Energy Spectrum Measurement of the Multipacting Electons in the SPS… · 2015-07-29 ·...

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN � SL DIVISION

Geneva, SwitzerlandMay 2000

SL-Note-2000-040 BI

Energy Spectrum Measurement of the MultipactingElectons in the SPS.

Analysis of the Possible Utilisation of the BGIP Monitor

Pivi, M. – LHC Division – VAC Group

Variola, A. – SL Division – BI Group

Abstract

See the introduction in next page.


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Energy Spectrum Measurement of the Multipacting Electrons in the SPS.Analysis of the Possible Utilisation of the BGIP Monitor

-------------------------------------------------------------------------------------------------------------------------

Mauro PiviCERN – LHC Division – VAC Group

Alessandro VariolaCERN – SL Division – BI Group

Introduction

An electron cloud cascade driven by the proton beam space charge field is expected to occur in theLHC. This charge cloud composed of secondary, ionisation and photo - electrons is acceleratedunder the influence of the proton beam electric field as a function of both bunch intensity andperiod. This process results in the production of a large heat load on the surface (especially on thedipole section beam screen at cryogenic temperature), multiplication of space charge in thechamber, coupling between electrons and the beam and increased vacuum pressure that couldultimately cause the loss of the proton beam itself.In-depth theoretical and experimental work has recently been carried out at CERN [1-1b]. In thisframework, a numerical simulation code has been developed [2-2b] following the analyticalapproach to the problem [3] and a deep research program has been executed. To test the validity ofthe simulation code, experimental results were obtained in a TW test chamber simulating the protonbeam by means of RF pulse trains. Good agreement was observed between experiments andsimulations when measuring the energy spectrum of the emitted electrons and the current as afunction of the RF pulse parameters. Furthermore, in the SPS-MD with an LHC-type beam, above abeam intensity threshold a pressure rise by more than a factor of 50 has been observed. Multipactinghas been measured and unambiguously confirmed, because preliminary test ruled ion-induceddesorption out. An extensive program is underway for the SPS in order to test possible remedies andto avoid the detrimental effect of the electron-cloud in the LHC.To validate the simulation code by testing it on a real accelerator, the missing but essentialmeasurement is the determination of the multipacting electrons energy spectrum produced in theSPS by the LHC – type proton beam.

1) Energy Spectrum Measurement

As discussed previously, the beam-induced electron cloud may produce substantial heat load in theLHC beam pipe. The cryogenic system cannot tolerate a heat load exceeding 1 W/m, and the currentcapacity based on a heat load induced by multipacting is just 0.6 W/m. Fig.1 shows the measuredSPS electron cloud intensity during multipacting.


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Fig. 1: Multipacting in the SPS during an MD with LHC type beam, proton intensity ca. 6.0 1012 protons per batch.Left: The multipacting signal is recorded in synchronism with the proton batch revolution time in the SPS of 23 Ps. Thehorizontal scale is 10 Ps/div, the vertical scale is 2mV/div.Right: measurement with an horizontal scale of 200ns/div. The multipacting signal increase is repeated at every protonbunch passage (25ns).

The heat load is linearly dependent on the average energy of the electrons hitting the beam pipeduring multipacting, and electron energy is therefore a parameter which needs to be estimated asaccurately as possible. The impact energy distribution of the electrons accelerated by the electricfield of the subsequent bunches towards the opposite wall is a function of beam intensity.To verify the validity of the estimated electron energy spectrum, the LHC multipacting computercode has been adjusted to the SPS chamber geometry, and has been run with a LHC-type beam,which will be used during the next MD sessions. The energy spectrum obtained by the simulation isshown in Fig. 2.

Fig. 2: Multipacting electron energy distribution obtained by simulations, with the SPS LHC-type proton beam, whenconsidering a 25mm chamber radius. Normalisation has been performed on the integrated intensity.


