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

Transcription

1 SL-Note BI Energy Spectrum Measurement of the Multipacting Electons 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. Geneva, Switzerland May 2000

2 Energy Spectrum Measurement of the Multipacting Electrons in the SPS. Analysis of the Possible Utilisation of the BGIP Monitor Mauro Pivi CERN LHC Division VAC Group Alessandro Variola CERN SL Division BI Group Introduction An electron cloud cascade driven by the proton beam space charge field is expected to occur in the LHC. This charge cloud composed of secondary, ionisation and photo - electrons is accelerated under the influence of the proton beam electric field as a function of both bunch intensity and period. This process results in the production of a large heat load on the surface (especially on the dipole section beam screen at cryogenic temperature), multiplication of space charge in the chamber, coupling between electrons and the beam and increased vacuum pressure that could ultimately cause the loss of the proton beam itself. In-depth theoretical and experimental work has recently been carried out at CERN [1-1b]. In this framework, a numerical simulation code has been developed [2-2b] following the analytical approach to the problem [3] and a deep research program has been executed. To test the validity of the simulation code, experimental results were obtained in a TW test chamber simulating the proton beam by means of RF pulse trains. Good agreement was observed between experiments and simulations when measuring the energy spectrum of the emitted electrons and the current as a function of the RF pulse parameters. Furthermore, in the SPS-MD with an LHC-type beam, above a beam intensity threshold a pressure rise by more than a factor of 50 has been observed. Multipacting has been measured and unambiguously confirmed, because preliminary test ruled ion-induced desorption out. An extensive program is underway for the SPS in order to test possible remedies and to 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 essential measurement is the determination of the multipacting electrons energy spectrum produced in the SPS by the LHC type proton beam. 1) Energy Spectrum Measurement As discussed previously, the beam-induced electron cloud may produce substantial heat load in the LHC beam pipe. The cryogenic system cannot tolerate a heat load exceeding 1 W/m, and the current capacity based on a heat load induced by multipacting is just 0.6 W/m. Fig.1 shows the measured SPS electron cloud intensity during multipacting. 2

3 Fig. 1: Multipacting in the SPS during an MD with LHC type beam, proton intensity ca protons per batch. Left: The multipacting signal is recorded in synchronism with the proton batch revolution time in the SPS of 23 Ps. The horizontal 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 proton bunch passage (25ns). The heat load is linearly dependent on the average energy of the electrons hitting the beam pipe during multipacting, and electron energy is therefore a parameter which needs to be estimated as accurately as possible. The impact energy distribution of the electrons accelerated by the electric field 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 computer code 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 is shown in Fig. 2. Fig. 2: Multipacting electron energy distribution obtained by simulations, with the SPS LHC-type proton beam, when considering a 25mm chamber radius. Normalisation has been performed on the integrated intensity. 3

4 In the case of the SPS vacuum chamber a secondary emission electrons yield was estimated at ~ 210^7-10^8 e - per mm 2 per second ( ~0.01 na per mm 2 ). Since during multipacting in the SPS the electron current signal is measured for less than 1 Ps, the whole energy spectrum should be measured during this time. Hence, for a single-energy spectrum analyzer to scan only a single electron energy at a time one would need to vary the applied voltage from 0 to 400V in this short interval. Furthermore one would be obliged to deconvolute the measured spectrum with the variation of the current signal. This being quite difficult to achieve, a spectrum analyser allowing the acquisition of the whole spectrum is more suitable. For this purpose, the use of the available BGIP 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 ions velocity spectrum measurement. In fact, while the beam is passing the rest gas ions are accelerated by the radial space charge field that for a gaussian shape (size V, r = radial coordinate) is: qe r 2 r e N ere mec ) ( z, t ) (1) r 2 V where q is the electron charge, m e its rest mass and r e its classical radius, N e is the number of particles and )(z,t) is the longitudinal distribution function with respect to the propagation direction z. Once the accelerated ions are extracted from the vacuum chamber, they are deflected by a magnetic field (converting velocities into positions) and detected by a system that allows the acquisition of a 2D image. By comparing the measured and the simulated spectra it is possible to obtain 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 long stainless steel channel, with a diameter of 153mm, was inserted and fixed by expansion. On one side of this channel there is a 20 x 3 mm window that allows ions extraction. Beyond the window the ions drift into a multilayer mumetal tube that, acting as a magnetic screen, prevents the parasitic magnetic 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 work on the external components of the BGIP without affecting the SPS vacuum operation. The ions pass then inside an iron box (fig. 3 / 3) that shields the parasitic B field and allows a sharp transition of the fringe field at the entrance of the magnet (fig. 3 / 10), which is a LEP corrector [6]. The field is measured 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, a phospor screen which performs the electron-photon conversion and a CCD camera that collects the light and gives an image of the particle velocity spectrum. 4

