The fill pattern in proton synchrotrons usually features an empty  gap, longer than the abort kicker raise time, for machine protection. This gap  is referred to as the "abort gap" and any particles, which may accumulate in  it due to injection errors and diffusion between RF buckets, would be lost  inside the ring, rather than in the beam dump, during the kicker firing. In  large proton rings, due to the high energies involved, it is vital to monitor  the build up of charges in the abort gap with a high sensitivity. We present a  study of an abort gap monitor based on a photomultiplier with a gated  microchannel plate, which would allow for detecting low charge densities by  monitoring the synchrotron radiation emitted. We show results of beam test  experiments at the Advanced Light Source using a Hamamatsu 5916U MCP-PMT and  compare them to the specifications for the Large Hadron Collide

Beche, Jean-Francois

Byrd, John

De Santis, Stefano

Denes, Peter

Placidi, Massimo

Turner, William

Zolotorev, Max

The fill pattern in proton synchrotrons usually features an empty gap, longer than the abort kicker raise time, for machine protection. This gap is referred to as the ''abort gap'' and any particles, which may accumulate in it due to injection errors and diffusion between RF buckets, would be lost inside the ring, rather than in the beam dump, during the kicker firing. In large proton rings, due to the high energies involved, it is vital to monitor the build up of charges in the abort gap with a high sensitivity. We present a study of an abort gap monitor based on a photomultiplier with a gated microchannel plate, which would allow for detecting low charge densities by monitoring the synchrotron radiation emitted. We show results of beam test experiments at the Advanced Light Source using a Hamamatsu 5916U MCP-PMT and compare them to the specifications for the Large Hadron Collide

