MODES OF THE COMPONENTS OF THE ESS LINAC
M. Eshraqi, R. De Prisco, R. Miyamoto, Y. I. Levinsen European Spallation Source, Lund, Sweden.
May 27, 2015
1 Introduction
In the following we report on the study of the failure of a single component, either RF cavity or quadrupole. For now it is assumed that the field in the element drops to zero instantly and makes it completely un-functional, even destructive to the beam, while the other elements are unchanged. This assumption is not necessarily very realistic since field of either a cavity or a quadrupole has a finite decay time determined respectively by their Q-value or inductance. Additionally, the machine protection system (MPS) should stop the beam at the front-end with a short delay in the order of few microseconds. Therefore, the losses shown in the following represent unrealistically bad cases even ignoring the decay of the field, and so only their patterns have meaning and their absolute scale should not be taken literally. The purpose of this study is to understand the worst case scenario in the absence of a functional MPS.
These simulations cover the failures of the quadrupoles and cavities of the MEBT, field in the DTL tanks, quadrupoles and cavities in the superconducting linac. For the case of cavities in the superconducting linac, a few representative cavities have been considered, 3 cavities in the spoke section, 2 cavities in the medium beta section and 2 cavities in the high beta section.
One should emphasize that no other errors are considered in these studies and inclusion of dynamic and static errors will affect the conclusions.
2 MEBT
2.1 Overview
Due to the lack of a periodicity, a pattern of losses caused by a failure of an element in the MEBT could differ for each element. Hence, we simulate losses caused by a failure of one element of the MEBT for all the buncher cavities as well as all the quadrupoles. Exception for the forth quadrupole surrounding the chopper since, as of February 2015, the layout around the chopper and its dump is likely to be changed with respect to the lattice used in this study.
A detailed study of beam losses was conducted and presented at the HB’14 Workshop [1]. The same lattice was used in the following study. The lattice version or name of each section is
• MEBT: 2014.v1
• DTL: v86
• HEBT: raster27
1 × 105 macro particles in a Gaussian distribution at the entrance of RFQ is transported through the RFQ (the version of 2013 baseline), and used as the input to the MEBT. Please note that this
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lattice is almost identical to one so-called “2014 Baseline”, the snapshot at the end of 2014, but some details may be slightly different. As presented in [1] and seen below, the MEBT scrapers have a significant impact on the beam losses cause by issues on the transverse planes. Hence, we always repeat a case of a simulation with and without the scrapers.
Figure 1 shows the total particle losses in the linac caused by a complete failure of each MEBT element. The schematic on the top represents the MEBT lattice and the bars are located at the location of the failed element. The complete failures of the quadrupole magnets have immediate effects and a large fraction of the beam is lost in the MEBT and the following DTL but almost no loss occurred in the SC linac or HEBT. For the failure of a buncher cavity however, even a complete failure does not create such a fast effect. A large fraction of the beam reaches the target, but failure of a buncher cavity causes the losses throughout the whole linac.
DTL Tank 1
Q1 Q2 Q3 Q5 Q6 Q7 Q8 Q9 Q10Q11 0
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Figure 1: The total losses in the whole linac in number of particles due to a complete failure of each MEBT lattice element, comparing the cases with and without the three MEBT scrapers. The bars are shown at the corresponding location of each element. The schematic on the top represents the MEBT lattice, where the blue boxes above (below) the line are the focusing (defocusing) quadrupoles, green boxes are the buncher cavities, and the red lines and triangles are the chopper and its dump. (Losses into the three MEBT scrapers are not shown.)
2.2 Failure of a buncher cavity
In this section, we look at the losses caused by a failure of each buncher cavity. Figure 2 shows loss densities in J/m in MEBT (left column), DTL (middle column), and the rest of the linac (SC linac and HEBT) for a complete failure of the first buncher (top row), second (middle row), and the third (bottom). As discussed above, the effect of a buncher failure is not immediate and only the failure of the first buncher causes losses within the MEBT itself, but all the three cases cause losses
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in the DTL, beginning of the spoke section, high-β section, and HEBT up to the dogleg. Since the problem is in the longitudinal plane, the scrapers have almost no impact for these cases.
