3 issues identified in review Some data for the cryogenic load and quench test in SM18 on an 11T magnet following a trip of the trim PC . 3 issues identified in review Overcurrent in trim lead if R_crowbar is too small (not an issue since R_crowbar is foreseen at 60 mOhm) During trim trip current will pass through the diode, depositing heat (discussed in this presentation) Control of two PC’s. (not discussed in this presentation) G. Willering, 22-04-2016

Trim circuit trip in the LHC Input 6 kA in main circuit Trim crowbar 60 mOhm Diode opening voltage 5.5 V Output Energy deposited = 1.6 kJ Maximum current in diode 158 A Current through crowbar 92 A Input 6 kA in main circuit Trim crowbar 60 mOhm Diode opening voltage 2.75 V (half) Output Energy deposited = 2.7 kJ (+ 70%) Maximum current in diode 204 A (+ Current through crowbar 46 A

Trim circuit trip in SM18 – MB in series with 11T Input Connecting 130 mH 11T in series with 100 mH MB magnet. 6 kA, assuming a trip in the main PC after trip in Trim circuit Trim crowbar 60 mOhm Diode opening voltage 5.5 V Output Energy deposited = 0.7 kJ Maximum current in diode 158 A Current through crowbar 92 A Trip of main PC Main PC disconnected Only trim Main PC stable Assuming no trip of the main PC and stable regulation at 6 kA the decay will be similar as in the LHC Trim current independent of main circuit current: Trim at -250, trip -> Same current profile in trim circuit as in the LHC.

Possibilities for testing cryogenic loads in diode container 1.Simplest ??, most versatile method (for this we could use any MB magnet with diode) Place temperature probe on the diode lead close to the SC busbar Put a 1 kW heater in the diode container 2. Simple but representative way of pushing current through diode: Disconnect main converter. Same decay as in LHC. Use T-probe to validate if a quench could happen or not. No magnet protection needed. No trim PC control test possible Only with 2*5.5m assembly (130 mH) Main dipole gets close (100 mH) Cold diode -250 A crowbar 3. More complex way, but allowing test of trim PC in the Main PC circuit by connecting 2 cold magnets in series – use watercooled cable to connect them. Same decay as in LHC if main PC does not trip. T-probe will be still important to validate the measurement. Modification of PLC and protection needed.

Can the heat deposition quench the magnet? The total heating is 1.6 kJ, if spread over 5 liter of helium (volume of diode container with diode installed) the helium warms up from 1.9 to 2.2 K. With the heat spread out over the whole magnet (about 250 liter) the delta T < 10 mK. No quench. Only helium is taken into account, no heat capacity of the cold mass nor the diode heat sinks. Energy Tmax diode container Tmax magnet 1.6 kJ 2.2 K <1.91 K 3 kJ 3.1 K <1.92 K

The decay of Vf will probably be considerably faster at 1.9 K due to What do we know about diode behavior at small currents at low temperature. Forward voltage with low currents in the magnet, see CSCM tests in all sectors. Slow diode opening with some 150 A lead to very slow decay of V_f, which indicates slow heating, see examples below. Start of diode conduction Start of diode conduction MQ magnet: (higher current, same diode, smaller heat sinks) MB magnet (bit lower current) The decay of Vf will probably be considerably faster at 1.9 K due to - lower heat capacity - higher dVf/dT - double heating power (double voltage).

Conclusion No quench will occur, so to make the cryogenic load test usefull a temperature probe should be placed on the diode bus bar, close to the T-junction with the SC leads In SM18 testing current through diode after a trip in the trim can be done easily with the main converter disconnected, even with a NbTi main dipole. T-probe is usefull. Connection of two magnets on two benches in series will mainly serve the test of the control of the main PC and the trim PC.