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Development of an Abort Gap Monitor forHigh-Energy Proton Rings*J.-F. Beche, J. Byrd, S. De Santis, P. Denes, M. Placidi, W. Turner,M. ZolotorevLawrence Berkeley National Laboratory, Berkeley, USAAbstract. The fill pattern in proton synchrotrons usually features an empty gap, longer than theabort kicker raise time, for machine protection. This gap is referred to as the “abort gap” and anyparticles, which may accumulate in it due to injection errors and diffusion between RF buckets,would be lost inside the ring, rather than in the beam dump, during the kicker firing. In largeproton rings, due to the high energies involved, it is vital to monitor the build up of charges inthe abort gap with a high sensitivity. We present a study of an abort gap monitor based on aphotomultiplier with a gated microchannel plate, which would allow for detecting low chargedensities by monitoring the synchrotron radiation emitted. We show results of beam testexperiments at the Advanced Light Source using a Hamamatsu 5916U MCP-PMT and comparethem to the specifications for the Large Hadron ColliderINTRODUCTIONLarge proton synchrotron rings are characterized by very high energies stored intheir beams and extensive use of superconducting magnets. Such magnets, as well asother delicate components, can easily be damaged by even a very small fraction of thebeam, if that should be hitting them. Special beam dumps, capable to absorb such highstored energies (~350 MJ/beam in the LHC, for example) are placed along the ringtogether with abort kicker magnets which can steer the entire beam to be lost in thedump in case of irregular operations. It is standard practice to leave a gap, longer thanthe abort kicker raise time, in the ring fill pattern to avoid dispersing particles on othermachine components during the kicker firing. It is vital to assure that no particlesaccumulate in the gap since those would receive only a kick of reduced strength, whenthe dump system is activated, and would hit machine components. The maximumcharge density admissible in the gap can be calculated taking into account the machineenergy and the damage threshold of the most sensitive components (usually quenchingin superconducting magnets sets the lower limit). The abort gap needs then to bemonitored continuously in order to dump the beam or activate gap clearing procedureswhen the safety threshold is reached.                                                 * This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.Several phenomena can cause the gap to become populated: timing errors atinjection; drifting of unbunched particles; diffusion of particles between RF buckets;debunching due to glitches in the RF. The time scale of such phenomena is varied,going from practically instantaneous for injection errors to tens of thousands of turnsfor slow diffusion mechanisms.In this paper we present a preliminary study of a simple instrument, based on agated photomultiplier, which is suitable to be included in a ring’s interlock chain.Experimental tests have been carried out at the Advanced Light Source and theirresults are reported here. Though our instrument is not designed with a particularmachine in mind, we will often make reference to the Large Hadron Collider forwhich an exhaustive study of the abort gap monitoring requirements is available [1].The LHC also makes for an interesting case study due to the necessity of monitoringthe abort gap at two different energies (450 GeV at injection and 7 TeV at finalenergy) with the relative consequences on the photon flux available and differentsafety thresholds.ABORT GAP MONITOR OUTLINEThe fundamental component of our instrument is a gateable microchannel platephotomultiplier tube (MCP-PMT) which we use for measuring the synchrotronradiation emitted by the circulating particles in a suitable diagnostic station (dipole orundulator). By selecting an appropriately long observation window and shifting italong the abort gap, it is possible to monitor the presence of particles continuously andto estimate their longitudinal density. Commercially available MCP-PMTs [2] are theideal instrument for such a task due to their high gain, fast gate, low noise and varietyof peak wavelength of their spectral response. Also the low voltage required for gatingand their reduced dimensions (Fig.1) are other beneficial features that make the use ofsuch a device particularly simple.The optimal time window used has to be determined on the basis of the particularconfiguration of the dumping kicker and dump line. Other aspects to take into accountare the MCP-PMT maximum duty factor, typically of the order of a few percent, theabort gap length, compared to the total length of the ring, and the maximum allowableintegration time, compared to the expected photon flux. As an example, therequirements for the LHC specify maximum number of protons of 4•109 (450 GeV)and 6•106 (7 Tev) over 100 ns intervals. With a 1% duty factor, it is possible to take100 ns long samples only at 10 µs intervals, which means one sample per turn giventhat the abort gap is 3 µs and the revolution period ~89 µs. Mapping the entire abortgap requires 30 of such samples and, therefore, 30 turns. CERN specifications allowfor a 100 ms integration time, which means that it is possible to average over 37distinct measurements for every 100 ns sample. This has to be compared to theexpected [3] intensity in the longitudinal diagnostic station which is ~40 photons/0.1%bandwidth/turn (450 GeV) and ~5 photons/0.1% bandwidth/turn (7 TeV) produced bythe maximum allowable number of protons at