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

2. 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

3. 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

4. History of trips
Voltage
Current

5. 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

6. Nov 9th
1-year trend
Probe 16
Probe 15

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

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

9. 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

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

11. 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

12. Krypton refill There should be enough reserve in the dewar
~ 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

13. 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

14. 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