Buncher1
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Figure 2: Losses in MEBT (left column), DTL (middle column), and SC linac and HEBT (right column) due to a complete failure of MEBT Buncher1 (top row), Buncher2 (middle row), and Buncher3 (bottom row). (Losses into the three MEBT scrapers are not shown.)
2.3 Failure of a quadrupole
In this section, we look at the losses caused by a failure of each quadrupole. For all cases of quadrupole failures, no loss is observed beyond DTL except for the ones in the very beginning of the spoke section. We can also conclude that the scrapers prevent losses in DTL. Exception for the losses caused by the quadrupoles towards the end of the MEBT, for which the scrapers cannot act effectively. This is good for DTL but such cases may have to be taken into account for the design of the scrapers.
Figure 3 is structured in the same way as Figure 2, but now for the first seven quadrupoles of the MEBT. Please note that the failure of the second quadrupole could cause losses all over the first half of the MEBT, and that the third quadrupole could cause losses into the chopper dump. As for the scrapers discussed in the previous paragraph, this might need to be considered for the design of the chopper dump. What should be noted is that, even with the scrapers, we could have losses in the third buncher cavity. The losses in the DTL are larger for the middle triplet (Q4-Q6).
Figure 4 shows the cases for the last four quadrupoles of the MEBT. Because the last and third scraper is located between the ninth and tenth quadrupoles, the scrapers cannot act on failures of these quadrupoles very well (or at all for the failure of the tenth and eleventh).
2.4 MEBT Conclusions
The failures of the buncher cavities do not have immediate effects and a large fraction of the beam actually reaches the target, but cause losses throughout the linac, including the SC sections, raising some concern. The failures of the quadrupoles have immediate effects and the losses are either in MEBT or DTL. The losses caused by the quadrupole failures could be improved by a lot with the MEBT scrapers for some cases. Some cases of the quadrupole failures may cause considerable
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Quad1
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Figure 3: Losses in MEBT (left column), DTL (middle column), and SC linac and HEBT (right column) due to a complete failure of a single quadrupole in the MEBT. From top to bottom are Quadrupole 1 to Quadrupole 7. Losses into the three MEBT scrapers are not shown.
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Figure 4: Losses in MEBT (left column), DTL (middle column), and SC linac and HEBT (right column) due to a complete failure of MEBT Quad8 (first row), Quad9 (second row), Quad10 (third row), and Quad11 (fourth row). (Losses into the three MEBT scrapers are not shown.)
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losses into the scrapers and the chopper dump so further studies, including the thermomechanical calculations of these devices themselves, may be needed. Finally, a more detailed study taking into account the decay of the field should be conducted.
3 Drift Tube Linac
The drift tube linac of the ESS is composed of five independently powered tanks. Each of these tanks is powered by a 2.8 MW klystrons that are fed by independent modulators. The failure of each modulator or klystron will result in a field decay in the DTL tank with a time constant that is inversely proportional to the quality factor, Q, of the DTL tank. In this study the fields are instantly dropped to zero and the transients are not included. This is worse than the real life, since the machine protection system should stop the beam within few micro-seconds. In this study 1×106
macro-particles are used for simulations that start at the beginning of the DTL and are tracked to the target. The power loss in case of an individual cavity failure, or an individual permanent magnet quadrupole (PMQ) failure are presented in the following.
3.1 DTL RF failure
All the five tanks of the DTL have been turned off one by one. Tank one failure results in a complete beam loss before the end of DTL. In other words, with no power to tank 1 the beam will not reach the SC linac. Tank 2 failure results in significant losses within the DTL and also the rest of the linac with complete beam loss before the target. Tanks 3 through 5 failures cause no losses in the DTL tanks, but all the beam will be lost in the linac. These losses are presented in Fig. 5 and the transmission is shown in Fig. 6.