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In the case of the SPS vacuum chamber a secondary emission electrons yield was estimated at ~210^7-10^8 e- per mm2 per second ( ~0.01 nA per mm2). Since during multipacting in the SPS theelectron current signal is measured for less than 1 �s, the whole energy spectrum should bemeasured during this time. Hence, for a single-energy spectrum analyzer to scan only a singleelectron energy at a time one would need to vary the applied voltage from 0 to 400V in this shortinterval. Furthermore one would be obliged to deconvolute the measured spectrum with thevariation of the current signal. This being quite difficult to achieve, a spectrum analyser allowingthe acquisition of the whole spectrum is more suitable. For this purpose, the use of the availableBGIP monitor was considered.

2) The BGIP Monitor concept

The BGIP is a monitor that gives the r.m.s value of the beam size by means of the rest gas ionsvelocity spectrum measurement. In fact, while the beam is passing the rest gas ions are acceleratedby the radial space charge field that for a gaussian shape (size��, r = radial coordinate) is:

r

etzcmrNqE

r

eeer

2

2

22 1

),(2V

�

�� (1)

where q is the electron charge, me its rest mass and re its classical radius, Ne is the number ofparticles and �(z,t) is the longitudinal distribution function with respect to the propagation directionz. Once the accelerated ions are extracted from the vacuum chamber, they are deflected by amagnetic field (converting velocities into positions) and detected by a system that allows theacquisition of a 2D image. By comparing the measured and the simulated spectra it is possible toobtain the r.m.s beam size. The BGIP principle was already proposed and verified in the FFTB [4]and further experimental evidence was given by the proton beam measurements in the SPS ring [5].

2.1) The SPS BGIP Monitor

Figure.3 shows the different BGIP components. In the SPS vacuum chamber a 250 mm longstainless steel channel, with a diameter of 153mm, was inserted and fixed by expansion. On oneside of this channel there is a 20 x 3 mm window that allows ions extraction. Beyond the windowthe ions drift into a multilayer mumetal tube that, acting as a magnetic screen, prevents the parasiticmagnetic fields from modifying the ions dynamics before they enter the spectrometer magnet.Immediately after a vacuum valve is installed to isolate the chamber, so that it is possible to workon the external components of the BGIP without affecting the SPS vacuum operation. The ions passthen inside an iron box (fig. 3 / 3) that shields the parasitic B field and allows a sharp transition ofthe fringe field at the entrance of the magnet (fig. 3 / 10), which is a LEP corrector [6]. The field ismeasured in the magnet by means of a Hall probe with a 0.1 % accuracy. The vacuum chamber(fig. 3 / 2) inserted into the magnet has a dedicated pumping station and ends with the ion detector(fig. 3 / 4). This device is composed of a double MCP stage that amplifies the charge signal, aphospor screen which performs the electron-photon conversion and a CCD camera that collects thelight and gives an image of the particle velocity spectrum.


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Fig. 3: SPS BGIP Monitor layout: 1) Spectrometer magnet, 2) Vacuum chamber, 3) Shielding iron box, 4) Detectoroutput, 10) Magnet entrance

Let us see in more detail the characteristics of the detector:

2.1.1) The Detector

The detector was set up in collaboration with the PHOTEK company [7]. Figure 4 shows the mainparts of the detector: at the entrance there is a polarisation grid, made of 50 microns diameter and0.6 mm spaced titanium wires. After 2mm there is the input of the double stage MCP amplifier.Every MCP plate is 0.43 mm thick with channels’ diameter of around 10 microns. The maximumgain for one MCP is approximately 10^4, but the double stage, in the “chevron” configuration, canincrease the gain up to 5 10^6. The measured impedance of the MCP is ~ 100 M� and maximumapplied voltage is 1.2 kV. The MCP plates are separated by a short space where it is possible toapply a voltage (up to 100 V), that stops the low-energy secondary electrons emitted by the firstMCP with a fast trigger, thus allowing the selection of pre-determined signal integration windows.The detector efficiency is estimated at 50-75% for electrons between 500 and 4000 eV, but maydecrease to 10 % for lower energies.The electrons emitted by the second MCP are collected by a P43 (Gd2O2S:Tb) phosphor plate. P43is characterised by a wide emission spectrum in the visible range (370 – 680 nm, peaked on green -545 nm), and a decay time of above 1 ms ( in the intensity range 100 % - 10 %). The efficiency canvary from 180 to 500 photons/e- as a function of the applied voltages (6 – 12 kV). The maximumvoltage that phosphor can stand is 10 KV, even though, in operation, good results were obtained inthe 6-7 kV range. The phosphor resolution is essentially a function of grains size and plate


