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MEASUREMENT OF PICOSECOND PULSES OF 35MeV TODAI-LINAC (II)Y.Katsumura,T.Ueda,S.Tagawa,Y.Tabata,K.Hasegaw a,and J.Tanaka*
Nuclear Engineering Reseach Lab.,Univ. of Tokyo
National Lab. for High Energy Physics*
To perform the pulse radiolysis study with high time resolution using picosecond pulses,it is necessary to characterize theprofile and the width of pulses.Several methods to determine thepulse width have been tried.Characteristics of the detection
methods have been compared each other and the results are mentioned.
Detection methods of picosecond electron pulses are .divided
into two groups(Table l).One is direct method and the other isindirect one.Assuming that the intensity of~Cfrenkov radiation
which is emitted from electron ()21MeV) travelling in air is proportional to electron density,the electron pulses can be detectedby optical detecters such as streak camera,photodiode and photomutiplier. On the other hand,electron itself or induced current
around beam is used as electrical methods including co-axial line
target and core moniters.
when sampling technique ~ith high time resolution is employed,a syncronous trigger having small jitter and high stability oflinac system are required. It must be noticed that time resolutinof detection is strongly dependent on the length of the signaltransfer cable. Attenuation of the cable is also dependent on thefrequency of signal.
Width of single pulse can be calculated from the energy dispersion,indirectly. From the output of cavities connected withcore moniter or co-axial line target,amplitude of higher orderharmonics of microwave frequency(2856MHz) are obtined. The pulse
shape can be reformed and analized based on Fourier analysis.
Measuring system of pulses is shown in Fig.l. A trigger accurately syncronized with electron pulse is easily obtained fromthe pulse generator(PG-lOP), before electron beam arrives to thedetecter head . In Fig.Z, it is clearly seen that a IOns macro
pulse contains a number of fine pulses with 350ps separation.
Fine structures of Zns pulse detected by streak camera, co-axial
line target and core moniter are shown in Fig.3. It is found thatthe larger the signal transfer cable, the poorer the time resolution. At present, it is believed that a streak camera has thehighest time resolution among our detecters. In addition, it has
an advantage that the profile of electron beam can be measured
more quickly than other optical detecters such as photodiodecombined with sampling scope. Bunch width of electron beam wasobtained to be 18ps by comparing the width with the separationof fine pulses,350ps,in Fig.4. Considering both poor focusing ofCerenkov radiation and resolution ofstreakcamera, the value of
18ps may be larger than true one. However, true pulse width isexpected to be ~lOps from theoretical calculation.

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TABLE 1 Detection methods ofpicosecond electron pulses• Direct method
Optical .•• Cerenkov radiation
streak camera,photodiodeElectrical ..• electron itself
or induced current
core moniter,coaxial linetarget• Indirect method
energy spectrum
Fourier analvsis -pulse width

1CONTROL ROOM

Fig.l Measuring system of Todai-Linac.

Fig.Z
lOns pulse measured
by co-axial type
Faraday cup.

Fig.3
Fine structure of Zns pulse measured
by streak camera(a) and sampling scopewith core moniter(b), and co-axial line
target (c), (d).

Fig. 4
Fihe structure oflOns pulse measured
by streak camera.
(a)

(c)

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7.5m 10D-2V(b) 15m 10D-2V(d) 30m lOD-ZV~- 350ps -:.-1