an 800 nm wavelength. The typicalquantum efficiency of an MCP-PMT is in the range of a few percents, but they have afairly large bandwidth so that it is possible to integrate photon counts over a 10%range which gives a very good signal to noise ratio.FIGURE 1. Hamamatsu gateable microchannel plate photomultiplier R5916U-50 Series.Monitoring the beginning and the end of the abort gap presents an extra challengedue to the presence of regular bunches, having a current higher by many orders ofmagnitude, right next to these regions. The 1 ns gate raise time in our HamamatsuMCP-PMT is fast enough to ensure that the filled bunches can be gated out.FIGURE 2. Instrument response function or the Hamamatsu R5916U-50 MCP-PMT.On the other hand, the instrument response function (roughly equivalent to the impulseresponse) of the MCP-PMT decays three order of magnitude in about 1 ns, so that thequestion arises on whether the large filled bunches could saturate our instrument,keeping in mind that the separation between RF buckets is only a few nanoseconds inmany rings (2.5 ns in the LHC and 19 ns in the Tevatron, for example). We alsowanted to verify that the intense radiation from the filled bunches wouldn’t saturatethe photocatode, even with the gate signal off. In that case a fast optical (Pockels cell)switch in front of the instrument would have been required, adding to the instrument’scomplexity. To this end we conducted experimental tests at the Advanced LightSource, which features a ~100 ns long gap in its fill and a bunch separation of 2 ns.Further tests on the instrument’s response to unbunched beam will be shortly carriedout at the Tevatron.EXPERIMENTAL RESULTSSome parameters of the ALS storage ring make it a good place for testing apossible utilization of a MCP-PMT as an abort gap monitor for the LHC. Wementioned the similar distance between RF buckets and a gap length equal to thelength of an LHC sample in the previous paragraph. The ALS ring also features aspecial high current bunch (referred to as camshaft) in the middle of its fill gap which,after a while, is followed by very low current parasitic bunches due to diffusion ofelectrons from the camshaft into the following RF buckets. This is particularlyinteresting from the point of view of studying eventual saturation phenomena in theMCP-PMT, which could not allow to detect a low charge density next to a muchlarger one.Our experimental station is at the ALS diagnostic beamline (BL 3.1) which uses thesynchrotron radiation generated in a bending dipole (1.3 T).FIGURE 3. Block diagram of the experimental setup.Figure 3 shows the block diagram of our experimental setup at BL 3.1. The 10 Vgating pulse, synchronized to the ring orbit clock (SROC), can be shifted around usinga delay box and the internal pulser capabilities.FIGURE 4. The ALS fill gap with camshaft as measured by an MCP-PMT.Figure 4 shows an image of the entire ALS fill gap. Four regularly filled bunchescan be seen at the beginning and at the end of the gap and the camshaft is the largersignal between them. This picture was obtained setting the oscilloscope on an infinitepersistence time, so that we are indeed observing data accumulated over a very largenumber of turns. This was done because, at the beginning of the experiment, we hadplaced a neutral density filter in front of the MCP-PMT, to be sure not to expose it toexcessive light.FIGURE 5. Image of the camshaft, trailing parasitic bunch and first 6 bunches of the ALS fill train. Noaverages.After this additional attenuation is removed, Fig.5 gives an idea of the instrumenttime resolution. This image was taken with no averaging and shows the good signal-to-noise characteristics of our instrument.FIGURE 6. a) Beginning of the ALS gap and the last 14 filled RF buckets. b) Filled RF buckets aregated out by shifting the gate signal. Vertical scale is enlarged to show parasitic bunches.In Fig.6 we show how it is possible to detect very low intensity bunches simply bygating out nearby large signals. The two parasitic bunches after the first one (which isalso visible in Fig.6a) shown in Fig.6b have an estimated population of roughly 107electrons, about 100 times less than the regular bunches, showing that photocatodesaturation is not an issue.Figure 7 further demonstrate that transit time spread in the MCP-PMT has onlylimited effects on the measurement. Comparing the left side of the trace (with the gateoff) to the rightmost side (gate on, no charges) shows the noise level of the instrument.Bunched particles are still easily detectable, even at low densities, since one knowswhere to look for them. Tests are being planned at the Tevatron to check theperformance with unbunched beam.FIGURE 7. Close up view of the last bunch of the ALS train and the nearby parasitic bunch.IN CONCLUSIONSWe have presented what we believe is a promising technique for monitoring thepresence of low charge densities in the abort gap of storage rings. We use a gatedmicrochannel plate photomultiplier for measuring the synchrotron radiation emittedand it is then possible to integrate the detected signal over the desired observationwindow to estimate the number of particles therein. The simplicity of the schemeproposed makes it suitable to be included in a machine interlock chain, whererobustness and high MTBF are paramount.Estimates of the signal levels obtainable in the LHC and experimental results at theALS have also been shown.REFERENCES1. C. Fischer, High Sensitivity Measurement of the Longitudinal Distribution of the LHC Beams, LHC-B-ES-0005, 2003.2. http://www.hamamatsu.com.3. M. Facchini and R. Jung, private communication.

