JAERI-Conf 95-021

PA05
Pulse Response of Y-796 Electron Gun and Space Charge Effects
Takahiro KO'ZAWA. Toshiaki KOBAYASID, Toru UEDA.
Mitsuru UESAKA, Kenzo MIYA, Hiromi Shibata* and Hitoshi Kobayashi**Nuclear Engineering Research Laboratory, Faculty of Engineering,
The University of Tokyo,2-22 Shiraknta~hirane, Tokai-mura, Nakn-gun, lbarald 319-11, Japan*Research Center for Nuclear Science and Technology,
University of Tokyo
2-22 Shirakata-Shirane, Tokai-mura, Nakn-gun, lbarald 319-11**National Laboratory for High Energy Physics
1-1 Oho, Tsukuba, lbarald 305
pulse in X-band linac whose electron source is athermionic gun, a shorter emission is required, because
an electron beam from the electron gun must becompressed within one period of the X-band RF (87.5ps), which is 1/4 period of S-band RF (350 ps). In
order to determine a frequency of SHB, it is importantto evaluate pulse response of a thermionic electron gunand space charge effects of high current short pulse in
low energy region. In this paper, the pulse response ofa thermionic electron gun and lengthening of its
emission in the drift space are discussed.

Abstract
For the purpose of the estimation of the characteristics
of low energy short pulse in an injector, the peakcurrents and the pulse duration were measured by coaxial beam catcher at the distance of 25mm, 225mm
and 475mm from the anode plane of Y-796 electrongun. The 200 ps duration pulse with the peak currentof 13A and the energy of 90 keV lengthened until 390
ps time duration at the 225 mm measurement point.
The computer simulation almost agreed withexperiment a results.

2. Experimental

1. Introduction
A test bench for measurement of waveform of anemission from a thermionic electron gun wasconstructed as shown in Fig. 1. It consists of anthermionic electron gun and a d-e biased grid-cathodepulse generator placed on a -90 kV high potential deck,
focusing system and so on. Y-796 electron gun(EIMAC) was used as a thermionic electron gun. The
accelerating potential of the electron gun is provided by
90 kV pulses of 8 J.I.S duration. The duration of flat top

The X-band linear accelerator, which can generate aultra short single pulse in the femtosecond domain, isunder design at Nuclear Engineering ResearchLaboratory, the University of Tokyo[l]. We planed theconstruction of the X-band linac, which consists ofthermionic gun, a subharmonic buncher (SHB), twoaccelerating tubes and achromatic magnetic pulse
compression system[2]. In order to generate a single

GridPulser-90kV HV Deck

25mm~...-_ _ _ _...:::.:.ii!..:..,_-Q

Sampling
Oscilloscope

Trig.
Fig. 1. Test bench for an thermionic electron gun.

-35-JAERI-Conf

95-021,_,c.

is 4 J.I.S. TI1e beam sizes in the transverse direction wascontrolled by solenoid coils. We used two kinds of grid
pulsers (Pulser I and Pulser II). Pulser I have presently
been used for nonnal operation of the UTNL linac.
Pulser II was purchased from Kentech Corporation.
Peak output voltages of each pulser are 300 V (PulserI) and 1.7 kV (Pulser II), respectively. Their rise times
are 1.2 ns and 100 ps, respectively. Their pulse width(FWHM) are 1.6 ns and 130 ps 1 respectively.
The waveform and total charge of emission
from the electron gun were mea8ured by using co-axial
beam catcher. The diameter of the beam catcher is 4> 14mm. The signals passing through semirigid cable were
measured by sampling oscilloscope (S-4, tectronix)
and DC microvolt animeter (PM-18R, TOAElectronics Ltd.). The time resolution of the co-axial
beam catcher is less than 50 ps. Measurement of theemission was carried out at the distance of 25 mm, 225mm and 475 mm from the anode.

Vp:90KV
Vh:9V
Vg:180V

Grid Pulserl

1.53!-

GunGrid Bias (V) Vb

Fig. 3. Relation between grid bias and emissioncurrent (Pulser I).

3. Results and DiscussionuQ)

3-1. Pulse response of Y-796 electron gun
Typical output waveforms of Pulser I and of theemission measured at the distance of 25 mm are shownin Fig. 2. Figure 3 shows the relation between ·gridbias and emission current. The peak current was 800
rnA. On the other hand, typical output waveforms ofPulser II and of the emission at the distance of 25 mm
are shown in Fig. 4. Figure 5 shows the relation
between grid bias and emission current. The peakcurrent was 8 A. If enough voltage is applied, Y-796
respond to the trigger with the pulse length less than 1ns.

<(8Vp 90KVVh 9VVg 1. 2KVi:...,"0....

~"'84/t.r.J

/c.

;:::

3:

~

~7><.--!/)

Grid Pu l ser2
Ken tech-*2/200Q)!/)......

~
200--/==t>.$.----- Tw_ll-100-1so

100-soGrid Bias (V) Vb

Fig. 5. Relation between grid bias and emission
current (Pulser II)

20V/div

500ps/div

EmissionFig. 2. Typical output waveforms ofPulser I and of the emission
measured at the distance of 25 mm.

Grid PulserEmission(KENTECH

SOOV/div50ps/div

lA/div 100ps/div

Fig. 4. Typical output waveforms of Pulser II and of the emission atthe distance of 25 mm.

-36-JAERI-Conf 95-021

3-2. Space charge effects
Lengthening process of the emission from the electron
gun were measured in the drift space as shown in Fig.
6. It is found that the pulse was lengthened by thespace charge effects. Lengthening by the space charge
effects are calculated based on the disk model(3]. The
results are shown in Fig. 7. The pulse widths almost
agree· with the experimental results. However, peakcurrent does not. This is because beam loss is ignoredin this simulation. Figure. 8 shows dependence ofbunch lengthening on accelerating voltage. It is foundthat the bunch lengthening of 200 keV beam by thespace charge effects is little at the distance of 225 mmfrom the anode. This distance is enough long to locate
subharmonic buncher.

-15$ 10=~ 5::su

000.250.75

0.5Time (ns)(a) 25 mm

4. Conclusion-

From the experimental results, it is found that Y-796
have pulse response was less than 1 ns. The
longitudinal lengthening of low energy short pulse wasevaluated.

~....

15~ 10.....c::

5:::suReference0[1] M. Uesaka, T.Kozawa et al., proc. 20th linear
accelerator meeting in Japan, 1995, p. 121.
[2] A. Takeshita, T.Kozawa et al., proc. 20th linear
accelerator meeting in Japan, 1995, p. 145.
(3] G. Mavrogenes et al., IEEE Trans. Nucl. Sci. NS20, 1973, p.919

00.25

0.75

0.5

1Time (ns)(b) 225 mm-

15~ 10.....
c::

I[A}

25mm

141210Exp~....

Calcu

520ps

5:::s086
40

0.25

0.75

0.5

1Time (ns)2S20pso~~~~~~~~~~~~~~~

0d5

1L5 2 25Time[ns}3~54Fig. 7. Bunch lengthening calculated based on the diskmodel.

(c) 475 mm
Fig. 6. Pulse shapes of 90 keV electron beammeasured at the distance of 25 mm, 225 mm and 475mm form the anode.
/{A/
109876-

Orlfilltl/ Pulse-

C11lt: Z(J(Jitefl'-

C11/t: ISOitefl'-

Ct~lc IJOiteV54J
210 ~~~~--~--~--~~~~nme~~00.1 ~2 ~J ~-~ ~s ~6 ~7-37-Fig. 8. Dependence of bunch lengthening on
accelerating voltage.