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Experience of High Voltage Conditioning of Kicker Magnets

Experience of High Voltage Conditioning of Kicker Magnets. M.J. Barnes Acknowledgements: G. Bellotto, P. Burkel, H. Day, L. Ducimetière, T. Kramer, V. Gomes Namora. Overview. Reminder: what is a transmission line type kicker magnet?

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Experience of High Voltage Conditioning of Kicker Magnets

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  1. Experience of High Voltage Conditioning of Kicker Magnets M.J. Barnes Acknowledgements: G. Bellotto, P. Burkel, H. Day, L. Ducimetière, T. Kramer, V. Gomes Namora RF Breakdown Meeting - MJB

  2. Overview • Reminder: what is a transmission line type kicker magnet? • HV Conditioning of MKE (SPS extraction kicker magnets): • Lab – dc x 2, then pulse x 2 • SPS tunnel – dc x 2, then pulse x 1 • HV Conditioning of MKI (LHC injection kicker magnets): • No dc conditioning • Lab – pulse conditioning x 2 • LHC tunnel – pulse conditioning x 1 • During operation – short (10 minute) pulse conditioning before each LHC fill RF Breakdown Meeting - MJB

  3. Transmission Line Type Kicker Magnet Dielectric (permittivity εr) The kicker magnetic field must rise/fall rapidly (in the time period between bunch trains): in addition the field pulse flattop must generally have very low ripple. Thus the basic idea of a transmission line type kicker magnet (MKE & MKI are both of this type) is that, electrically, it is similar to coaxial cable: ideally a pulse will travel through and be undistorted. Coaxial cable: Simplified equivalent circuit of a transmission line type kicker magnet : (1) (2) (n-1) (n) Flattop Field Rise time Fall time RF Breakdown Meeting - MJB

  4. SPS Extraction: MKE4 & MKE6 Extraction from SPS, at MKE4, towards LHC: 2560 A  51.2 kV PFN (5 magnets, each has its own PFN, and terminator) Extraction from SPS, at MKE6, towards LHC: 3310 A  33.1 kV PFN (3 magnets in series, 1 PFN, short-circuit) Bipolar Magnet Voltage , -17kV+17kV: Unipolar Magnet Voltage, -2kV ~+26kV: Ferrite blocks ~1.7m Serigraph of ferrite blocks RF Breakdown Meeting - MJB Capacitors (13nF each) outside magnet tank

  5. MKE (SPS Extraction) – dc Conditioning • 2 stage conditioning: dc and then pulse. • MKE magnets can be used at either MKE4 or MKE6, and are thus conditioned for both installations; • MKE magnets have capacitors mounted outside the vacuum tank; • The MKE capacitors are removed for DC high voltage conditioning (to limit energy into breakdown, to ~10’s of Joules); • Serigraphy is NOT conditioned by dc. • DC conditioning: • Voltage is set to maximum value; current is slowly increased (in specified steps), by an automated system, until the specified limit. Hence voltage increases (to specified limit) as conditioning occurs. • To +33 kV (current limited to 20 µA) • Then to -23 kV (current limited to 20 µA) • If the magnet fails to condition with 20 µA, current limit can be manually increased to 100 µA. • In the event of a spark (e.g. vacuum > 10-6 mbar), current is automatically reduced to 0µA, and then slowly re-incremented. • An MKE is considered dc conditioned when it holds the specified voltage, with less than 20 µA for between 12 and 24 hours. • For a “good” MKE magnet +ve dc conditioning and -ve dc conditioning requires ~2 days and +ve ~0.5 days, respectively. RF Breakdown Meeting - MJB

  6. MKE (SPS Extraction) – Pulse Conditioning (MKE6) (MKE4) ~1.4kJ stored in PFN ~0.6kJ stored in PFN (short-circuit after ~195m of coaxial cable  2 x ~1µs delay) • Pulse conditioning is carried out manually (no computer control). In lab – terminated then short-circuit; • Magnet is non bake-able; normal vacuum ~10-8 mbar; • Vacuum during conditioning is recorded on a chart recorder. • Vacuum generally reduces during conditioning – for a given magnet (pressure x voltage) can indicate safe level; • In general 100 pulses, at 1 pulse/6s, per PFN voltage setting (many more pulses at night and weekend); • For a terminated MKE: • Voltage increment with a (constant) pulse length of ~8 µs; • 1000+ pulses at 55.2 kV (4 kV above nominal operating value); • Subsequently pulse elongation to ~16 µs, ~500ns per increment and 20+ pulses per increment, at a PFN voltage of 53.2 kV. • For a short-circuit MKE: • Voltage increment with full pulse length (no clipper can be used); • 1000+ pulses at a PFN voltage of 36.1 kV (3 kV above nominal operating value). • Generally more sparks/conditioning with a short-circuited MKE. RF Breakdown Meeting - MJB

  7. MKE (SPS Extraction) – Pulse ReConditioning Following a spark (vacuum > 5x10-7 mbar), during extraction from the SPS, the set of magnets are pulse re-conditioned (at ~1 pulse per 9s). Before starting reconditioning, wait for the magnet vacuum to reduce to within a factor of 3 of the pre-spark level. Carefully watch magnet vacuum during the re-conditioning: if there is a significant pressure spike, caused by pulsing, do NOT increase the PFN voltage until these pressure spikes have reduced to a normal level (few % of base pressure) – it may even be necessary to temporarily reduce the PFN voltage if the pressure spikes remain at a significant value. Finally operate for a minimum of 50 pulses at the maximum voltage shown above. RF Breakdown Meeting - MJB