5 Fig. 3: SPS BGIP Monitor layout: 1) Spectrometer magnet, 2) Vacuum chamber, 3) Shielding iron box, 4) Detector output, 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 main parts of the detector: at the entrance there is a polarisation grid, made of 50 microns diameter and 0.6 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 maximum gain for one MCP is approximately 10^4, but the double stage, in the chevron configuration, can increase the gain up to 5 10^6. The measured impedance of the MCP is ~ 100 M: and maximum applied voltage is 1.2 kv. The MCP plates are separated by a short space where it is possible to apply a voltage (up to 100 V), that stops the low-energy secondary electrons emitted by the first MCP 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 may decrease to 10 % for lower energies. The electrons emitted by the second MCP are collected by a P43 (Gd 2 O 2 S:Tb) phosphor plate. P43 is characterised by a wide emission spectrum in the visible range ( nm, peaked on green nm), and a decay time of above 1 ms ( in the intensity range 100 % - 10 %). The efficiency can vary from 180 to 500 photons/e - as a function of the applied voltages (6 12 kv). The maximum voltage that phosphor can stand is 10 KV, even though, in operation, good results were obtained in the 6-7 kv range. The phosphor resolution is essentially a function of grains size and plate 5

6 thickness. The P43 has a resolution of ~ 75 lp/mm. If one also take into account the two-stage MCP, the resolution drops to lp/mm (~ microns at 90 % of the intensity) with unit magnification ratio. The light emitted from the phosphor is transmitted across a quartz window and collected on a 12 bit CCD camera by means of a camera lens. The CCD is an array of 288 x 384 square pixels that gives a total surface of x mm 2 and it is cooled by a Peltier cell. The camera lens has a 25mm focal 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 E radioactive source (Ni63). The emitted particles had a spectral range from 0 to 60 kev with an average value of 20 kev. The emission rate is ~ 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 MCP and 6 kv on the phosphor plate. Different measurements have confirmed the linear behaviour of the detector 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 ratio mm/pixel with the present optical configuration giving a scaling of 200 Pm/pixel at the phosphor plane. 3) Multipacting electrons energy spectrum measurement To measure the multipacting electrons energy spectrum the utilisation of the BGIP monitor in the present configuration was initially considered. The only essential adjustement needed is the polarity inversion of the magnet power supply. In this context it is necessary to calculate the radius of curvature for electrons in the energy range determined by the simulations (see fig 1). The results for 6

7 this estimation, shown in fig. 5, indicate that, to obtain curvature radius in the order of 10 cm (the BGIP s present radius), we have to apply very low magnetic fields (from 2 to 15 Gauss). Curvature Radius for the Electron Cloud Radius Centimeters Energy ev Field Gauss 500 Fig.5: Curvature radius in case of magnetic deflection. The radius is calculated as a function of the applied magnetic field and of electron energy. In this case there are two main difficulties: the magnet and the power supply are not built for these low values ( typical values for the ion experiments are ~ 200 Gauss) and, what is more important, in the SPS tunnel there is a measured background magnetic field of ~ 3 Gauss. This can affect the measurements since it is impossible to internally shield the parasitic fields without perturbing the B field homogeneity. The absolute error is surely important owing to the low value of field necessary to maintain the 10 cm curvature radius. The only possibility is to shield the entire monitor with considerable effort. 3.1) BGIP with electrostatic deflectors. Owing to the previous considerations, the BGIP deflection principle should be modified by using an electrostatic field instead of a magnetic one. Two different kinds of electrostatic deflecting plates were taken into consideration: a) a simple system of plane parallel electrodes, with a deflecting angle given by: - arctan( El / pe ) (2) where E is the electric field, l the electrode length, p the particle momentum and E the relativistic ratio v/c. Fig. 6 shows the deflection with a 100 V applied voltage and an energy range of the multipacting electrons from 30 to 150 ev. 7

8 Fig.6: Parallel electrodes. Trajectories for 30 ev and 150 ev electrons. The upper line represent the grid of the BGIP detector. 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 selecting spectra windows by properly choosing the voltage applied to the plates. Optimizations are required for a specific experimental configuration. In the case of parallel plates some Poisson simulations were performed to calculate the distortion of the field lines between the plates due to the detector grid. Good results (less than 2% of distortion in the last 10 mm) were obtained with little wings at the 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 consisting of two concentric hemispherical electrodes was considered, as shown in Fig. 8 8

