Observation of the ion trapping phenomenon with bremsstrahlung

Nuclear Instruments and Methods in Physics Research A248 (1986) 565-568 North-Holland, Amsterdam

565

Letter to the Editor OBSERVATION OF THE ION TRAPPING PHENOMENON

WITH BREMSSTRAHLUNG

H. K O B A Y A K A W A , K. H U K E , M. I Z A W A , Y. K A M I Y A , M. K I H A R A , M. K O B A Y A S H I a n d S. S A K A N A K A

National Laboratory for High Energy Physics, Oho-machi, Tsukuba-gun, lbaraki-ken, 305 Japan Received 10 January 1986

The ion trapping phenomenon was studied at the Photon Factory storage ring. The accumulating process of ions in the electron beam orbit was directly observed by detecting bremsstrahlung from electron-ion collisions.

A circulating beam in the storage ring ionizes molecules of the residual gas in the vacuum chamber. In the electron storage ring, created positive ions can be trapped i n t h e electron beam orbit by an attractive potential. This phenomenon, called ion trapping [1], sometimes limits beam performances, leading to a beam blow-up and a short beam lifetime. At present, experimental knowledge of the ion trapping is quite poor. Because it depends on a large number of quantities, of which precise measurements are often difficult, experiments related to this effect sometimes have large uncertainties. However, because of increasing requirement of a low emittance machine for the brilliant synchrotron radiation source, precise study for this phenomenon is particularly important. So far the following experiments have been reported: (1) Measurements of the betatron tune shifts and tune spreads [2,3,4]. (2) Anomalous beam size blowup [3,4]. (3) Effects of cleating electrodes [5]. (4) Vertical beam pulsation due to ion trapping, which depends on the stored current, the vacuum pressure and the vertical betatron tune [6]. We report in this short note a study of the ion trapping by means of bremsstrahlung measurement which was carried out at the Photon Factory storage ring [7] in the National Laboratory for High Energy Physics (KEK). Because of a direct detection of bremsstrahlung that is produced by collisions of electrons with trapped-ions, counting rate represents the local gas density at the source point right on which the beam traverses [8]. The process of the ion accumulation was observed in a variation of the bremsstrahlung yields synchronized with the vertical pulsation due to the ion trapping. Behavior observed in the present experiment can be understood qualitatively in terms of the two-beam instability model developed by Keil and Zotter [9]. The Photon Factory storage ring, dedicated to the synchrotron radiation experiments, is nominally oper0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

ated in an electron beam energy of 2.5 GeV, and in a multi-bunch mode. The parameters of the storage ring are given in table 1 together with symbols used in this report. The experimental arrangement [8] is shown schematically in fig. 1. The source point was in the vacuum duct of the bending magnet B20. Synchrotron radiation emitted from the bending arc was absorbed in the blank flange. The flange was made of stainless steel (3 mm in thickness) with a radiation absorber. High energy bremsstrahlung passed through the flange and hit on a gamma ray detector which was placed 3530 mm apart from the source point. The gamma ray detector consisted of a lead glass block (300 x 100 x 100 mm3) and a photomultiplier. A veto-counter made of a plastic scintillator was placed in front of the lead glass to eliminate unwanted charged particles. The entire counter system was covered by lead blocks except for an opening slit with an aperture of 7.6 mm in height, 50 mm in width. The aperture was wide enough to capture all the bremsstrahlung, even when Table 1 Principal parameters of the PF ring Energy Circumference Bending radius Rf frequency Harmonic number Revolution frequency Momentum compaction factor Horizontal tune Vertical tune Horizontal damping time Vertical damping time Horizontal emittance Vertical emittance

E L 0 fRF h fr a ~x py ~'Ox ~'~y cx Cy

2.5 GeV 187.07m 8.66 m 500 MHz 312 1.6026MHz 0.04 5.2-5.5 4.1-4.2 9.1 ms 7.8 ms 4.1~r X 10 - 7 mrad 1.2~r × 10 -8 mrad

H. Kobayakawaet aL / Observationof the ion trappingphenomenon

566

B20 Pole Piece ~ Vacuam Chamber .......... - Orl~t - -_-_-_-_-__-_- - -- O r b / , ~eod-Glos, ~, ~ ..... ~ Cererll~v Counter

7-------,'-------~ ~__ I

Sou\'rce Pain, 0 L

Flange

tm

Fig. 1. Schematic drawing of experimental system set at the bending magnet B20. Bremsstrahlung was detected by a lead glass Cherenkov counter placed at 3530 mm apart from the source point.

