The High Voltage problem R. Fantechi 13/12/2016

Onset of HV problems First HV trips noticed around Sep 16th First guess: related to higher intensity Higher current in the channel which groupes 30 out of 32 LKr HV columns Max current on a single HV channel 1 mA Overcurrent seen already in 2015 at 100% intensity Split the common channel to multiple ones At first problems with the power supply Change HV card -> no way Change Mainframe ok, 9 channels split on Oct 13th Operations with GECO, DCS panel doesn’t have control of all the channels

No change Trips continued See chart next slide Also happening without beam Start blaming HV cables, then current/voltage monitors in series with individual cables

History of trips Voltage Current

The LKr level Around Nov 8th, there were questions by Italo Could it be there a lower level of the LKr? Are there level probes? Are they readout and monitored? Quick answer from Valeri The probes are there, a series of zener diodes whose current varies between gaseous and liquid phase No info about absolute positions and spacings Alarms not set A first trend plot of the last year produced on Nov 9th History of the last 5 years provided on Nov 11th by Johan Bremer

Nov 9th 1-year trend Probe 16 Probe 15

The calorimeter is alive… A zoom of one of the previous pictures showing the breathing of the Argon cooler

Nov 11th Normal level as recorded in the ‘98 filling logbook ~ Oct 2012

The level is low The level is assumed to be now such to have the first row of cells in the gas The discharges are of two types: On the HV cable, before the RC circuit of each cell Observed trips on the HV channels In the cell itself No visible effect on the PS, but destruction of the preamplifier There is a clear correlation with the behavior of the uppermost row, looking at the data Starting from Sep 16th, more and more channels in that row show zero signal Confirmed also by the calibration data taken after the end of the run Indeed on Nov 9th, the HV has been decreased to 2 kV ~10% less signal, to be compensated in the analysis

The upper row of cells Run 6202 burst 699 3/9 00:22

After the end of the run HV immediately switched off Start to setup a procedure to search for the leak Evaluation of the lost krypton is around 150 liters of liquid i.e. 1 cm3 of gas/second Working group setup with the participation of NA48 cryogenics old timers (Bremer, Gonidec, Schinzel) Eventually found a leak on the big flange on the top Repair strategy under definition (see Hans) It should have been worse if it was on the signal FTs

Krypton refill There should be enough reserve in the dewar ~300-600 liters depending on the measuring instrument New purification filters in the gaseous circulation path are needed: old ones already dismounted Filling procedure well known Start to discuss whether we need additional reserve Define the quantity Investigate price vs gas purity Setup a purity control and liquefaction station

LKr purity LKr purity related to electron lifetime Interest to have a continuous monitor Verify that after the refill the purity is still ok Monitor during the run to spot possible variations Possible correlation with a 3% decrease in the energy scale seen by Giuseppe wrt to NA48 and NA62/1 data A device was installed during the assembly of the calorimeter 3 alpha chambers, .5 cm gap, Am source Tested at the beginning of NA48 operations, then left behind because of problems Some work has started (Italo, myself) to check whether these devices are still operational Dig inside old documentation to find the details of the cabling, etc. Just started to investigate, no results at the moment

HV system New system bought CAEN SY5527 with 3 12-channel boards (+1 spare) Possibility to power each column from individual channels Spare channels for the alpha chambers if they will be useful Need to integrate the new system in the DCS Crate and boards already supported by the DCS team User panel to be modified