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thickness. The P43 has a resolution of ~ 75 lp/mm. If one also take into account the two-stageMCP, the resolution drops to 20-30 lp/mm (~ 30-50 microns at 90 % of the intensity) with unitmagnification ratio.The light emitted from the phosphor is transmitted across a quartz window and collected on a 12 bitCCD camera by means of a camera lens. The CCD is an array of 288 x 384 square pixels that givesa total surface of 6.624 x 8.832 mm2 and it is cooled by a Peltier cell. The camera lens has a 25mmfocal length and a 0.8 N.O.

Fig. 4: SPS BGIP Monitor detector layout

2.1.2) Detector test and calibration.

The Photek detector was tested at CERN by means of a radioactive source (Ni63). The emittedparticles had a spectral range from 0 to 60 keV with an average value of 20 keV. The emission rateis ~ 3.5 MBq but, considering the solid angle range intercepted in the experimental configuration,the hitting rate was ~ 5 10 ^ 5 e-/s. A good signal was obtained with 2.2 kV on the two-stage MCPand 6 kV on the phosphor plate. Different measurements have confirmed the linear behaviour of thedetector if the voltage applied to the MCP is raised from 2 to 2.2 kV.Several tests, both in the tunnel and in the laboratory, have been performed to calibrate the ratiomm/pixel with the present optical configuration giving a scaling of 200 �m/pixel at the phosphorplane.

3) Multipacting electrons energy spectrum measurement

To measure the multipacting electrons energy spectrum the utilisation of the BGIP monitor in thepresent configuration was initially considered. The only essential adjustement needed is the polarityinversion of the magnet power supply. In this context it is necessary to calculate the radius ofcurvature for electrons in the energy range determined by the simulations (see fig 1). The results for


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this estimation, shown in fig. 5, indicate that, to obtain curvature radius in the order of 10 cm (theBGIP’s present radius), we have to apply very low magnetic fields (from 2 to 15 Gauss).

Curvature Radius for the Electron Cloud

100200

300

400

500

Energy eV

6

8

10

1214

Field Gauss

0

5

10

15

Radius Centimeters

100200

300

400

500

Energy eV

Fig.5: Curvature radius in case of magnetic deflection. The radius is calculated as a function of the applied magneticfield and of electron energy.

In this case there are two main difficulties: the magnet and the power supply are not built for theselow values ( typical values for the ion experiments are ~ 200 Gauss) and, what is more important, inthe SPS tunnel there is a measured background magnetic field of ~ 3 Gauss. This can affect themeasurements since it is impossible to internally shield the parasitic fields without perturbing the Bfield homogeneity. The absolute error is surely important owing to the low value of field necessaryto maintain the 10 cm curvature radius. The only possibility is to shield the entire monitor withconsiderable effort.

3.1) BGIP with electrostatic deflectors.

Owing to the previous considerations, the BGIP deflection principle should be modified by using anelectrostatic field instead of a magnetic one. Two different kinds of electrostatic deflecting plateswere taken into consideration:

a) a simple system of plane parallel electrodes, with a deflecting angle given by:

)/arctan( �� pEl� (2)

where E is the electric field, l the electrode length, p the particle momentum and the relativisticratio v/c. Fig. 6 shows the deflection with a 100 V applied voltage and an energy range of themultipacting electrons from 30 to 150 eV.