Development of an abort gap monitor for high-energy proton rings

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Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleDevelopment of an abort gap monitor for high-energy proton ringsPermalinkhttps://escholarship.org/uc/item/8pw350bmAuthorsBeche, Jean-FrancoisByrd, JohnDe Santis, Stefanoet al.Publication Date2004-05-03eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaDevelopment of an Abort Gap Monitor forHigh-Energy Proton Rings*J.-F. Beche, J. Byrd, S. De Santis, P. Denes, M. Placidi, W. Turner,M. ZolotorevLawrence Berkeley National Laboratory, Berkeley, USAAbstract. The fill pattern in proton synchrotrons usually features an empty gap, longer than theabort kicker raise time, for machine protection. This gap is referred to as the “abort gap” and anyparticles, which may accumulate in it due to injection errors and diffusion between RF buckets,would be lost inside the ring, rather than in the beam dump, during the kicker firing. In largeproton rings, due to the high energies involved, it is vital to monitor the build up of charges inthe abort gap with a high sensitivity. We present a study of an abort gap monitor based on aphotomultiplier with a gated microchannel plate, which would allow for detecting low chargedensities by monitoring the synchrotron radiation emitted. We show results of beam testexperiments at the Advanced Light Source using a Hamamatsu 5916U MCP-PMT and comparethem to the specifications for the Large Hadron ColliderINTRODUCTIONLarge proton synchrotron rings are characterized by very high energies stored intheir beams and extensive use of superconducting magnets. Such magnets, as well asother delicate components, can easily be damaged by even a very small fraction of thebeam, if that should be hitting them. Special beam dumps, capable to absorb such highstored energies (~350 MJ/beam in the LHC, for example) are placed along the ringtogether with abort kicker magnets which can steer the entire beam to be lost in thedump in case of irregular operations. It is standard practice to leave a gap, longer thanthe abort kicker raise time, in the ring fill pattern to avoid dispersing particles on othermachine components during the kicker firing. It is vital to assure that no particlesaccumulate in the gap since those would receive only a kick of reduced strength, whenthe dump system is activated, and would hit machine components. The maximumcharge density admissible in the gap can be calculated taking into account the machineenergy and the damage threshold of the most sensitive components (usually quenchingin superconducting magnets sets the lower limit). The abort gap needs then to bemonitored continuously in order to dump the beam or activate gap clearing procedureswhen the safety threshold is reached.                                                 * This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.Several phenomena can cause the gap to become populated: timing errors atinjection; drifting of unbunched particles; diffusion of particles between RF buckets;debunching due to glitches in the RF. The time scale of such phenomena is varied,going from practically instantaneous for injection errors to tens of thousands of turnsfor slow diffusion mechanisms.In this paper we present a preliminary study of a simple instrument, based on agated photomultiplier, which is suitable to be included in a ring’s interlock chain.Experimental tests have been carried out at the Advanced Light Source and theirresults are reported here. Though our instrument is not designed with a particularmachine in mind, we will often make reference to the Large Hadron Collider forwhich an exhaustive study of the abort gap monitoring requirements is available [1].The LHC also makes for an interesting case study due to the necessity of monitoringthe abort gap at two different energies (450 GeV at injection and 7 TeV at finalenergy) with the relative consequences on the photon flux available and differentsafety thresholds.ABORT GAP MONITOR OUTLINEThe fundamental component of our instrument is a gateable microchannel platephotomultiplier tube (MCP-PMT) which we use for measuring the synchrotronradiation emitted by the circulating particles in a suitable diagnostic station (dipole orundulator). By selecting an appropriately long observation window and shifting italong the abort gap, it is possible to monitor the presence of particles continuously andto estimate their longitudinal density. Commercially available MCP-PMTs [2] are theideal instrument for such a task due to their high gain, fast gate, low noise and varietyof peak wavelength of their spectral response. Also the low voltage required for gatingand their reduced dimensions (Fig.1) are other beneficial features that make the use ofsuch a device particularly simple.The optimal time window used has to be determined on the basis of the particularconfiguration of the dumping kicker and dump line. Other aspects to take into accountare the MCP-PMT maximum duty factor, typically of the order of a few percent, theabort gap length, compared to the total length of the ring, and the maximum allowableintegration time, compared to the expected photon flux. As an example, therequirements for the LHC specify maximum number of protons of 4•109 (450 GeV)and 6•106 (7 Tev) over 100 ns intervals. With a 1% duty factor, it is possible to take100 ns long samples only at 10 µs intervals, which means one sample per turn giventhat the abort gap is 3 µs and the revolution period ~89 µs. Mapping the entire abortgap requires 30 of such samples and, therefore, 30 turns. CERN specifications allowfor a 100 ms integration time, which means that it is possible to average over 37distinct measurements for every 100 ns sample. This has to be compared to theexpected [3] intensity in the longitudinal diagnostic station which is ~40 