8 Accelerator Engineering and Technology: Accelerator Technology
463
the dump switch end. If the kicker magnet is terminated in its own characteristic
impedance there is no reflection of the pulse which propagated through the magnet.
However if the kicker magnet is short circuited at its output the pulse which
propagated through the kicker magnet is reflected and travels backwards through
the magnet, doubling the current and hence the field in the magnet; the fill time is in
this case also increased by a factor 2.
8.7.1.4 Power Switching
Various technologies are used for power switching [177]. Thyratrons are still
commonly used for fast, high power, switching applications. These deuterium filled
gas tubes are characterized by a short turn-on time, high rate of rise of current and
generally good reliability. They need however careful tuning and supervision of
parameters such as reservoir voltage and grid bias voltages, as well as requiring
occasional HV conditioning, to ensure optimum performance. Thyratron faults
include failure to become conducting when required (so called “missing” pulses),
or on the contrary becoming conducting before being triggered (so called “erratic”
turn-on). High power thyratrons are also relatively expensive (up to several 10 k$
each) and have a limited lifetime which is strongly determined by the conducted
charge (typically several 100 thousand pulses for very high current and long duration
(tens of μs) pulses, to 100 million pulses for lower current and shorter pulses). To
lower the risk of erratic turn-on the anode-cathode voltage is only applied to the
thyratron a short time (typically few ms) before the kick is required. Although
not many companies produce thyratrons anymore they are still widely used in
accelerators.
For many years high power semiconductors have been used as switches in
those kicker applications which do not require the high rate of rise of current of
thyratrons. Semiconductors have the advantage of requiring little or no maintenance
in comparison with gas tubes and they have a less critical dependence on the
anode-cathode voltage, but generally have a lower rate of rise of current rating.
An example where these switches are preferred are the LHC extraction (dump)
kickers: the associated PFNs need to be charged over hours and the switches must
therefore withstand the high voltage over extended periods without erratic turnon. Semiconductors used for power switching in kicker systems include GTOs
(Gate Turn-off Thyristors, which have been improved for fast switch-on; they are
also called Fast High Current Thyristors (FHCTs)), IGBTs (Insulated Gate Bipolar
Transistors), and MOSFETs (Metal-Oxide Semiconductor Field-Effect Transistors).
Although in principle semiconductors require no maintenance and have a very
long lifetime when used in normal conditions, their application in accelerators is not
completely straightforward. Since the typical breakdown voltage of a representative
GTO is in the order of a few kV, the high voltages used in kicker systems requires
the assembly of the semiconductors in series stacks (in practice up to 10–15 per
stack, including redundancy) (Fig. 8.72). The distribution of the total voltage within
the stack of series GTOs and the synchronous injection of a sufficiently high trigger
463
the dump switch end. If the kicker magnet is terminated in its own characteristic
impedance there is no reflection of the pulse which propagated through the magnet.
However if the kicker magnet is short circuited at its output the pulse which
propagated through the kicker magnet is reflected and travels backwards through
the magnet, doubling the current and hence the field in the magnet; the fill time is in
this case also increased by a factor 2.
8.7.1.4 Power Switching
Various technologies are used for power switching [177]. Thyratrons are still
commonly used for fast, high power, switching applications. These deuterium filled
gas tubes are characterized by a short turn-on time, high rate of rise of current and
generally good reliability. They need however careful tuning and supervision of
parameters such as reservoir voltage and grid bias voltages, as well as requiring
occasional HV conditioning, to ensure optimum performance. Thyratron faults
include failure to become conducting when required (so called “missing” pulses),
or on the contrary becoming conducting before being triggered (so called “erratic”
turn-on). High power thyratrons are also relatively expensive (up to several 10 k$
each) and have a limited lifetime which is strongly determined by the conducted
charge (typically several 100 thousand pulses for very high current and long duration
(tens of μs) pulses, to 100 million pulses for lower current and shorter pulses). To
lower the risk of erratic turn-on the anode-cathode voltage is only applied to the
thyratron a short time (typically few ms) before the kick is required. Although
not many companies produce thyratrons anymore they are still widely used in
accelerators.
For many years high power semiconductors have been used as switches in
those kicker applications which do not require the high rate of rise of current of
thyratrons. Semiconductors have the advantage of requiring little or no maintenance
in comparison with gas tubes and they have a less critical dependence on the
anode-cathode voltage, but generally have a lower rate of rise of current rating.
An example where these switches are preferred are the LHC extraction (dump)
kickers: the associated PFNs need to be charged over hours and the switches must
therefore withstand the high voltage over extended periods without erratic turnon. Semiconductors used for power switching in kicker systems include GTOs
(Gate Turn-off Thyristors, which have been improved for fast switch-on; they are
also called Fast High Current Thyristors (FHCTs)), IGBTs (Insulated Gate Bipolar
Transistors), and MOSFETs (Metal-Oxide Semiconductor Field-Effect Transistors).
Although in principle semiconductors require no maintenance and have a very
long lifetime when used in normal conditions, their application in accelerators is not
completely straightforward. Since the typical breakdown voltage of a representative
GTO is in the order of a few kV, the high voltages used in kicker systems requires
the assembly of the semiconductors in series stacks (in practice up to 10–15 per
stack, including redundancy) (Fig. 8.72). The distribution of the total voltage within
the stack of series GTOs and the synchronous injection of a sufficiently high trigger