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Fig.6: Parallel electrodes. Trajectories for 30 eV and 150 eV electrons. The upper line represent the grid of the BGIPdetector. The plates are separated by 64 mm and they are 70 mm long.

The absolute resolution depends on the electron energy range and could be increased by selectingspectra windows by properly choosing the voltage applied to the plates. Optimizations are requiredfor a specific experimental configuration. In the case of parallel plates some Poisson simulationswere performed to calculate the distortion of the field lines between the plates due to the detectorgrid. Good results (less than 2% of distortion in the last 10 mm) were obtained with little “wings” atthe end of the plates (see fig.7)

Fig. 7: Poisson simulation. Electric field lines for parallel plates. 'V = 600 V

a) In order to increase the energy resolution selecting central momentum trajectories (furthermore,detector efficiency depends on the electrons incidence angle), a second configuration consistingof two concentric hemispherical electrodes was considered, as shown in Fig. 8


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Fig. 8: Hemispherical electrodes. Trajectories for 20 eV and 85 eV electrons. The upper line represent the grid of theBGIP detector. The plates are separated by 64 mm.

The principle of the energy analyzer, when used in the concentric hemispherical configuration, isthe following: two cylindrical plates of radii R1 (inner) and R2 (outer) are positioned concentrically.A positive potential U1 is applied to the inner electrode while the outer one is grounded. The radialpotential between the two electrodes is thus

)ln()ln(

)ln()ln(

)( 2

1

2

1

1

2

1 R

R

RU

r

R

RU

r �� (3)

To measure the whole energy spectrum of the SPS multipacting electrons, shown in fig. 1, both lowand high energy spectra are acquired by applying different voltages to the inner electrode, selectingthe central momentum trajectory.The estimation of the relative resolution which can be obtained for two energy windows 20-85 eV(range I) and 85-350 eV (range II), for plates separated by 64 mm and a curvature radius ofrespectively 30 and 94 mm, was calculated. The results are shown in the following table:

Energy Applied Voltage RelativeEnergy Resolution

Range ILower limit

20 eV 120 V 1 %

Range ICentral trajectory

56 eV 120 V 0.6 %

Range IHigher limit

85 eV 120 V 0.3 %

------------------------------------------------------------------------------------------------------------------------Range IILower limit

85 eV 500 V 1 %

Range IICentral trajectory

262 eV 500 V 0.5 %

Range IIHigher limit

350 eV 500 V 0.3 %


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The size of the entrance slit should be optimised to get enough multipacting current signal (ca. 510^6 e-/sec gives a resolved signal). The detector angle with respect to the beam direction will bechosen to reduce the error due to the size of the entrance slit. The angular spread for which theextracted multipacting electrons remain in the resolution range (200 �m at the phosphor plane) wasalso calculated. The simulation result shows a maximum angle of ~ 10 mrad at the vacuum chamberexit. This can be obtained, for example, by a two-diaphragm system separated by 10 cm with anacceptable slit size of 1 x 1 mm.

