645 cards in aVX141 Omainframedigitizer #1 digitizer #2 digitizer #3 digitizer #4fZTWW-~~~~~;;atra~~~~z~line W Y Yinputs similar to digitizer #14,50 n outputsA 4 lensea\R1328UNote: Preamp beama ahown.Amplifier & target beamanot ahown.graded fiber bundle beam 4,90 ns delaydelay lines; 30,60 &90 nsFigure 3. Power measurement3.4 DigitizerThe digitizer is VXI Bus compatible, the sample rate is 5 GS/s (5.E9 samples/second), and it has a bandwidtb of 1GHz. in order to realize the 1 GHz bandwidth (350 ps rise time), each of the four input channels must have its full scalesensitivity set between 100 mV and 10 V. The digitizer has only an eight bit converter with 6.3 bits effective resolution, butit has four identical channels per chassis. Even though the 20/. accuracy point of the digitizer is exceeded for signals less than63% of full scale, the accuracy requirement is for the average over any 2 ns time interval, which permits averaging to be usedto increase the individual digitizer’s effective dynamic range. llre four channels arc used in parallel, with each channel set toa different full-scale sensitivity in order to cover the required dynamic range by amplitude multiplexing the input signal, asshown in tbe lower right portion of Figurs 2. Signal processing will be used to reconstruct the full dynamic rcnge of thesignal.The digitizer is mounted in a supporting VX1 mainframe with power supply. There is room for a control card (localcomputer), a communications card and a total of five digitizer cards, four of which will be the Tek&onix TVS 645 and thefifth, a slower TVS 641 for the backreflection measurement. Including labor, the fiber bundles and the electronics, theprojected cost is about $6200 per measurement. Fiber bundles are required to reduce speckle modulation, which would bcexcessive if a single fiber were used. The fiber bundles are a major labor expeme because optical delays of a]] 20 fjber~ mustbe within 50 ps of each other in order not to affect the overall system rise time significantly. This requires a tediousprocedure of measuring and trimming each of the large number of fibers using an optical time-domain-reflectometer and thenbundling and adding connectom and a protective shield.Preliminary evaluation of the digitizer gives indications that tbe device meets or exceeds all of tbe manufacturer’sspecifications that we checked, Of concern was crosstalk among channels, which was specified at 30:1 (3 V on one channelwould produce less than 0,1 V on the other channels). During testing, we saw no crosstalk signal on three of the channels at5 mV sensitivity when the fourth channel was driven with a 6-V, fast-rise pulse. Other tests also produced encouragingresults.Comment Although the specified risetime requirement is not met by the system described,and computerprocessingwill have to be used with some compromise in performance on dynamic range, it is likely that improvements in commercialdigitizers will be realized in time for implementation on NIF. An increase in bandwidth to 1.5 GHz would allow us toachieve our rise-time requirement. Although commercial manufacturers refuse to speculate on future improvements, onemanufacturer now has a 10 GS/s digitizer, and there are commercial RF amplifiers available with bandwidths of several Ghz.slot 1&2digitizer #1TVS 645 Cardm4I 2:1transformer I—- —slot 3&4 slot 6&6 slot 7&8 slot 9&1 o slot 11digitizer #2 digitizer #3 digitizer #4 digitizer #5 communicationscard7VS 645 Card TVS 645 Card TVS 645 Csrd TVS 641 Cardqppm%&sc re e on slot 12electronics PD HVsee below powersuppliesw2KV 1Oov+,-, towfor16kam,inesFO bundles from OSPS.“Vat. Si.PDs PDsFigure 4. Poweribackreflection sensor in a VX I410 mainframe3.5 Mainframe chsssisFigure 4 shows the block diagram for one complete system to measure three samples each of 16 beam lines, as well astheir backreflections. All of the electronic components are mounted within the TektronixVX1410 mainframe. The lenses,fiberoptic delays and photodiodes (PD) are enclosed within two chassis below. The 50 Q photodiode outputs are fed to two-to-one, coaxial-line, high-speed transformers with four identical 50 Q outputs, which drive each of the four inputs of onedigitizer. Each digitizer occupies two of the 13 slots. The fifth digitizerisaTVS641, a slower unit, for the backreflectionmeasurements. The communications card and a high voltage (HV) power supply for the photodiodes occupy slots 11 and 12.Not shown is the control card in slot O.3E2A ferrite heeds/ ‘“1 \ \ “hem’’”series5oflINPUTJ.en9msW2=W3&W4SW5= W6=W750QUT85 25fJparallelconnectionBNC - Layout@‘1iW3W4Figure 5. 2:1coaxial-line transformer3.6 TransformerThe two-to-one transformer is shown in Figure 5. Semi-rigid, copper-jacketed coaxial cable withpolytetrafluoroethylenl dielectric is used for its high frequency characteristics and stability. WI is a 50 Q cable. W2 and W3are identical lengths of 25 Q cable and arc connected in series at their input (Jul ) to match WI and in parallel at their outputjunction (Ju2) in order to provide a 12.5 Q impedance. W4 to W7 are identical lengths of 50 L?cables connected in parallelat the output junction (Ju2), providhg a match to the 12.5 Q impedance of W2 and W3 at this point. The cable from thephotodiode is connected to J1, and J2 through J5 are connected to the four input channels of the digitizer. 