the beam oscillated vertically. Threshold level of the gamma detection was set approximately to 1.9 GeV. In the Photon Factory storage ring, a vertical beam blowup could be observed under a certain machine condition as an almost regular pulsation appeared in the signal of the beam profile monitor [7], as shown in fig. 2. The profile monitor consists of arrayed photodiodes. Dips observed in the picture correspond to the time that the beam blows up vertically. When we take notice of the highest signal, photon density that hits the corresponding diode decreases as a result of beam size enlargement. After the beam blowup the beam size shrinked with a time constant comparable with the radiation damping time. This phenomenon can be explained [6] by the two-beam instability model, that is, in this case a dipole oscillation was executed between the trapped ions and the electron beam. This vertical blowup [7] appeared almost regularly with a period which varies linearly with the stored beam current. The period was dependent upon the average vacuum pressure. In this experiment, bremsstrahlung was counted by a multi-channel analyser in the multi-channel scaler (MCS) mode, with a trigger started by the dip of the

i

il

¸ i

ii

i

!i

i!

Fig. 2. Vertical pulsation caused by the ion trapping observed with the beam profile monitor in sweep time of 50 ms/division.

photo-diode signal. The highest signal of the photo-diode array was picked out and used to generate the trigger pulse as shown in fig. 3. Bremsstrahlung yields thus measured in the MCS mode are shown in fig. 4, where two examples taken under different conditions are presented. The data were accumulated in multi-time scanning. The vacuum pumps in the storage ring were partly turned off to obtain rather bad vacuum pressure as shown in table 2, in which the case (a) and (b) correspond to the conditions in fig. 4(a) and (b), respectively. The yield of bremsstrahlung strongly relates to the gas composition. A typical mass spectrum is presented in table 3, in which the data were taken at the vacuum chamber near the source point of bremsstrahhing. Table 3 also gives the ionization cross sections [10] for 2.5 GeV electrons. As shown in table 3, the gas components in the chamber are mostly CO and H 2. Taking into account the ionization cross sections, the ionization rate of CO is seven times larger than that of H2. Moreover, because of a larger bremsstrahlung cross section, we can simply assume that mainly CO and CO + contributes to the ion trapping and also to the bremsstrahlung yields. In fig. 5, the bremsstrahlung yields of two examples are plotted in the same time scale. In the case (a), the behavior of counting rate is governed by two different time constants: (1) Rapid drop of the counting rate and increase with a time constant of about 10 ms right after the beam blowup. (2) Slow rise in an interval of 200 ms, until the start of the next blowup. Region (1) corresponds to the process that the beam blew up and shrinked in the radiation damping time as observed in the beam profile monitor (fig. 3). This time varying behavior of the counting rate implies an existence of an ion cloud in which the gas density is higher than that of the residual gas area. Because of large mass differences, electrons move faster than ions when the beam is oscillating, and overlapping of the electron beam and the trapped-ion cloud becomes small. As a result, bremsstrahlung yields decreased. Trapped-ions were partly released from the beam potential during this process, and then .the beam oscillation stopped. After this, as the beam size was damped by the radiation damping, the counting rate increased again. Slow rise in the region (2) can be interpreted as an accumulation process of ions. Increasing rate d Y / d t is 34.6 s -2. Assuming that only CO + contributes to this mechanism and the density distributions of ions and electrons are both Gaussian having the same rms beam size ox and oy, we can estimate the trapping rate of ions (in unit length) by using the following relation:

dni dt

41reax%(dY) fiOBl --~ '

H. Kobayakawa et al. / Obseroation of the ion trapping phenomenon

Vertical

Profile

567

Honitor

Peak S i g n a l o f Vertical

Pzofile

Trigger

btonitor

Pulse

Fig. 3. Triggler pulse generated from the dip, that is, the timing of the beam blowup.

