Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

IMPROVEMENT OF LASER WIRE MONITOR AT NEWSUBARU
S.Hashimoto#Laboratory of Advanced Science and Technology for Industry, University of Hyogo
NewSUBARU, 1-1-2 Koto, Kamigori, Ako, Hyogo, 678-1205 JAPANFig. 1, which shows time chart of the stored beam current
and the number of gamma photons per second. Thesegamma photons are generated due to Bremsstrahlung,because laser power is stopped. And this noise degradedthe accuracy of the measurement as shown in Fig. 2.

400

Stored current
350300200€€

are standard deviation of the transverse
distributions of the gamma photons, the electron beamand the laser beam, respectively. Therefore by measuring€ gamma photons with €the distribution ofa detector, σ ecan be precisely measured.
We have developed a compact laser wire monitor
system suitable for small synchrotron radiation sourcesuch as NewSUBARU, and succeededin the€measurement of small vertical beam size [4]. In the topup operation, however, undesirable spike noises wereobserved in synchronism with beam injection as shown in______________________________________________#hashi@lasti.u-hyogo.ac.jp-743-

199150100

γ photons
050100150

198

200

time[sec]
Fig. 1. Time chart of stored current and gamma
photons in the top-up operation. Spike noise in γ
measurement is observed in synchronism withbeam injection.

85

Number of photons

σe, σL

200

250

INTRODUCTION
It is important to non-destructively measure transverse
profiles of an electron beam stored in synchrotron lightsources for their stable operation.
One of thesemeasurements is laser wire monitor [1,2,3], which canmeasure horizontal and/or vertical beam size bymeasuring gamma photons generated by Compton backscattering. The measurement principle of the monitor isvery simple. A thin laser wire is scanned transverse to anelectron beam. When laser photons collide with beamelectrons, high-energy gamma photons are generated.
Assuming that the transverse distributions of both thelaser beam and the electron beam are Gaussian, thedistribution of the measured gamma photons should be2
22also Gaussian distribution, σ γ = σ e + σ L , where σ γ ,201

STored current [mA]

The laser wire monitor developed in NewSUBARU canmeasure the vertical beam size accurately in decay mode.
In top-up mode, however, spike noises in the γmeasurement are observed in synchronism with beam
injection and impair the measurement accuracy. To
resolve this problem we have made a masking circuit,which masks the γ signal just after beam injection bystopping the counter for several ms. By setting mask
width comparable to damping time of betatron oscillation,
the spike noise disappears and the vertical beam size canbe accurately measured even in top-up operation.
Measured beam size and emittance are in good agreementwith calculations.
In the laser wire monitor,
Bremsstrahlung is the origin of both background noiseand spike noise. We found that, in background noise, γyield by the collision of lost electrons with vacuum
chamber is comparable to that by the collision ofcirculating electrons with residual gas molecules. On theother hand, in spike noise, γ yield is mainly from theformer.

γ [photons/sec]

Abstract

Top-up mode
Without mask
80757065-270-180-90090180270

Vertical Position[μｍ]Fig. 2. Distribution of gamma photons by thelaser wire monitor in top-up operation (WithoutMask).

Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

MASEREMENT IN TOP-UP MODE WITH
MASKING NOISE SIGNAL

The vertical beam size measurement with mask in thetop-up operation is shown in Fig. 4, where the effect ofspike noise is disappeared. Appling Gauss fitting to themeasured data, we can find that σ γ is 74 micron.

Because the ring can be operated in top-up mode tokeep the stored current constant, the monitor is preferred
to measure the beam size accurately during a beam
injection. To reduce these spike noises we masked thegamma signal just after beam injection by stopping thecounter for several ms. Figure 3 shows the dependence ofmask width on the noise strength, which is defined as theheight of spike noise from the average value in Fig.1.
The noise strength decreases with an increase in mask
width and diminishes at 20 ms, which almost corresponds
to the damping time of betatron oscillation. This result€
indicates that an extremely small part of a circulating
beam is lost just after beam injection, when the amplitude€of horizontal betatron oscillation is relatively large.

Height of spike noise / background

0.5

0.4

0.3

0.2

0.1

005

1015
Mask width(ms)2025Fig. 3. Strength of spike noise vs. mask width.

Number of photons

75

Top-up mode
With mask (20ms)

Assuming that σ L is 14 micron, it is found that σ e is 69micron.
The vertical betatron function β y at theinteraction point calculated by MAD is 10 m.