3E2A ferrite beadsplaced near the junction points provide a high common mode impedance, preventing losses to the surrounding air andextending the low frequency characteristics of the transfomrer. Rise time can he in the order of 50 ps if proper care is takento make the lead connections as short as possible at the junctions and to keep the beads close to the junctions. The outputvoltage is half of the input; the four outputs are identical since the output cables are all tied to the same point (Ju2) at one end.~’-dlamter. Use central 8 mm only.. ... . . ..-ges, movableFtf=8‘%”~~~g~~ml+-, Wa?l,●wadge, fixed.PAM AMP 3 m19 to 20 gradedfiber bundle foror for 3f“1(0One of the three eete of Ianeea laJ- 50pCoreahown here ee an example. Thera 125t0145pia one aet of four Ienaea each for cladding.the PAM, the AMP and the 3 cdy,les.-l.=Figure 6. Lens and filter system3.7 Lens and FilterThe amplitudes of the beams flom the four preamps (PAM) being sampled will lMadjusted in the respective inputsensnrs with fixed filters to prnduce equal intensity outputs from the fiberoptic when they are sent to the lens and filtersystem in the power sensor, as shown in figure 6. The same holds true for the four amplifier (AMP) and the four 3rJIsampleshorn the output sensors. However, the intensity ratios of the PAM, AMP and 3ro samples vary with overall laser power,which can be different for different laser shots. Therefore, independent control over the groups of samples must be achieved.This is accomplished by use of three independently stepper-motor-driven attenuator wedges. The wedges have greaterattenuation at one edge than the other, requiring a fixed wedge to be used for compensation. An anti-reflection (AR) coatedlens is used to image the outputs of each fiber bundle onto the photodiode. The figure shows only one of the three sets oflenses needed to image the 12 samples onto the PD.3.8 IssuesIf reflections occur within the beam line and become mixed in time with any of tbe desired signals, data could beaffected or obliterated. This effect will be investigated on Beamlet, our working, one-beam NIF prototype laser. There arcother questions concerning the operating characteristics of the digitizers which will have to be answered by prototypeevaluation, which is underway.We have contmcted with a foreign vendor to supply us with tibcroptics for use at 3(o. The expected attenuation ishigh, but acceptable for our application, as are the other important parameters. The contmct has just started and there is tbepossibility that it maybe unsuccessful in accomplishing the design goals, Alternatives will undoubtedly be more expensive.3.9 BackreflectimrSlots 9 and 10 (see figure 4) of the mainframe arc used for the TVS64 1 digitizer for the backrcflection measurement.Backrcflection refers to any light that is reflected backwards down the beam path, It is of concern because the opticscompress the size of the beam in the back direction while amplifiers add more energy. This can increase the fluence abovethe damage threshold for optics in the PAM. The TVS 64 I has a digitizing rate of 1 GS/s and an analog bandwidth of 250Mhz, which are adequate for this application. Single 100-pm fibers instead of bundles and silicon diodes with their lowerbandwidth can be used here, both of which reduce costs. Since the damage threshold is a constant, a lower dynamic rangeconfiguration using only one digitizer channel can be used. Attenuation of the sample for each beam line can be fixed at thepickoff point, which is located at the main laser backreflection beam dump. Four backreflection samples are combined usingappropriate fiber delays, sent to one diode and used as the input to one of the digitizer’s four channels. In this fmhion,backreflection from 16 beams can be recorded f.bm one digitizer,4. CONCLUSIONAn economical method of simultaneously measuring and recording 768 time-resolved power samples from the NIFlaser has been described. The main constituents are commercially available items.5. ACKNOWLEDGEMENTThiswork wasperformed under the auspices of the U.S. Department of Energy by the Lawrence Livermore NationalLaboratory under Contract No. W-7405-Eng-48. Reference to a company or product name does not imply approval orrecommendation of the product by the University of California or the U.S. Department of Energy to the exclusion of othersthat may be suitable.Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

Temporal multiplexing for economical measurement of power versus time on NIF

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Temporal multiplexing for economical measurement of power versus time on NIF

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