Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 51 (2018)

Electron energy stabilization at the electron gun in RTM∗1∗2RIKEN Nishina Center
Department of Physics, Hiroshima Univ.

60

100 nF oil capacitor
8.4 nF ceramic capacitors
without capacitor

40200-20-40
-60
02468

Time (µsec.)
Fig. 1. Voltage drop of the H.V. stage of the electron gun
in the time range of the grid pulse of 5 µs.

HV-stage voltage drop (V)We have been upgrading1) a racetrack microtron2)(RTM) as an electron injector for the electron storage ring SR2 and ISOL-type nuclei source, ERIS3)(electron-beam-driven RI separator for SCRIT), at theSCRIT facility.4) For both the apparatuses, a stableelectron beam in two speciﬁcations of current and energy is important to accomplish reliable and continuous
measurement of electron scattering in unstable nuclei.
In addition to stability, a high power beam is necessary
to maintain a good signal-to-noise ratio in the electron
scattering measurement results.
The electron beam energy of 150 MeV from the RTM
is determined by the mechanical design and magnetic
ﬁeld of the dipole magnets of the RTM. If the electronacceleration by the RF cavities is insuﬃcient or unstable, the beam at the middle points in the RTM do notreach the designed energy and fail to exit the RTM. In
order to accelerate the electron beam up to the designed
energy, the RF power for the acceleration and electron
beam current from the DC electron gun should be balanced at a certain ratio. Therefore, the stability of theDC electron gun with a maximum energy of 80 keV isimportant for the RTM and all of the facility.
We measured the stability of the DC electron gunof the RTM. The black line in Fig. 1 shows the timedependence of the voltage of the electron gun cathode,which corresponds to the electron beam energy at atest voltage of 1.45 kV, extractor-grid voltage of 200 V,
cathode ﬁlament voltage of 6.1 V, and grid pulse widthof 5 µs without any special treatment for stability. The
voltage drop is 20 to 40 V during the grid pulse, except
for the pulse switching noise. This voltage drop directly
corresponds to the drop in energy of the electron beam,
which results in diﬀerent electron beam passes in thetransport line and ﬁnally changes the beam current going into the RF cavities of the RTM. As a consequence,
the outgoing 150 MeV electron beam becomes unstable
Simple solutions to reduce the instability include theinstallation of a high-current power supply as the DC
voltage source of the gun, or the installation of a capacitor with high capacitance at the cathode to maintainthe voltage. We tested the eﬀects of the capacitances.
The blue line and red line in Fig. 1 show the results of
installing the 8.4 nF ceramic capacitor and 100 nF oilcapacitor, respectively. The voltage drops were apparently reduced by these capacitors. The 8.4 nF ceramic
capacitor reduces the voltage drop by half, while the
100 nF oil capacitor reduces it by more than 10 times.
In general, the oil capacitor is not good at high-speed
time ranges; however the result shows that it still works
suﬃciently in the microsecond time range.

HV-stage voltage drop (V)M. Watanabe,∗1 A. Enokizono,∗1 T. Hori,∗1,∗2 and M. Wakasugi∗160

100 nF oil capacitor
8.4 nF ceramic capacitors
without capacitor

40200-20-40-60-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Time (µsec.)
Fig. 2. Voltage drop of the H.V. stage in a short time range
around the switching noise of the grid pulse.

In Fig. 2, the same data shown in Fig. 1 are shown fora diﬀerent time range of 600 ns. The high speed oscillations of the data originate from the switching electronic
circuit of the grid pulse. Compared to the black line,
both the 8.4 nF ceramic and 100 nF oil capacitors reduce the high speed noise of more than 10 MHz by similar amounts. This means that the ceramic capacitors
are good in the speed range of 10 MHz.
This result shows that the installation of a capacitor at the electron gun cathode electrode stabilizes theelectron beam voltage of the RTM. Therefore, we areplanning to install a larger oil capacitor of 500 nF at
the electron gun cathode.
References
1) M. Watanabe et al., RIKEN Accel. Prog. Rep. 49, 188(2015) and 50, 190 (2016).
2) T. Hori, Doctor thesis (2002).
3) T. Ohnishi et al., Nucl. Instrum. Methods B 317, 357(2013).
4) M. Wakasugi et al., Nucl. Instrum. Methods B 317, 668(2013).

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