Fig. 4. Bremsstrahlung yields in MCS mode under different conditions: (a) Stored current I = 58 mA and Vy= 4.15. Abscissa: 512 ms full scale. (b) Stored current 1 = 135 mA and py = 4.10. Abscissa: 30.7 ms full scale.

where I is the stored b e a m current, f the transmission factor of b r e m s s t r a h l u n g through the stainless steel flange, l the length of the source point, o B the brems-

Table 2 Beam currents and vacuum conditions in the present experiment Case (a) I (mA) 58 Beam current uy 4.15 Vertical tune pay(Tort) 1.2)<10 -9 Average pressure (min) 930 Lifetime uniform Beam filling Vacuum pumps 1/4 off Distributed ion pumps all on Sputter ion pumps

strahlung cross section of CO + (1.9 G e V < E v < 2.5 GeV). Giving following values; I = 0.058 A, o B = 0.27 × 10 -24 cm 2, ox = 0.15 cm, Oy = 0.038 cm, f = 0.35, l = 12.2 cm a n d d Y / d t = 34.6 s -2, we obtain the trapping rate as d n i / d t = 5.9 × 106 s -1 cm -1. On the other hand, the ionization time for CO, ~'co,is estimated to be about 1.0 s under this vacuum condi-

Case (b) 135 4.10 1.8×10 -9 1070 uniform 1/2 off B19, 20 off

Table 3 Typical spectrum of residual gas in the beam chamber near the source point Molecule M Partial pressure (~) Ionization cross Section [10] ( × 10-1s cm 2)

2 41

CH 4 CO 16 28 8 48

0.30

2.04

H2

Ar 40 2

CO 2 44 1

1 . 7 7 1 . 8 2 2.77

568

H. Kobayakawa et aL / Observation of the ion trappingphenomenon

5000

~

(b)

I :135mA 4000

~=4.1o

3000

(o) I =58mA Pv =4.t 5

g 2000

7

ee e

~000

/-'r

@

eeeee

~

OQ

Another example, shown in fig. 5(b), is a case. of higher beam current, I = 135 mA. At high current the threshold of the instability is low. Therefore, the amount of trapped-ions quickly reaches the threshold and it gives rise to a high repetition beam blowup. Bremsstrahlung observation synchronized with the timing of the beam blowup is a very promising method for the investigation of the ion trapping mechanism. In the present experiment, we observed the existence of trapped-ions and accumulating process of them in the beam orbit. Observed behaviors can be qualitatively interpreted in terms of the two-beam instability theory. By this method, we can measure the real density of the residual gas and ions right on the beam orbit of the source point which is in the bending magnet field in this experiment.

rod

I

°o

3oo Time (ms)

Fig. 5. Bremsstrahlung yields in MCS mode. Two examples in fig. 4 are plotted in the same time scale.

Acknowledgements The authors wish to thank T. Katsura and S. Shibata for the construction of the beam monitor system. We are grateful to S. Ban, H. Hirayama and C.O. Pak for preparing the gamma-ray detection system.

tion, i.e., the partial pressure of CO was 5.8 × 10 -1° Torr. The average increasing rate of ions is dni dt

1 I %0 efrL'

where L is the ring circumference. The increasing rate thus obtained is 1.2 × 107 s -1 cm -1. This value is in fairly good agreement with the trapping rate that is estimated above, although there is a large ambiguity [1] in the amount of ions in the magnetic field. It should be noted that the vertical beam size measured with the profile monitor gradually enlarged during this ion accumulation process, as seen in fig. 3. When the density of accumulated ions reached a certain value that is the threshold of the two-beam instability, the beam started to blow up again. This process repeated. The reason of a rather gentle dip at the time of - 300 ms was that the pulsation was not exactly periodic.

References [1] Y. Baconnier, CERN/PS/PSR 84-24 (1984). [2] M.E. Biagini, S. Guiducci, M. Preger, M. Serio and S. Tazzari, 11th Int. Conf. on High Energy Accelerators (1980) 687. [3] T. Kasuga, H. Yonehara, T. Kinoshita and M. Hasumoto, Jpn. J. Appl. Phys. 24 (1985) 1212. [4] M. Kihara et al., KEK Report KEK 83-5 (1983). [5] A. Poncet, PS/ML/Note 83-1. [6] Y. Kamiya, M. Izawa, T. Katsura, M. Ki_hara, H. Kobayakawa and S. Shibata, Proc. of 5th Symp. Acc. Sci. Tech. (1984) 292. [7] Photon Factory Activity Report (1982/83). [8] M. Kobayashi, K. Huke, S. Ban and H. Hirayama, Proc. of 5th Symp. Acc. Sci. TEch. (1984) 148. [9] E. Keil and B. Zotter, CERN-ISR-TH/71-58 (1971). [10] F.F. Rieke and W. Prepejchal, Phys. Rev. A6 (1972) 1507.