0 50 100 150 200 250 300 350 400 450 500 550 600
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Figure 5: Losses due to individual DTL tank failure
Without power in tank 1 the loss pattern will not be affected by the field value in the other tanks since all the beam is lost before leaving tank 1. However, having tank 1 fully powered beam is almost fully transported through the DTL even if all the downstream cavities are turned off. This is explained by the smooth phase advance variation along the DTL.
3.2 Demagnetized PMQ in the DTL
Increased temperature of the PMQs above their Curie temperature demagnetizes them. Although this does not happen instantly in this study we assume an instant demagnetization of few of the PMQs along the DTL to look at the resulted loss. The losses are presented in Fig. 7.
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Figure 6: Transmission as a result of individual DTL tank failure
0 5 10 15 20 25 30 35 40 45 50 10−2
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Figure 7: Losses due to individual PMQ failure. Each failed quadrupole is marked with vertical dashed lines and the corresponding losses have the same color. The vertical dotted line at ∼43 m indicated the end of DTL.
Failure of each PMQ results in losses that happen very locally within few meters of the failed quadrupole, either in the same tank or within the DTL.
Tank 1 Tank 2 Tank 3 Tank 4 Tank 5 DTL loss Linac loss Case 1 OFF ON ON ON ON 100% – Case 2 ON OFF ON ON ON < 0.1 W ∼100% Case 3 ON ON OFF ON ON 0 100% Case 4 ON ON ON OFF ON 0 100% Case 5 ON ON ON ON OFF 0 100%
Table 1: DTL RF failure
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3.3 DTL Conclusion
Considerable amount of losses are observed inside the DTL (Tank 1 and 2) only if the Tank 1 is off its nominal voltage. If the Tank 1 is at its nominal voltage and the following tanks are completely off, only negligible amount of losses occur within the DTL itself but all the beam is lost in either SC linac or HEBT. These situations are summarized in Table ??.
In the unrealistically pessimistic cases of completely destroyed PMQs, the downstream part of the DTL acts as scrapers and a part of the beam is lost in the DTL. The particles which arrive to the end of the Tank 5 reach the target without causing any further losses.
4 Superconducting Linac and HEBT
In the superconducting linac the magnet fields have been turned off instantaneously from their nominal values to zero. The reason behind this study is to determine if there are hot spots where losses will be focused for any failed magnet in the spoke, medium beta, high beta sections or beam transport system. All the magnets have been turned off one by one (no two magnets simultaneously) and losses have been recorded. Since these were extreme failure cases, which do not define a regular operation of the accelerator the figures of merit of beam like beam emittance, halo or spatial distribution are not being analyzed here.
4.1 Cavity failures in the SCL
Upon failure of the RF system the field in the cavity will decay to zero in the absence of any beam over the time constant of the cavity which is inversely proportional to the Q-value of the cavity. In the presence of a high current beam though, the beam interaction with the cavity will excite the cavity and eventually a field with 50% of the amplitude in the decelerating phase will affect the following bunches. In the first part of the study of the cavity failures, the transients are excluded and the worst case where the beam is decelerated by the cavity is presented to have an idea of the maximum beam loss and activation. The transmission of the linac is shown in Fig. 8 and losses are shown in Fig. 9 for few cases where individual cavities are failed. As is shown in Fig. 8, failure of a cavity from the middle of medium β section where beam is energetic enough does not cause significant losses in the linac and HEBT, however, since the beam energy is below the acceptance of the dogleg the remaining beam is entirely lost in the bend area. Energy gain after the failed cavity is almost zero since beam does not arrive at the right synchronous phase, Fig. 10.
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4.1.1 Correction of the failed cavities
In this part of the study it is assumed that the failed cavities are de-tuned not to be excited through interaction with the passing beam. As a result the field in the cavity remains zero, i.e. cavity act as a drift space. The neighbor cavities (one on each side) are used to match the beam envelope in longitudinal plane. But the beam energy will be different as there is no margin reserved in the cavities to increase the gradients and compensate for missing cavity. As a result the beam velocity downstream of the failed cavity is reduced resulting in an increase in the time of flight to the downstream cavities. Therefore phases of the downstream cavities should be adjusted to have the same synchronous phase including delayed arrival of the beam. Transmission, losses and energy gain after correction of each cavity are shown in Figures 11-13.
