Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 52 (2019)

Electron-beam-current control at RTM injector
M. Watanabe,∗1 A. Enokizono,∗1 and M. Wakasugi∗1Beam Current Ie (mA)

100
10Grid voltage Vg
200 V50 V

1Cathode heater power Ph = 21.1 W

400Perveance = Ie / Vg

161820Cathode Heater Power Ph (W)Fig. 1. Electron beam current depending on cathode heater
power. It saturates around the heater power of 20 W.
RIKEN Nishina Center

3/2: 0.2 x 10-3300
200
1000050100

150200Grid Voltage Vg (V)Fig. 2. Electron beam current depending on the grid voltage.
The figure shows the case of cathode heater power of
21.1 W, which is saturated.

graph forms straight lines in the logarithmic y-axis up
to Ph = 18 W. This is a common behavior of cathodetype electron source. In this range, it should be notedthat the electric ﬁeld by Vg does not signiﬁcantly affect the beam current, indicating that the electric ﬁeld
used to extract the electrons does not have a dominanteﬀect on extraction. However temperature plays a signiﬁcant role in the extraction of thermal electrons. Thisis consistent with the exponential functions that produce
straight lines in the logarithmic scale because the Fermi
function is composed of an exponential function.
In contrast, Ie is saturated above Ph = 19 W. This
is interpreted as the space charge between the cathodeand grid manages the maximum beam current because
the space charge suppresses the electron extraction byscreening the electric ﬁeld.
Figure 2 shows the dependence of electron beam current Ie on the grid voltage Vg . It seems that it is easy
to extrapolate the line toward the higher Vg range toproduce a higher beam current Ie . The original designof RTM has a maximum Vg of 200 V. However, we havealready installed a grid voltage power supply of 500 Vfor this purpose.
According to these results, the heater power Ph >19 W for the cathode-type electron source NJK2305 is abetter option to obtain a controllable beam current withVg < 200 V.
References

0.1
14

∗1500

Beam Current Ie (mA)

SCRIT facility1) is a unique facility to measure electron scattering oﬀ from short-lived nuclei. In theSCRIT facility, a racetrack microtron (RTM2) ) is usedas the electron injector of the energy of 150 MeV forthe electron storage ring (SR2) and ISOL-type nuclei
source which is the electron-beam-driven RI separator
for SCRIT (ERIS3) ). We have been working toward upgrading4) the RTM to operate ERIS for eﬃcient photoﬁssion and run SR2 in a stable condition. Especiallyfor ERIS, a higher electron beam current is desired forhigher ion yield of short-lived nuclei.
RTM is equipped with a cathode-type electron sourcewith the energy of 60 keV. Before adjusting and upgrading the RTM accelerator, it is important to know thespeciﬁcations of the 60 keV cathode-type electron sourceof RTM.
It is evident that the electron beam current depends
on the temperature of the cathode, which is controlled
by the cathode heater power Ph and the electric ﬁeld
around the cathode to extract thermal electrons fromthe surface of the cathode. In this study, we examinedthese relationships.
The commercially available NJK2305, provided by
New Japan Radio Co., Ltd., is used as the cathode-type
electron source. The maximum electron beam current is
4.25 A at a heater voltage of 6.3 V. It has a grid meshin front of the cathode surface to make electric ﬁeld forthe extraction of thermal electrons. The beam current
ranges from 20 to 70 mA with a grid voltage of 20 V.
Figure 1 shows the cathode-heater-power Ph dependence of the electron beam current Ie. Unfortunately,
we do not have information about the temperature of
the cathode; however, it is considered that temperatureis nearly proportional to Ph . The two lines in the ﬁgure
represent the grid voltage (Vg ) of 50 and 200 V. The

1) M. Wakasugi et al., Nucl. Instrum. Methods Phys. Res.
B 17, 668 (2013).
2) T. Hori, Doctoral Thesis, the Graduate University for
Advanced Studies (SOKENDAI), 2002.
3) T. Ohnishi et al., Nucl. Instrum. Methods Phys. Res. B
317, 357 (2013).
4) M. Watanabe et al., RIKEN Accel. Prog. Rep. 49, 188(2015), 50, 190 (2016), and 51, 177 (2017).

- 143 -