LHC BLM threshold modification reports
Chair: E.B. Holzer
Members: Chiara Bracco, Serge Claudet, Bernd Dehning, Eduardo Nebot Del Busto, Stefano Redaelli, Mariusz Gracjan Sapinski, Rudiger Schmidt, Arjan Verweij, Jorg Wenninger, Markus Zerlauth.
Information: Ralph Wolfgang Assmann, Wolfgang Bartmann, Brennan Goddard, Malika Meddahi, Andrzej Siemko, Laurent Tavian, Jan Uythoven.
|2012-01-18||Revision of Direct Dump thresholds||E. Nebot||WG meeting 1|
|2012-01-18||Noise/threshold comparison at LIC locations||E. Nebot||WG meeting 1|
|2012-02-15||Revision of Direct Dump BLM thresholds II||E. Nebot||WG meeting 2|
|2012-02-15||LIC dump thresholds||E. Nebot||WG meeting 2|
|2012-02-15||Preliminary results of simulations of the 3.5TeV quench test||A. Priebe||WG meeting 2|
|2012-05-16||Studies on BLM thresholds for 200kW||B. Salvachua and D. Wollmann||WG meeting 3|
|2012-05-30||Preliminary iteration for changes of BLM thresholds in IR7||E. Nebot||WG meeting 4|
|2012-07-11||BLM thresholds in IR6 and IR7||E. Nebot||WG meeting 5|
|2012-07-11||BLM thresholds in IR3||B. Slavachua||WG meeting 5|
consistency of sanity checks
Increase thresholds at the tertiary collimators (luminosity losses)
Injection region (non LIC) thresholds (increase with MF?)
Arc thresholds increase for testing UFO-quenches with normal physics beam (use MF or new master thresholds?)
Ralph's request to increase collimator thresholds
Direct Dump thresholds
noise levels with respect to thresholds:
lower limit on thresholds?
effect of filters, monitor type
re-check operational thresholds vs noise
new threshold calculation code should become the operational code (compare with specification)
regular test of monitors flags (connected to BIS, filters etc.) (extend Audun's code)
MF=0.05 - why?
injection region cold magnets to be revised (including MF), including triplet magnets, including MQY
TCT in IP8 lumi induced losses; increase last 2 RS? 33% of threshold with ~2E32 in LHCb and 480 bunches
IP7 currently 20 kW loss ok with thresholds - have to increase (in steps) to 200 kW design value
Threshold family of BLMEI.08L8.B2I23_MBA is sanity check applied? (RS 1-4 and RS6 triggered 18.4.2011 00:06, but not RS5)
check for possible limitations all around the ring for the second half of 2011 (signal/threshold less than 10 and MF bigger than 0.1)
collimator thresholds to be increased (in steps) up to the nominal losses on collimators (MD results)
MQTLH thresholds (currently set to 10 time the MQY thresholds) --> QP3 simulations
direct dump monitors (continuation)
thresholds on TDC (nothing had been specified so far)
Barbara: injection region TCTVB (2010 changes due to conflicting requirements of injection and collimation)
Arjan and Mariusz: update on MB and MQ (and MQM) thresholds
Eduardo: cross comparison of simulations of various cold magnets, warm magnets and collimators
BLM signal per lost proton (as function of energy)
maximum #protons (transient) and #protons/s (stead state) allowed (as function of energy)
Eduardo: analysis IP7 and IP3 MBW magnets: can/shall we set thresholds for local protection?
2012-07-11 E. B. Holzer, E. Nebot, B. Salvachua, R. Schmidt and M. Zerlauth.
A proposal for an increase of thresholds in IR6 (monitors protecting Q4 and Q5, MQY magnets) and IR7 (MQW) was presented by Eduardo (analysis based on LHC fills 2800, 2803, 2804 2808 and 2818 that were dumped due to slow losses). In both cases it was agreed that the simplest way to increase the dump threshold is via the Monitor Factor (MF). For MQY magnets the threshold will be increased by a factor 5 (MF from 0.1 to 0.5) except for 4 BLMs in which the MF was previously increased from 0.1 to 0.2 (in this case MF will go from 0.2 to 0.5). For MQW monitors, it was agreed that thresholds will be increased by a factor 2 via the MF (from 0.5 to 1.0). In this case, the worry is that allowing for this extra losses will decrease the lifetime of the magnets. However, a preliminary analysis by Rudiguer indicates that allowing for these losses (before going into collisions) will only increase the total received dose over a whole fill by a few percent. It was agreed that a more extensive analysis (dose received by the magnets over a few weeks) is required to confirm this.
A preliminary analysis to determine the new dump thresholds in IR3 was presented by Belen. Losses in fill 2660 (~80% of dump threshold estimated to correspond to a power loss of ~ 60kW) were compared with off-momentum loss maps to find a good agreement (maximum differences of the order of a factor 2). Therefore, off-momentum loss maps at 450GeV are used to estimate the new thresholds. It was agreed to follow the same approach as for IR7, i.e to modify the thresholds to allow for a power loss of 200kW with the ability to further increase them via the MF (will be set at 0.4) in order to allow for the 'nominal' power loss of 500kW on the collimators. It was found that a set of 14 monitors protecting TCSG and TCLA will need to be increased by up to a factor of 5.4. (to allow for 200kW of losses). A set of 4 BLMs protecting cold magnets (Q7) would also require a threshold increase of up to a factor 3.3. Since off-momentum losses are observed at the beginning of the ramp, it was agreed that the BLM thresholds for cold elements will be only increased for the first LHC energy levels, keeping unchanged the thresholds for top energy.
