High Voltage Testing at the 35t Long Baseline Neutrino Committee (LBNC): ProtoDUNE-SP CE & TPC Sarah Lockwitz, FNAL June 23, 2017 ANL (eng), W&M (prof., student, tech), LBNL(scientist, eng), KSU (prof, post doc, student), U.C. Davis (prof., post doc), U of Houston (prof), UCLA (prof), BNL (scientists), FNAL (scientists, techs)
Motivation • Design is new • Resistive cathode • Aluminum profiles • Cable-based HV bus • …more • And how do these pieces work together? Photos from B. Yu LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Motivation • Predicting HV behavior in LAr is not easy / designing stable & robust LArTPCs with V cathode =180 kV is not well-trodden territory • Breakdown can break components • it can also set the operating limit of the detector limiting the physics reach Long Bo TPC • Instabilities in the system (glitches, “blips”) are annoying, and may affect data quality • Possible light production… HV issues can This is why you should cause light affecting our light detection system care! This ring is open LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Introduction to the Test • The goal of the HV test in the 35t was to test the ProtoDUNE-SP design with regard to high voltage • The test was carried out in the 35t in PC4 https://inspirehep.net/record/1397920 • The 35t cannot fit the full size of the ProtoDUNE-SP field cage. • We used a partial field cage with full-scale components (e.g. the profile curvature was the same, etc.) LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Introduction to the Test • The goal of the HV test in the 35t was 1.5 m to test the ProtoDUNE-SP design with regard to high voltage • The test was carried out in the 35t in 1.5 m 1.5 m PC4 • The 35t cannot fit the full size of the Cathode ProtoDUNE-SP field cage. Resistor • We used a partial field cage with to ground full-scale components (e.g. the Potential profile curvature was the same, etc.) Relative (kV) Anode LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
HV Issues We Were Preparing for Feedthrough went here • We expected HV problems to be something like a spark • Fast discharge/high current drawn momentarily • Visible light LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
HV Issues We Were Preparing for Feedthrough went here • We expected HV problems to be something like a spark • Fast discharge/high current drawn momentarily • Visible light LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Monitoring ✴ Power supply: voltage and current monitoring ✴ Current resolution ~10s nA ✴ Two toroids on the HV cable ✴ Sensitive to fast changes in current ✴ Two pick-off points near the end of the resistor network ✴ Current going down the resistor chain ✴ Three PMTs. No wavelength shifter on them — Just visible HV PS ✴ Cameras — RPi LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Note: The supply was a current-limiting supply. When • the output current from the supply reached a set limit, the supply reduces the voltage to stay What We Saw below the current threshold. The voltage listed in the following slides is the • voltage at the power supply unless otherwise noted. The voltage at the cathode was 5% less.
Initial Turn On • Purge, cool down, and fill went well • On March 31st, just after we were filled with not TPC-qualty (7 ppb) LAr we began to ramp. • We tripped at 160 kV • It appeared to be a real “fast discharge” trip. • Let it sit over night at 150 kV, and ramped without issue to 190 kV the next morning • It held until we ramped down Monday morning LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Running with Purification Current Read Back (and any jumps) • The pump started without any issue Pumps On No Circulation • We then ramped back up to 190 kV and noted it seemed to be getting noisier • We then saw periods of elevated current; we called this “streamer mode” • Current was noisier coming out of the supply & less current was going to the end of the resistor network • → Alternate path to ground Pick-o ff points (near end of chain) • We lowered the voltage and saw this behavior also at 150 kV LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Running with Purification • The pump started without any issue • We then ramped back up to 190 kV and noted it seemed to be getting noisier Power Supply Read • We then saw periods of elevated current; we called back (current = green) this “streamer mode” • Current was higher coming out of the supply & less current was going to the end of the resistor network • Alternate path to ground • We lowered the voltage and saw this behavior Pick-o ff Points — also at 150 kV voltage /current near the end of the resistor network LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Continuing with the Run: Contamination • We then wanted to better check the effect of purity on the current features Current • We bypassed the filters and returned the condensed liquid to the cryostat • Still had current features, but their magnitude was smaller • Running at 175 kV in ~350 μ s LAr Pick-o ff s (end of chain) LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Purity Effects LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
What Happens at 100 kV? • Let’s re-purify and answer this • Ran at 100 kV in 3 ppb O 2 LAr for a baseline and restarted purification • Ran at a steady ~4 ms lifetime • Still observed streamer-like events LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Running in Stable ~4 ms LAr • Lowered the current limit of the supply to 24% over the expected current draw; considered this a “tight”-er limit • Still “jumps/spikes”, 1-2/hour, no streamers • Changed to 90, 70 120, 140, and 160 kV (with corresponding current limit),— still spikes, similar rate • (Note the order) • After running at 70 kV, the current limit was raised from 21 μ A to 40 μ A, and a two minute streamer occurred • Tighter current limits quench the long-streamer behavior LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
How do the features behave in time? Are there charge-up effects? • We turned the voltage to 0 for ~67 hours and then ran at 150 kV • We observed charge-e ff ects. No activity for 4.5 hours (previously 1-2 features/hour). • More followed. Appeared to get busier in time. • Saw this for other voltages too • There is a hysteresis — one can go to a higher voltage and have a stable period, but going down did not lead to a stable period. • This was not simply a current limit effect • We record spikes even in the tight current limit running • We relaxed the current limit and did not observe streamers after being off LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
Where are the Sparks? A Quick Note on What We Were Seeing • We were expecting HV issues to be like sparks we’d seen in other test setups • Light produced & relatively fast discharge (resistive cathode may have slowed this) producing a signal on the toroid • While ramping up the first day, we experienced a trip of the PS. This appeared to be like something we expected — there was a signal on the toroid • We had something similar while ramping the day after the pumps were turned on (4/4) • The PMTs were instrumented shortly after we purified • We did not see light or toroid signals for any of the streamer or current activity in the month after April 4th LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
A Note on Diagnostics on These HV Events — Dark Sparks • We have 3 PMTs in the cryostat (no TPB on them) Evidence the • The PMTs were running after the first purification PMTs are • We do not see light associated with these events (H.Y. Liao’s work in our weekly working meeting) — they do see noise • The PMTs were working — they see the purity monitors and Cherenkov light from Cherenkov light from shower showers Photo: G. Horton-Smith • Toroids do not see activity for the streamer events (slow developing). • See two types of signals on the toroids — small signal (10s mV) 25 V 30 mV • Large (multiple Vs) signals we expect from breakdowns — only a few of these (1st day, 200 kV event, one while ramping…) 200 μ A current jump ~95 μ A current jump • It seemed to be hard to get a fast-discharge-like at 170 kV on 4/25 at 180 kV on 4/4 event in the pure[r] states LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
What’s Going On? Going Out with a Bang • Toward the end of the run, after not seeing traditional sparking behavior, we went to 200 kV — the maximum of the supply • Current limit was loose (200 μ A limit; ~48 μ A exp.) • Ran for 8 hours — streamers/activity • Then, we suffered a major E&M event (30 kV swings) • Probed issues that could occur • Instrumentation tripped off (Keithleys, scopes, PMT HV, cameras) — some didn’t recover • Our LAr pump was turned off • Lesson: Look at conditions for cryo-system — make it resistant to sparks • The field cage, resistor chain, and HV feedthrough + filters were not damaged. LBNC: HV Test in the 35t Sarah Lockwitz June 23, 2017
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