Conclusion

The electron cloud is a serious detrimental phenomenon for the LHC beam. Important progress instudying this effect has already been made in both experimental and theoretical fields. A majorimprovement in the understanding of multipacting can be given by the SPS measurements with theLHC-type proton beam during the next MD periods. In this framework it is important to measurethe multipacting electrons energy spectrum. Since the BGIP monitor is already operational in theSPS ring, measuring velocity spectrum of the rest gas ions. The re-utilisation of this device for thispurpose was considered. It was demonstrated that the principle of the BGIP can be applied and thatthe efficiency of the detector is sufficient to yield useful results. Some important aspects have beentaken into account to analyse the relationship between the BGIP configuration and the electronenergy spectrum range to be measured. In this context deflection trajectories for three differentsystems that having different advantages have been analysed. Some more important work onoptimisation of the system configuration as a function of all the parameters (first of all theextraction slit width) can be carried out to optimise the measurement principle. Precise calibrationtest, especially in the case of electrostatic deflectors, can be performed by means of an existingelectron gun.There is clear evidence in favour of BGIP application to spectrum measurement.1) This kind of measurement makes fast acquisition of a large spectrum window possible. It alsomakes single energy measurements unnecessary, which would require applied voltage with steepramps of a duration in the order of the batch length.2) MCP signal amplification provides good dynamics and detects even weak signals in the order of~ 0.1-0.05 pA.3) Table 1 shows that in both cases energy resolution is in the order of 1%, which fully meets ourexpectations.Changes and time schedule may vary according to the solution chosen. A crucial change is insertinga vacuum chamber with the typical diameter of LHC sections (radius ~ 25 mm) into the SPSsection. This requires breaking vacuum, which means a two or three days’ shutdown.If we opt for the magnetic field solution, the only major change is replacing magnet power supply soas to have better sensitivity. As has already been said, it would take longer to calibrate, measure(and probably screen) residual magnetic field in the SPS tunnel.In the case of electrostatic plates, besides optimising monitor geometry for measurement, it isnecessary to build a system made up of a vacuum chamber, feedthroughs and deflecting plates. Thismay take around one and a half months. Calibrating the system may take roughly a week, but it canalso be done after the measurements.


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References

1] V. Baglin, I.R. Collins, J. Gómez-Goñi, O. Gröbner, B. Henrist, N. Hilleret, J-M. Laurent, M.Pivi -R. Cimino - V.V. Anashin, R.V. Dostovalov, N.V. Fedorov, A.A. Krasnov, O.B. Malyshev,E.E. Pyata, “Experimental investigations of the electron cloud key parameters”, CERN LHCProject Report 313 (1999), presented at e+e- Factories’99 conference, KEK Tsukuba, Japan, 21-24Sep 1999.

1b] O. Brüning, F. Caspers, I.R. Collins, O. Gröbner, B. Henrist, N. Hilleret, J.-M. Laurent,M. Morvillo, M. Pivi, F. Ruggiero and X. Zhang “Electron Cloud and Beam Scrubbing in the LHC”,CERN LHC Project Report 290 (1999), presented at the Particle Accelerator Conference(PAC’99), New York, 29 Mar - 2 Apr 1999.

2] F. Zimmermann, ''A simulation Study of Electron-Cloud instability and Beam-InducedMultipactoring in the LHC'', LHC Project Report 95 (1997).

2b] O. Bruning ''Simulations for the Beam-Induced Electron Cloud in the LHC beam screen withMagnetic Field and Image Charges'', LHC project Report 158, 7 November 1997.

3] F. Zimmermann, Electron cloud effects in high-luminosity colliders, Proc. 14th AdvancedICFA Beam Dynamics Workshop, Frascati, 20-25 October 1997, eds. L. Palumbo andG. Vignola (Frascati Physics Series, Vol. X, INFN Laboratori Nazionali di Frascati, 1998),pp. 419-424.

4] J.Buon et al "A Beam Size Monitor for the Final Focus Test Beam", Nucl.Inst.Meth A306(1991) 93-111.

5] A.Arauzo Garcia, C.Bovet, I. Koopman, A. Variola "First Results of th Beam Gas IonisationProfile Monitor (BGIP) Tested in the SPS Ring" to be published in the BIW (BeamInstrumentation Workshop) proceedings – Boston 2000.

6] P.Lebrun “Design of the Dipole Magnets for Orbit Correction in LEP” Journal de Physique,supplement au n 1, Tome 45, Janvier 1984.

7] Photek reference n 90073.

http://wwwslap.cern.ch/collective/electron-cloud/electron-cloud.html

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Energy Spectrum Measurement of the Multipacting Electons in the SPS… · 2015-07-29 ·...

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