EUSO-BALLOON DESIGN REVIEW, 18.12.2012, CNES TOULOUSE

1. EUSO-BALLOON DESIGN REVIEW, 18.12.2012, CNES TOULOUSE HV status and prototype results.Compliancewith performance and technicalrequirements • P. Gorodetzky APC, Paris • Workdonewith C. Blaksleyat APC • HV wereconceived and produced by J. Szabeski and J. Karczmarczyk, Lodz, Poland

2. REQUIREMENTS: • The High Voltage unit shouldbe: • Stable (HV voltage variations should have an effect on PMT output resolutionnegligiblecompared to PMT resolution). • remotelycontrolable • protected • protecting the PMTsto lessthan 100 µA output (Hamamatsu requirements) • allowdynamicsfrom 0.01 photo-electron (pe) to 200 pe • the dynamicswith the switches shouldbearound 106 (ratio between a lightning and the background light). • - PMT output shouldbelinearwith light flux falling on the PMT • If the light islargerthanwhatissafe for PMT, the HV shouldswitchin a mode allowing to continue measuringthatstrong light. • the switching time shouldbe short and no longer than a GTU (2.5 µs) • size and weightshouldbesmallenough for Jem-Euso requirements (to hold in PDM system) • powershouldbelessthan 0.5 W per PDM for Jem-Euso and < 1 W for the balloon. • The Gate Time Unit (GTU) is 2.5 µs. The gain of the PMT isfixed (requirement of the ASIC) to 1.0 106. The light background of the balloonis about 1 pe / GTU / pix, thatis 0.16 pC in 2.5 µs. Then the averagecurrentproduced by the PMT (which has 64 pixels) is 64 x 64 nA = 4.1 µA. • A strong light (lightning, city…) islimited in spatial extension, corresponding to some 8 pixels (a KI is 8 pixels). Then, we have 64 – 8 = 56 pixels with 64 nAeach (3.6 µA), and 4 pixels with 200 pe, thatis 102 µA. The total (106 µA) is close to 100 µA, and acceptable. • As one CW will power 4 PMTs in parallel (an EC), the current to bedelivered by the CW has to beslightlyhigher than110 µA (the strong light fallsonly on 8 pixels of the EC). HV Status

3. REQUIREMENTS: Comparison to the classic voltage divider: Wecan have 100 µA in the PMT, in a linearway, but onlysometimes. This meansthat the current in the voltage divider must consume continuouslyat least 50 times 100 µA, thatis 5 mA. At 1000 V, thisis a power of 4.5 W per PMT. IPMT andIdividerare in parallel IPMT Idivider For the balloon: 162 W, but, for Jem-Euso, whoseballoonprojectis a patfinder:, itwouldbe 22 kW. Exit the divider. Weneedsomethingthatprovides to the dynodes exactly the currenttheyneed, gettingrid of resistors. The best is a voltage multiplier, alsoknown as a Cockroft-Walton (CW), takingintoaccount the factthat the inter-dynode voltage is constant: here about 60 V. A bigadvantage of thisschemeisthat, the impedanceis the lowest close to the anodes, where the currentis the biggest, and at the cathode level , wherewe deal withindividualelectrons, the impedanceis the highest. HV Status

4. HV Status

5. The prototype in the black box. It isconnected to 4 PMTs in parallel, simulating an EC. The light source for background is a LED in a sphereequippedwith a NIST photodiode. The light source for intense and localized light ishere: The switches The CW. Their size is 7 x 3 cm, hence 9 of themcanbeinstalled in one PDM. The switches command HV Status Command for switches Cockroft-Walton and switches

6. Oscilloscopeview of the 4 PMTS (anode 21 in each). All of them are illuminateduniformly on their full surface by the sphereat background level (1.2 pe/pix/GTU) The third one isalsoilluminated by the strong light on 8 pixels onlyat 150 pe/pix/GTU. The CW gain is 1.2 times largerthan the Hamamatsu divider gain. HV Status