  8. MKI: LHC Injection ~800ns @ 10% Ceramic capacitors (inside magnet). Magnets are NOT dc conditioned • Point 8: 5130 A  51.3 kV PFN • (4 magnets, each has own PFN). • “SoftStart” to 54.3 kV PFN. • Point 2: 4960 A  49.6 kV PFN • (4 magnets, each has own PFN). • Both points: HV conditioned to 56.4 kV. Nominal 7.8µs flattop. RF Breakdown Meeting - MJB

  9. MKI– Pulse Conditioning in Lab • Each pulse conditioning in 2 phases: • PFN voltage increment to 56.4 kV (@ 1.5 µs pulse flattop); • 1200 pulses at each of 53.1kV and 57.1kV. • Pulse elongation to 9 µs flattop (@ 55.4 kV PFN; Δ25 ns per 30 pulses); • 1200 pulses at 9 µs flattop, 55.4 kV. Idealized conditioning 35pls/inc Starting point after 2ndbakeout 40pls/inc ~1.9kJ stored in PFN 40pls/inc 35pls/inc 40pls/inc 40pls/inc 60pls/inc Starting point after 1stbakeout 60pls/inc 60pls/inc 60pls/inc • Conditioning is carried out automatically (under computer control). NOTE: Interlock on pressure – values based on experience! • Magnet is baked to ~300˚C (plateau for ~84 hrs), in an oven, before HV pre-conditioning; • HV pre-conditioning is at 1pls/13s (chosen by experience to avoid vacuum pressure increasing at each pulse); • Ideally ~66,000 pulses; • Magnet is baked to ~300˚C (plateau for ~84 hrs), with jackets, before HV conditioning; • HV conditioning is at 1pls/10s; • Ideally ~44,000 pulses. RF Breakdown Meeting - MJB

  10. MKI: Pulse Conditioning in Lab 5% Vreduction 5% Vreduction 4% Vreduction Controls bug! PFN Voltage PFN Voltage Controls OK! 6% Vreduction 1 hour 1 hour 1 hour Controls bug! 1 hour Vacuum Vacuum MKI06-T07-HC12PreHV MKI07-T08-MC08HV • Following a “strong spark” (4e-8 mbar) voltage is reduced and pulsing is stopped for 1 hour for vacuum to recover: • With a 4% Vreductiona controls bug incremented the voltage during the stop-state (no pulse) – so restarted at ~1.5% (700V) below previous breakdown level: cluster of 4 breakdowns (~6% overall reduction before successfully recommencing); • With the 5% Vreduction, first restart after 1 hour gave a 2nd breakdown – next restart OK; • With the 6% Vreduction, first restart after 1 hour OK. • Vacuum activity after restart suggests conditioning. Rate of increase of voltage seems OK, because no further breakdown occurs. PFN Voltage Controls OK! Vacuum Conditioning, but no breakdown MKI06-T07-HC12HV RF Breakdown Meeting - MJB

  11. Strong Spark Summary MKI: Pulse Conditioning in Lab If a breakdown occurs in the magnet, or beam screen, accompanied by a reasonably high level of energy dissipation, significant out-gassing normally occurs: the pressure can takes several tens of minutes to a few hours to recover to its pre-breakdown level “Clusters” of strong sparks correspond to a significant percentage of total… RF Breakdown Meeting - MJB

  12. MKI: Diagnostics Capacitive pickup on HV output plate Capacitive pickup on HVinput plate • Capacitive pickups, one at magnet input and one at magnet output, give the timing of the input and output edges of the voltage pulse. • When a electrical breakdown occurs, the relative timing of the (rapidly falling) edges gives the position of the breakdown: • delay input to output ~700ns • (Tin – Tout)/2 breakdown 216nsfrom centre, towards input side. TMR RF Breakdown Meeting - MJB

  13. MKI: Micro-Discharge Test Micro-Discharge test: 40kV to 55.3kV @ 8.6µs With 2 ion pumps per MKI tank, a pressure rise is considered a micro discharge (energy dissipated in the magnet or beam screen is relatively low) when the pressure takes a few minutes to recover to its pre-breakdown level: 3 minutes is used. Thus a micro-discharge is defined by the duration of the pressure rise recovery time. PFN Voltage Micro-discharge test Vacuum RF Breakdown Meeting - MJB

  14. Spare Slides RF Breakdown Meeting - MJB

  15. 4% Vreduction 1 hour RF Breakdown Meeting - MJB

  16. Vin Vout For a magnet terminated with a matched resistor: field rise time starts with the beginning of the voltage pulse at the entrance of the magnet and ends with the end of the same pulse at the output. Field riseis given by the sum of the voltage rise time and the magnet filling time : Flux The field builds up until the end of the voltage rise at the output of the magnet. Hence it is important that the pulse does not degrade while travelling through the magnet. Thus the magnet cut-off frequency is a key parameter, especially with field rise times below ~100 ns. Cut-off frequency (fc) depends on series inductance (Ls) associated with the cell capacitor (Cc): Thus, Ls should be kept as low as possible and the cell size small. However cells cannot be to small (because of voltage breakdown and cost). Transmission Line Kicker Magnet: Transmission line kicker magnets have much faster field rise time than equivalent lumped magnets. However, design and construction is more complicated and costly. RF Breakdown Meeting - MJB

  17. Charging end of line Load end of line • At t=0, when the ideal switch closes, the load potential (VL) is given by: A voltage pulse of “(α-1)V” propagates from the load end of the line towards the charging end. • At the charging end the reflection coefficient ( ) is +1 and hence “(α-1)V” is reflected back towards the load end of the line. • At the load end of the line: Time say. Impedances need to be matched to avoid reflections !, (i.e. β=0  ZL=Z0) and hence “β(α-1)V” is reflected back towards the charging end of the line. • Etc. Pulse Forming Circuit: General Case RF Breakdown Meeting - MJB

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