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Description of cavity quench detection
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TABLE OF CONTENT
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PAGE
1.
INTRODUCTION .................................................................................. 3
2.
QUENCH PROTECTION METHODS....................................................... 4
2.1.
Evaluate the cavity voltage ................................................................. 5
2.2.
Evaluate the loaded quality factor ...................................................... 6
3.
IMPACT ON ACCELERATOR OPERATIONS ........................................... 3
4.
SUMMARY ................................... ERROR! BOOKMARK NOT DEFINED.
5.
GLOSSARY........................................................................................... 7
6.
REFERENCES ....................................................................................... 7
DOCUMENT REVISION HISTORY ....................................................................... 7
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INTRODUCTION
This document shows a possible mechanism to detect the thermal breakdown – so-called
quench - by monitoring the accelerating field/voltage in the cavity. The immediate event
is usually not harmful for the cavity itself, but it may have consequences due to the
interaction with other systems, e.g. particle beam, couplers, high power RF. It is stated by
several facilities operating superconducting accelerators, that a quench needs to be
detected fast to shut off RF and interrupt beam delivery (e.g. [1,2]).
Furthermore it collects information from different stakeholders that require a protection
system in case a superconducting cavity quenches during accelerator operation at ESS.
2.
IMPACT ON ACCELERATOR OPERATIONS
The comparison with other linear accelerator facilities shows the following:
2.1.
Impact on the beam
For electron accelerators (in particular E-XFEL), the impact of ‘losing’ RF in one cavity
does not affect the beam too much, as the missing energy might be compensated with
the other RF cavities. This is due to the fact that electrons are ultra-relativistic almost
‘immediately’ (some MeV) – the velocity does not change that much and thus the bunch
will see the same RF phase either with more or less energy. Nevertheless if the energy
loss cannot be compensated with the subsequent cavities, the beam optics (quadrupoles,
steering magnets) may perturb the beam, as the magnetic fields are set for the nominal
energies.
For proton machines (example SNS, ESS) it is different. The mass of the proton is
approximately 1900 times the mass of an electron. So the protons will still considerably
gain velocity until reaching an energy of several GeV, which exceeds the design of current
linear proton machines (SNS 1 GeV, ESS 2 GeV). The ‘loss’ of one cavity in the accelerator
cannot be compensated easily, as the particle bunch will arrive at the wrong RF phase
(minimum 5 degrees phase shift at the high energy end), resulting in ‘undefined’
acceleration or even deceleration. This error propagates through all the accelerator
resulting in losing control of the beam. This effect is studied from the beam physics point
of view and simulations comfirm that as upon a cavity failure the arrival time to the next
cavities is affected the beam will be rapidly lost in the ESS linac [REF-X (2<X<3)]. In
addition the magnets are then powered for the nominal energy and thus will perturb the
beam, and eventually the beam will propagate in an unknown way. However, the effect
of magnets on the beam with slightly different rigidity is negligible compared to the effect
of the RF.
2.2.
Impact on the RF
A simulation study for a 1.3 GHz cavity in operation at ARIEL [3] shows the evolution of Q0
of the cavity if a quench occurs. While Q0 is in the 1010 range during regular operations, it
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drops to values of some 105 within a few milliseconds after quench. The quench spot
radius increases, according to the study, with 14.1 mm/ms, and thus leads to an excessive
RF power consumption, as the detuning due to the warm up of the cavity is very limited,
especially as it is not the whole surface, not immediately to room temperature and
probably only within a cell (example: 1.3 GHz cavity f(2K)=1.299 GHz to f(300K)=1.297
GHz). The change of Q0 and the small change in frequency will still result in continuous
feeding of RF power into the cavity, which increases the heat deposition. In the end
either the cavity f will be detuned sufficiently that no more power from the power
coupler is accepted. Before achieving this state, the feeding of power continues and will
result in a massive energy deposition in the niobium and thus the surroundings. In the
previous example [3] the cavity is fed with 100 kW of RF power, which then will be
dissipated, while for ESS it may be even up to 1.1 MW.
2.3.
Acting of RF and/or MPS *** to be filled
**** BLM faster than quench detection to pull beam?
**** Nevertheless RF needs to be switched off as well.
3.