photons/0.1%bandwidth/turn (450 GeV) and ~5 photons/0.1% bandwidth/turn (7 TeV) produced bythe maximum allowable number of protons at an 800 nm wavelength. The typicalquantum efficiency of an MCP-PMT is in the range of a few percents, but they have afairly large bandwidth so that it is possible to integrate photon counts over a 10%range which gives a very good signal to noise ratio.FIGURE 1. Hamamatsu gateable microchannel plate photomultiplier R5916U-50 Series.Monitoring the beginning and the end of the abort gap presents an extra challengedue to the presence of regular bunches, having a current higher by many orders ofmagnitude, right next to these regions. The 1 ns gate raise time in our HamamatsuMCP-PMT is fast enough to ensure that the filled bunches can be gated out.FIGURE 2. Instrument response function or the Hamamatsu R5916U-50 MCP-PMT.On the other hand, the instrument response function (roughly equivalent to the impulseresponse) of the MCP-PMT decays three order of magnitude in about 1 ns, so that thequestion arises on whether the large filled bunches could saturate our instrument,keeping in mind that the separation between RF buckets is only a few nanoseconds inmany rings (2.5 ns in the LHC and 19 ns in the Tevatron, for example). We alsowanted to verify that the intense radiation from the filled bunches wouldn’t saturatethe photocatode, even with the gate signal off. In that case a fast optical (Pockels cell)switch in front of the instrument would have been required, adding to the instrument’scomplexity. To this end we conducted experimental tests at the Advanced LightSource, which features a ~100 ns long gap in its fill and a bunch separation of 2 ns.Further tests on the instrument’s response to unbunched beam will be shortly carriedout at the Tevatron.EXPERIMENTAL RESULTSSome parameters of the ALS storage ring make it a good place for testing apossible utilization of a MCP-PMT as an abort gap monitor for the LHC. Wementioned the similar distance between RF buckets and a gap length equal to thelength of an LHC sample in the previous paragraph. The ALS ring also features aspecial high current bunch (referred to as camshaft) in the middle of its fill gap which,after a while, is followed by very low current parasitic bunches due to diffusion ofelectrons from the camshaft into the following RF buckets. This is particularlyinteresting from the point of view of studying eventual saturation phenomena in theMCP-PMT, which could not allow to detect a low charge density next to a muchlarger one.Our experimental station is at the ALS diagnostic beamline (BL 3.1) which uses thesynchrotron radiation generated in a bending dipole (1.3 T).FIGURE 3. Block diagram of the experimental setup.Figure 3 shows the block diagram of our experimental setup at BL 3.1. The 10 Vgating pulse, synchronized to the ring orbit clock (SROC), can be shifted around usinga delay box and the internal pulser capabilities.FIGURE 4. The ALS fill gap with camshaft as measured by an MCP-PMT.Figure 4 shows an image of the entire ALS fill gap. Four regularly filled bunchescan be seen at the beginning and at the end of the gap and the camshaft is the largersignal between them. This picture was obtained setting the oscilloscope on an infinitepersistence time, so that we are indeed observing data accumulated over a very largenumber of turns. This was done because, at the beginning of the experiment, we hadplaced a neutral density filter in front of the MCP-PMT, to be sure not to expose it toexcessive light.FIGURE 5. Image of the camshaft, trailing parasitic bunch and first 6 bunches of the ALS fill train. Noaverages.After this additional attenuation is removed, Fig.5 gives an idea of the instrumenttime resolution. This image was taken with no averaging and shows the good signal-to-noise characteristics of our instrument.FIGURE 6. a) Beginning of the ALS gap and the last 14 filled RF buckets. b) Filled RF buckets aregated out by shifting the gate signal. Vertical scale is enlarged to show parasitic bunches.In Fig.6 we show how it is possible to detect very low intensity bunches simply bygating out nearby large signals. The two parasitic bunches after the first one (which isalso visible in Fig.6a) shown in Fig.6b have an estimated population of roughly 107electrons, about 100 times less than the regular bunches, showing that photocatodesaturation is not an issue.Figure 7 further demonstrate that transit time spread in the MCP-PMT has onlylimited effects on the measurement. Comparing the left side of the trace (with the gateoff) to the rightmost side (gate on, no charges) shows the noise level of the instrument.Bunched particles are still easily detectable, even at low densities, since one knowswhere to look for them. Tests are being planned at the Tevatron to check theperformance with unbunched beam.FIGURE 7. Close up view of the last bunch of the ALS train and the nearby parasitic bunch.IN CONCLUSIONSWe have presented what we believe is a promising technique for monitoring thepresence of low charge densities in the abort gap of storage rings. We use a gatedmicrochannel plate photomultiplier for measuring the synchrotron radiation emittedand it is then possible to integrate the detected signal over the desired observationwindow to estimate the number of particles therein. The simplicity of the schemeproposed makes it suitable to be included in a machine interlock chain, whererobustness and high MTBF are paramount.Estimates of the signal levels obtainable in the LHC and experimental results at theALS have also been shown.REFERENCES1. C. Fischer, High Sensitivity Measurement of the Longitudinal Distribution of the LHC Beams, LHC-B-ES-0005, 2003.2. http://www.hamamatsu.com.3. M. Facchini and R. Jung, private communication.

Development of an abort gap monitor for high-energy proton rings

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