9 Fig. 8: Hemispherical electrodes. Trajectories for 20 ev and 85 ev electrons. The upper line represent the grid of the BGIP detector. The plates are separated by 64 mm. The principle of the energy analyzer, when used in the concentric hemispherical configuration, is the 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 radial potential between the two electrodes is thus U1 U1 I ( r) ln( r) ln( R2 ) (3) R2 R2 ln( ) ln( ) R R 1 1 To measure the whole energy spectrum of the SPS multipacting electrons, shown in fig. 1, both low and high energy spectra are acquired by applying different voltages to the inner electrode, selecting the central momentum trajectory. The estimation of the relative resolution which can be obtained for two energy windows ev (range I) and ev (range II), for plates separated by 64 mm and a curvature radius of respectively 30 and 94 mm, was calculated. The results are shown in the following table: Range I Lower limit Range I Central trajectory Range I Higher limit Energy Applied Voltage Relative Energy Resolution 20 ev 120 V 1 % 56 ev 120 V 0.6 % 85 ev 120 V 0.3 % Range II Lower limit Range II Central trajectory Range II Higher limit 85 ev 500 V 1 % 262 ev 500 V 0.5 % 350 ev 500 V 0.3 % 9

10 The size of the entrance slit should be optimised to get enough multipacting current signal (ca. 5 10^6 e - /sec gives a resolved signal). The detector angle with respect to the beam direction will be chosen to reduce the error due to the size of the entrance slit. The angular spread for which the extracted multipacting electrons remain in the resolution range (200 Pm at the phosphor plane) was also calculated. The simulation result shows a maximum angle of ~ 10 mrad at the vacuum chamber exit. This can be obtained, for example, by a two-diaphragm system separated by 10 cm with an acceptable slit size of 1 x 1 mm. Conclusion The electron cloud is a serious detrimental phenomenon for the LHC beam. Important progress in studying this effect has already been made in both experimental and theoretical fields. A major improvement in the understanding of multipacting can be given by the SPS measurements with the LHC-type proton beam during the next MD periods. In this framework it is important to measure the multipacting electrons energy spectrum. Since the BGIP monitor is already operational in the SPS ring, measuring velocity spectrum of the rest gas ions. The re-utilisation of this device for this purpose was considered. It was demonstrated that the principle of the BGIP can be applied and that the efficiency of the detector is sufficient to yield useful results. Some important aspects have been taken into account to analyse the relationship between the BGIP configuration and the electron energy spectrum range to be measured. In this context deflection trajectories for three different systems that having different advantages have been analysed. Some more important work on optimisation of the system configuration as a function of all the parameters (first of all the extraction slit width) can be carried out to optimise the measurement principle. Precise calibration test, especially in the case of electrostatic deflectors, can be performed by means of an existing electron 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 also makes single energy measurements unnecessary, which would require applied voltage with steep ramps 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 ~ pa. 3) Table 1 shows that in both cases energy resolution is in the order of 1%, which fully meets our expectations. Changes and time schedule may vary according to the solution chosen. A crucial change is inserting a vacuum chamber with the typical diameter of LHC sections (radius ~ 25 mm) into the SPS section. 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 so as 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 is necessary to build a system made up of a vacuum chamber, feedthroughs and deflecting plates. This may take around one and a half months. Calibrating the system may take roughly a week, but it can also be done after the measurements. 10

11 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 LHC Project Report 313 (1999), presented at e + e - Factories 99 conference, KEK Tsukuba, Japan, Sep b] 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 ] F. Zimmermann, ''A simulation Study of Electron-Cloud instability and Beam-Induced Multipactoring in the LHC'', LHC Project Report 95 (1997). 2b] O. Bruning ''Simulations for the Beam-Induced Electron Cloud in the LHC beam screen with Magnetic Field and Image Charges'', LHC project Report 158, 7 November ] F. Zimmermann, Electron cloud effects in high-luminosity colliders, Proc. 14th Advanced ICFA Beam Dynamics Workshop, Frascati, October 1997, eds. L. Palumbo and G. Vignola (Frascati Physics Series, Vol. X, INFN Laboratori Nazionali di Frascati, 1998), pp ] J.Buon et al "A Beam Size Monitor for the Final Focus Test Beam", Nucl.Inst.Meth A306 (1991) ] A.Arauzo Garcia, C.Bovet, I. Koopman, A. Variola "First Results of th Beam Gas Ionisation Profile Monitor (BGIP) Tested in the SPS Ring" to be published in the BIW (Beam Instrumentation Workshop) proceedings Boston ] P.Lebrun Design of the Dipole Magnets for Orbit Correction in LEP Journal de Physique, supplement au n 1, Tome 45, Janvier ] Photek reference n