QUENCH PROTECTION METHODS
ESS is operated in pulsed mode at a pulse frequency of 14 Hz. Each RF pulse is up to
3.5 ms long, while the proton beam pulse is only 2.86 ms long. During the beam pulse
time, the cavity should operate in a ‘steady state’, thus at a constant electric
field/voltage. At any point in time along the RF pulse, a quench of a superconducting
cavity may occur. There are several methods to detect a quench.
A simple method – that only works during ‘steady state’ (thus is fine for cw operation,
which is not the case for ESS) and only partially during the power rise and power decay,
would be to monitor the reflected power at the fundamental power coupler. As it is
mandatory to monitor the cavity all along the RF pulse, another option has to be chosen.
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Evaluate the cavity voltage
In Fig. 1 the cavity voltage during an (arbitrary) RF pulse is shown in black. The maximum
voltage Vmax is applied during the beam pulse, and the RF system has to make sure that
it is stable. During a quench, a lot of additional energy is dissipated in the walls of the
superconducting cavity, which leads to an anomaly in the cavity voltage. This anomaly
can be detected by assuming a ‘voltage envelope’ (red). If the real cavity voltage differs
from the expected cavity voltage by more than 10% from Vmax and thus leaves the
voltage envelope, it can be assumed that there is a quench or an issue.
Figure 1: RF pulse - nominal voltage (black), detection envelope for +/-10% of Vmax (red)
Note that this method requires an adaptation of the reference curve, whenever the
operating voltage has to be changed due to e.g. other beam requirements. Furthermore
the monitoring of the voltage can be used to avoid a quench or unstable operation, by
adapting the set point and/or analysing the RF data whenever the cavity voltage varies by
some percent within the ‘detection envelope’.
A similar quench detection method is used at SNS: if the cavity field does not reach the
‘low limit’ set in the LLRF at flat-top, the pulse is truncated. If this happens twice within a
second, the MPS trips RF and beam. It has to be noted that the operational parameters of
SNS are different (pulse rate 60 Hz, pulse length flat top 1 ms).
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Evaluate the loaded quality factor
Figure 2: Cavity gradient profile for three successive RF pulses, before (0) and during quench (1,2). [5]
Another possibility is to analyse the loaded quality factor Qload of the cavity [5,6]. In this
case a quench (or another issue) becomes evident, when Qload is more than 10% below
the expected set point.
Note: Qload is dominated by the external coupling Qext:
1
𝑄𝑙𝑜𝑎𝑑
=
1
1
+
𝑄𝑒𝑥𝑡 𝑄0
Qload is in the order of 105-106, while Q0 is in the order of 1010, thus the measurement/
calculation of Qload has to be very precise to conclude if there is a quench or similar
problem.
The current process at FLASH/E-XFEL only computes Qload at the end of each RF pulse
using the decaying signal from the pick-up probe, which may result in 1-2 undetected
quenches before the end of the pulse. DESY looks into computing the Qload in the FPGA
continuously, and it is stated that using this approach, the quench can be detected within
50us [6], thus well within a beam pulse and before an additional quench occurs, which
takes several 100us.
4.
CONCLUSION
With respect to the influence of a quenching cavity to the beam, and the possible damage
that may occur to the cavity due to continuous feeding of RF power after a quench, the
implementation of a quench detection system from the RF side is mandatory. Both
methods (monitoring the RF field, monitoring of Qload) can be implemented in the LLRF
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system as all signals are available in the system. In addition, if such a system is available
and has a fast response time, the losses in the downstream parts of the linac due to the
quenched cavity will be significanly reduced.*** to be completed with the information
from MPS about the speed necessary, or if BLMs are actually faster, nevertheless the RF
has also to be pulled then.
5.
GLOSSARY
Term
Definition
<<Sample term>>
<<Sample explanation >>
6.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
D. Kostin, DESY, private communication, 2016
C. Sibley (SNS), ROPB001, PAC2003, 2003
Z. Yao et al., TUPB013, SRF2015, 2015
S.-H. Kim, private communication, 2016
J. Branlard et al., THPPC072, ICALEPS2013, 2013
J. Branlard, private communication, 2016
[x] M. Eshraqi et al, “PRELIMINARY STUDY OF THE POSSIBLE FAILURE MODES OF THE
COMPONENTS OF THE ESS LINAC”, ESS-0031413
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