4.2 Electromagnet failure in the SCL
The failure of electromagnets in the LWUs will cause significant mismatch and losses happen rather locally. Losses due to failure of every 10 quadrupole (one by one) is plotted in Fig. ??. The first three quadrupoles are located in the spoke section, the second two in medium β, the third four quadrupoles in high β, followed by three between the last cryomodule and first dipole, one in the dogleg and another in the A2T. In the absence of any power supply the ramp and decay time of field in the quadrupoles in ∼ 4.5 s. However, when the power supply is connected (which is the case) the rise and fall time of the magnet is determined by the architecture of the power supply and is ∼ 4 ms. This means that by the time the machine protection systems acts, ∼ 11 µs, the field in the magnets are only 0.5% below the nominal field.
The losses due to quad failure in, as shown in Fig. 14, happen mainly very locally within 20 to 40 m of the failed quadrupole, however, several of the quad failures result in significant loss in the A2T area, due to local tighter apertures in this area.
5 SCL and HEBT Conclusions
A sudden failure of the cavity causes a mismatch between the arrival time of the bunch to the following cavities with respect to the RF wave, which results in null or inefficient acceleration. These unaccelerated particles will be lost, depending on the energy at the failed cavity, locally or in the dogleg 10.
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Figure 10: Energy of the beam when a cavity is failed.
Majority of the single cavity failure cases could be corrected as represented in Fig. 11. In the SC linac, a few cavities in one cryomodule or a few cavities linked to one modulator could potentially fail simultaneously, such cases including their correction scheme should also be studied. While for the cavities hosted in the very last cryomodule of the high β the loss of power would only reduce the final energy, after detuning and matching without any further studies, the other cases need a detailed investigation. On the other hand, as the transmission after the correction of the spoke cavity with highest energy gain is not recovered completely using only two cavities, failure of a pair of high energy gain spokes cavities should be studied further by including larger number of cavities in the correction scheme.
While the risk of a electromagnet failure is much lower than a failure of a cavity, these cases have been studied and the conclusion is that while for low energy quads the losses happen more locally, the quads in the high energy part of the linac cause losses mainly in the A2T, and the amplitude of these high energy losses are a function of the phase-advance between the failed quad and the A2T.
6 Further Studies
The results of the study presented in this note identify the components critical to the losses and the corresponding patterns of the losses. As discussed in the Introduction, however, an estimation of the real integrated losses due to failure of a component requires to take into account the loaded Q-value of the cavity or the decay time constant of the quadrupole together with the response time of the MPS. The total dose level over a long period of time may be also estimated based on the trip rate of a similar component used in other operational accelerators.
Loss of several random single cavities is another topic that should be studied. Possibility of operation with failed components by readjusting the lattice and lowering the beam current should be also studied.
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Figure 11: Transmission after the correction
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Figure 13: Energy after the correction
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Figure 14: Power lost due to QUAD failure. Vertical dashed lines indicate the position of failed quadrupoles and similar solid color bars indicate the losses associated to those. The two vertical dotted black lines represent the position of dipoles.
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Figure 15: Power lost before target due to each quad failure in the SCL and HEBT. Vertical dotted lines indicate the start of Spoke, Medium-β, High-β, HEBT, DogLeg and A2T sections.
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References
[1] R. Miyamoto, “An ESS Linac Collimation Study”, HB’14, to be published.
The latest version of this document could be downloaded from:
https://chess.esss.lu.se/enovia/tvc-action/showObject/dmg_TechnicalReport/ESS-0031413/valid
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Introduction
MEBT
Overview
Failure of a quadrupole
DTL Conclusion
SCL and HEBT Conclusions