7. Results for the CW In abscissa, the number of photoelectrons per pixel in the PMT #3, the highlyilluminated. Only 8 pixels receive the strong light. In ordinate, the current in the last dynode D12, adding the 4 PMTs. The linearitysatisfies the requirement: the HvPSwill support strong lights withoutloss of linearity. The power consumed (28 V and 3.3 V) is 70 mW, so 630 mW for the balloonwithrequirements < 1 W. The requirements of Jem-Euso: about 500 mW for a PDM. HV Status

8. Voltage to apply to a resistivedivider (Caen volts) to have the same gain thanwith the CW, versus the CW control voltage. The gain canberemotelyvariedfrom 0.8 106 to 3.2 106. The requirementis to bearound 1.0 106. Gain of one of the PMTs versus the control voltage (DAC acting on the multiplier frequency) HV Status

9. The switches: how theywork The basic ideabehind the switches is to vary the cathode voltage whilekeeping all other voltages on dynodes constant. Then, the electrostaticsbetween the cathode and dynode 1 are modified, and the collection efficiencyisdiminished. The real gain of the PMT iskeptatits original value (around 106), sothat the tube isalwaysworking in single photoelectron mode, whatever the light intensity (no pile-upat the anodes). On the ordinate, the word "gain" isimproper, but itiseasier to understand. The dynamicsallowable are > 106, which corresponds to the requirement. The switches are actuated by the KI (integrator part of the front-end ASIC) through a simple algorithm in the PDM board FPGA. HV Status

10. Timing properties of the switches When we test the switching, we apply squares pulses (1.2 s at high gain and 1.2 s at low gain) to the 4 PMTs of the EC. The light is DC. In the next scope pictures, the anode pulses are shown in violet, and the command in green. Vcw = 2.005 V (G = 1.4 106), switchingfrom 900 V (G = 1.4 106) to 739 V (G = 1.4 104) and vice-versa HV Status

11. Vcw = 2.005 V (G = 1.4 106), switching down from 900 V (G = 1.4 106) to 739 V (G = 1.4 104) Veryfast, very important Samepictureswhengoingfrom 739 v to 250 V HV Status

12. Vcw = 2.005 V (G = 1.4 106), switching up from 739 V (G = 1.4 104) to 900 V (G = 1.4 106) Herewe pulse also the light, thatis the light is back to the background levelwhenweswitch the gain up. Actually, the bckglevel has been increased by 10 (ID12 = 160 µA instead of 16 µA). This is close to whathappens in reality. Now the recovery time is < 4 µs Samepictureswhengoingfrom 739 v to 250 V Bckgmultiplied by 10 and strong light (100 times Bckg on. Low gain (739 V) Bckgmultiplied by 10, and no strong light High gain (900 V) Conclusion: good PMT protection AND no dead time HV Status

13. Difference between switches and conventional gain reduction: Weilluminatewith 20000 pe/pix/GTU "Gain" is 1.4 104with the switch: wesee pulses at the amplifier output of the ASIC, hence the KI (whois a "time over threshold" integrator in the ASIC), works. 1.0 µs Weilluminatewith 20000 pe/pix/GTU Gain is 1.4 104without the switch: Wereduce the voltage of all dynodes proportionally: Onlypile-up, -8mV DC level, no pulses and the KI does not work. 1.0 µs HV Status

14. Conclusion on switches • Theywork as intended • Gains are OK • Going down in gain in lessthan 2. 5µs • Going up isslower (4 µs), but perfectly OK • b) Dynamics: at 900 V, we have from 0 to 200 pe (per pix and per GTU) emitted by K • at 739 V, we have from 150 to 20000 pe • at 250 V, we have from 1.5 104 to 2 106pe • at 0 V, we have from 1.5 106 to 3.4 106pe •  total dynamics > 3 106 HV Status

15. Where are the HVPS 2 potted HVPS boxes (6 CW in one, 3 CW + connection to HK in the second). Cables go here HV Status

16. Cablesbetweenpotted HV boxes and potted HV boardunderPMTs9 x 14 cables 1 2 Tyco-Raychem TE: 44A0312-24-9 Ratedat 2000 V Diameter: 1.45 mm Connectors: 2 x PD-9, from Mac8, phi = 0.45 mm PTFE tube: 1.6 x 3.0 mm x 3 cm 3 HV Status