2012-05-30 E. Nebot, M. Sapinski, B. Salvachua, and D. Wollmann
2012-05-16 R. Assmann, B. Dehning, E. B. Holzer , S. Redaelli, M. Sapinski, B. Salvachua, R. Schmidt, A. Verweij and D. Wollmann
TCPs in IR7/IR3. When specifying a certain number of Watts over a certain time in (IP7 for example), it means the sum of the energies of the primary beam particles hitting the primary collimators (in IP7) during this time interval. This does not mean that all the power is absorbed in the primary collimators. But rather that most of the power is absorbed over several turns in the cleaning insertions (mostly IP7), because the leakage to other regions in the LHC is small..The table below is from the LHC design report:
Hence, the collimation regions ( all collimator made out of carbon) have been designed to dissipate:
500kW over 10sec (IP7 and IP3)
100kW continuously (IP7 and IP3)
1.2MW over 1 sec (IP3 -
In 2011 during an MD 500kW in horizontal losses in IP7 over 1-2 second did not lead to any magnet quench.
TCTs. With 500kW on the primary collimators, the TCTs will receive 500W. They are equipped with a cooling power of 3-7 kW. The temperature interlock is set to ~50degC. The thungsten melting point is above 1000degC.
TCAPs. TCAPA, TCAPB and TCAPC are conical tungsten absorbers which reduce the beam pipe inner diameter. They protect the MQW. The MQWs have been shown (by simulation?) to be fine up to 500kW on the TCCP. The aperture diameter at the TCP is ~2mm, while at the MQW it is about 40mm.
Guideline for changing
IP7 thresholds (and other locations in the ring if concerned):
2012-02-15 W. Bartmann, C. Bracco, E. B. Holzer, E. Nebot, A. Priebe, R. Schmidt, A. Verweij and M. Zerlauth
An analysis of the signals observed in the direct dump monitors during the second part of 2011 was presented. Only the cases in which a beam dump was not requested are considered. Most of the BLM readings are within ~100 BITS which correspond to the noise level of these monitors. A few signals between 100 and 500 BITS were observed during injections. One single event recording relatively high signals (2200 BITS) corresponds to losses generated during an MD (non linear chromaticity measurements). It was decided to further analyze this event before coming to a conclusion in the reduction of the dump threshold. It should be checked that the direct dump threshold will not be decreased bellow the threshold of the standard BLM system (some details of this event and the proposed dump threshold can be found here). It was pointed out that it is necessary to check how the XPOC would react to a dump triggered by the BLM Direct Dump system ( Jan and Chiara found out the following: a) If the BLMDD triggers, but it is not the first TSU trigger, no XPOC latch will take place. b) If the BLMDD is the first TSU trigger, the TSU module in the XPOC will latch. It will need an LBDS expert to reset the XPOC. c) At start-up no fast logging of the BLMDD will be available and there is only the 1 second interval logging of the BLMDD in Timber to look at the event.)
Master threshold tables for LIC detectors were presented. The thresholds allow for the same doses that standard ICs except in two cases: a) The thresholds were limited by the electronic saturation. In this case, due to the reduction in sensitivity, the allowed dose increases up to a factor 60. b) The thresholds are enforced to be a factor 5 higher than the BLM noise level in order to avoid beam dumps due to noise spikes (this affects two monitors out of 7).
Agnieszka presented preliminary results from simulations of the quench test with orbital bump performed in cell 14R2. A good agreement between measurements and simulation (within a factor 2) was found. Several effects not taken into account in the simulations and that may have a significant effect were mentioned. Rudiger pointed out that the magnets may be tilted up to angles of ~100 u rad and that could have a severe impact on the presented results. Two comments from Arjan: 1) on the estimation of the quench levels the B field distribution should be taken into account. At the end of the magnet where the field is larger is more probable to produce a quench. 2) Only the energy depositions in the MQs and MBs are currently investigated. If the MQT magnets were included in the simulations we may be able to get some information regarding quench levels for this magnets.
Markus asked about dump thresholds in TCT collimators. An increase of a factor 2 in luminosity may bring some BLMs close to the dump threshold in the long integration windows. It was agreed to contact the collimation team to provide a proposal (Since we are increasing beam energy for 2012, Stefano and Ralph have suggested to leave the thresholds from 2011. A factor 2-10 is available from MF that may be used in case of reaching critical conditions).
2012-01-18 C. Bracco, E. B. Holzer, E. Nebot R. Schmidt, J. Uythoven and M. Zerlauth
Revision of Direct Dump thresholds. The signals recorded by the 4 Direct Dump BLMs during the end of 2011 were shown (from 01/08/2011 until 31/12/2011). Only in one case the signals exceeded the dump threshold during standard LHC operation. Therefore, the question is whether the Direct Dump system would protect in case of failure of the BIS or BLM system. The largest signals (> 5000 ADC counts which correspond to 0.01 Gy in 138 us ) observed after the 5th of September were analyzed. All of this 61 events corresponded to beam dumps. The signal distribution for those events that do not match a beam dump will be further investigated in order to come to a conclusion on a less conservative dump threshold.
Noise/threshold comparison at foreseen LIC locations. The noise is assumed to come from the analog cables rather than the detectors themselves. The 7 locations where the LIC detector are foreseen have been investigated and the noise has been compared to the applied dump thresholds at 5 TeV. Two out of the seven monitors have been found to be limiting. The threshold to noise ratios are expected to be 5-10 but for detectors BLMQI.08R8.B1I10_MQML and BLMQI.03R8.B1I30_MQXA are 2.3 and 0.7 respectively. Since these two magnets are (over)protected by other BLMs it is not an issue to increase the dump thresholds in the short integration times to 5-10 times the noise level.
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