€Usingσ e = εy β y , the vertical emittance can be estimated as€€εy =4.9e-10 m-rad, which corresponds to 1.2% coupling.
€ORIGIN OF BREMSSTRAHLUNGGAMMA PHOTONS

Without laser input, measured gamma photons aregenerated by Bremsstrahlung, i.e., electromagnetic
radiation produced by the deceleration of a high-energy
electron when deflected by an atomic nucleus. In thelaser wire monitor, gamma photons produced byBremsstrahlung are a cause of both background noise andspike noise in synchronism with beam injection. Then
what is an origin of Bremsstrahlung gamma ? Dose all
gamma yield come form collision with residual gas
molecules ?
Measured γ yield Y can be expressed as follows,
Y = Yloss + Ygas
where Yloss denotes the yield generated when an electron
lost from a bunch by gas scattering or Touschek effect€ the surroundings such as vacuum chamber,€ collides withabsorber, and so on. And Ygas denotes the yield due to€collisions of circulating electrons with molecules ofresidual gas in vacuum chamber. Assuming that Yloss is
proportional to the€ number of electrons lost from bunchedbeams per unit time, it is written asdIIYloss ∝∝ ,€dt
τwhere I is the beam current and τ the beam lifetime. AsYgas is proportional to the beam current and the density of€ residual gas molecules dg , we have70

€


dP 
Ygas ∝ I ⋅ dg€∝ I ⋅  p0 +
⋅ I,
dI 

€

where p0 the vacuum pressure at off beam, dP dI the€pressure increase per stored current. Thus the total yieldof Bremsstrahlung gamma Y can be expressed as a
€ function of I and τ by the following equation,€65€
60-390 -270 -150-30

90

210

330

Vertical Position[μｍ]Fig. 4. Distribution of gamma photons by thelaser wire monitor in top-up operation (Withmask of 20ms).

I+ b ⋅ I 2 + c ⋅ I€,τ€constants.
where a , b , c are€
€To estimate
the origin of the background noise in theY = a⋅laser wire monitor, we have measured gamma yield at€ various beam current as functions of I τ and 1 τ as€ €shown€ in Fig. 5 and 6. Fitting the above equation to thesedata, we can express the yield of background noise as-744-€€

Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

YBG ≈ 34 ⋅SUMMARY

I+ 0.022 ⋅ I 2 + 5.5 ⋅ I .
τWe find that the first term is comparable to the sum of thesecond and third terms.
Thus the origin of thebackground noise in the laser wire monitor is in halves of€ gas scattering and collision of lost electron with chamber.
In the case of spike noise, on the other hand, the yield
can be expressed asYspike ≈ 49 ⋅I+ 0.031⋅ I 2 + 19 ⋅ I .
τIn the above expression the first term is greater than theother terms. The fact shows that the spike noise comesfrom mainly the small amount of beam loss just after€ injection, which corresponds to dependency of spikenoise to mask width shown in Fig. 3.

7000
Sh 0W
Sh 1W

6000

Sh 2W
Sh 4W
Sh 10Wγ[count]

5000

Sh 17W
Sh 30W

4000

ACKNOWLEDGEMENTS

3000

The author thanks to Dr. K. Aoki and Mr. S. Ogawa
for their experimental cooperation. The author also
thanks Prof. A. Ando for his support and encouragementin this work.

2000
100000204060

80

100120I/τ[mA/hrs]

REFERENCES[1] H. Sakai et al., Phys. Rev., ST. Accel. Beams,
4 ;022801(2001)[2] H. Sakai et al., “Performance studies of a laser wire
beam profile monitor”, Jpn. J. Appl. Phys. 41 (2002)
p. 6398[3] K. Kubo et al., “Extremely low vertical emittancebeam in accelerator test facility at KEK”, Phys. Rev.
Lett. 88 (2002) p.19
[4] S.Hashimoto et al. LASTI Annual report vol. 8 (2006),
p. 17

Fig. 5. Gamma photons vs. I/tau.

7000
6000
5000

γ[count]

We have developed the compact laser wire monitor tomeasure small electron beam size in NewSUBARU. The
monitor can measure the vertical beam size accurately indecay mode. In top-up mode, however, spike noise
observed in synchronism with beam injection impaired
the measurement accuracy. In order to resolve thisproblem, we have made a masking circuit, which masks
the gamma signal just after beam injection by stopping
the counter for several ms.
By setting mask widthcomparable to the damping time of betatron oscillation,
spike noise disappeared and the vertical beam size can be
accurately measured even in top-up operation. Measured
beam size and emittance are in good agreement withcalculations.
In the laser wire monitor, Bremsstrahlung is the originof both background noise and spike noise in synchronism
with beam injection. We found that, in background noise,
the gamma yield by the collision of lost electron withvacuum chamber is comparable to that by the collision ofa circulating electron with residual gas molecular. On theother hand, in spike noise, the yield is mainly from thecollision of lost electron with vacuum chamber.

4000
3000
I=44mA2000

I=73mA
I=109mA
I=144mA

1000

I=175mA
I=202mA

00

0.1

0.2

0.3

0.4

0.5

0.6

1/τ[1/hrs]

Fig. 6. Gamma photons vs. inverse lifetime.

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