Chapter 4
Experiments of EBIS Charge Breeder
The EBIS charge breeder’s primary role is to efficiently produce the highly charged ions with the electron beam and transport them with 10 keV/u to the post-accelerator. The properties of the electron beam in this process are important, so the performance test of electron beam transportation from the electron gun to the collector was conducted with the SC solenoid magnet up to 6 T. And, using the electron beam up to 1 A, the ionization and charge breeding of the residual gases in the breeding region was performed before the test with the ion injection. Through injecting the test ion beam, the highly charged Rb and K ions were produced with the electron beam of 1 A and a breeding time of 30 ms. The properties of charge breeding of Cs ions extracted from the test ion source were measured using various electron beam currents and breeding times. Additionally, to check the possibility of the pulse stretching of the ejected beam, its pulse length was measured by applying the time-dependent voltage of the drift tube in the breeding region during the ejection step. The charge breeding and transportation experiments with stable ions, which are Cs, Sn, and Na, transported from the ISOL beamline were performed to satisfy the energy requirement of the post-accelerator.
current. The cathode HV platform is set to -5 kV, and the voltage of drift tubes in the breeding region is 8 kV, which makes the energy of the electron beam∼13 kV.
4.1.1 Electron Beam Transmission in 2 T
At first, the test for the single shot of electron beam with the short pulse was conducted with following conditions; a dwell time of 1 ms, a rising and falling time of 0.1 ms, and an extraction voltage of 5 kV with the power of the cathode heater of 82 W. The current of the electron beam was measured by the Direct Current - Current Transducer (DC-CT) at the cathode and the collector, and its result is shown in Fig. 4.1a. The beam current at the cathode and the collector are 0.571 A and 0.5 A, respectively,
- 3 - 2 - 1 0 1 2 3
- 0 . 8 - 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8
Beam Current (Cathode) [A]
T i m e [ m s ] C a t h o d e
C o l l e c t o r
- 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4
Beam Current (Collector) [A]
(a)
- 3 - 2 - 1 0 1 2 3
- 3 - 2 - 1
012Beam Current (Cathode) [A]
T i m e [ m s ] C a t h o d e
C o l l e c t o r
- 1 01234 Beam Current (Collector) [A]
(b)
Figure 4.1: Electron beam current of 1 ms with Vpp of (a) 5 and (b) 10 kV in 2 T magnetic field.
with a transmission efficiency of 87.6%. The signal of DC-CT at the cathode is transported through
the analog-to-digital converter, the optical cable, and the digital-to-analog converter. So, the pulse of the cathode current is delayed rather than the collector current. Additionally, the signal of the cathode current has lower noises by the lowpass filtering in the analog-to-digital converter.
Figure 4.1b is the result of changing Vpp to 12 kV with the same conditions for other settings. The electron beam from the cathode is 1.5 A, and 96% of it, 1.44 A, is picked up at the collector. The
- 1 0 0 - 5 0 0 5 0 1 0 0
- 3 - 2 - 1
012Beam Current (Cathode) [A]
T i m e [ m s ] C a t h o d e
C o l l e c t o r
- 1 01234 Beam Current (Collector) [A]
Figure 4.2: Electron beam current of 50 ms with Vpp = 10 kV in 2 T magnetic field and when rising time is 0.2 ms.
step-shaped signal of the collector current in Fig. 4.1b comes from the limited slew rate of the anode amplifier. In later experiments, the rising time is changed to 0.2 ms to solve this problem.
- 1 0 0 - 5 0 0 5 0 1 0 0
- 3 - 2 - 1
012Beam Current (Cathode) [A]
T i m e [ m s ] C a t h o d e
C o l l e c t o r
- 1 01234 Beam Current (Collector) [A]
Figure 4.3: Multi-shot electron beam current for repetition rate of 1 Hz.
After the short pulse test, the electron beam of the long pulse is measured with the rising time set to
0.2 ms, as mentioned above. The result in Fig. 4.2 shows that the transmission efficiency is 91.2% when the cathode current is 1.48 A with a dwell time of 50 ms. The electron beam has been singly shot in the magnetic field of 2 T until now, but its high repetition rate and duty are essential in the charge breeding operation, so the multi-shot test is performed.
- 0 . 2 - 0 . 1 0 . 0 0 . 1 0 . 2
- 3 - 2 - 1
012Beam Current (Cathode) [A]
T i m e [ s ]
C a t h o d e C o l l e c t o r
- 1 01234 Beam Current (Collector) [A]
Figure 4.4: Multi-shot electron beam current for repetition rate of 7.1 Hz.
The beam pulses in Fig. 4.3 and Fig. 4.4 are the results measured when the repetition rates are 1 Hz and 7.1 Hz, respectively, with other settings same as in Fig. 4.2. As the electron beam is repeated faster, its duty increases up to 36%, and the collector current decreases from 1.35 A to 0.9 A. When the
- 0 . 2 - 0 . 1 0 . 0 0 . 1 0 . 2
- 3 - 2 - 1
012Beam Current (Cathode) [A]
T i m e [ s ]
C a t h o d e C o l l e c t o r
- 1 01234 Beam Current (Collector) [A]
Figure 4.5: Electron beam current for 7.1 Hz repetition rate when cathode heater increases to 96 W.
temperature of the cathode is low, the emission of electrons from the cathode is limited [47], and thus the decrease in the beam current is due to the lack of heat in the cathode.
By removing the temperature limitation as the power of the cathode power supply increases to 96 W, the beam current increases, as shown in Fig. 4.5. The electron beam current of 1.51 A at the cathode and the transmission efficiency of 94.7% are achieved, similar to the single shot result with the short pulse.
These results show that the electron beam of the EBIS charge breeder in the magnetic field of 2 T can be operated to ionize and breed the ions..
4.1.2 Electron Beam Transmission in 6 T
The performance of the electron beam in 2 T was checked so that it can be used for the stable operation of the charge breeding of the EBIS. However, it is well known that the charge breeding efficiency depends on the beam current density rather than the beam current. The current density can increase when the magnetic field in the breeding region increases. It makes the electrons radially more compressed, even if the electron beam currents are the same. So, the experiments of the electron beam in a 6 T magnetic field are conducted like the case of a 2 T field, using a short pulse of a single shot and repeated long pulses to get higher current density.
Like the process of the previous tests, the electrons are extracted as the short pulse with a dwell time of 0.5 ms firstly, and the single shot result is shown in Fig. 4.6. The power of the cathode heater is 93 W, and the extraction voltage is 10 kV in this test. The currents at the cathode and collector measured by the DC-CT are 1.62 A and 1.58 A, respectively, and the transmission efficiency is 97.5%, which is similar to the result of the test with 2 T.
- 1 0 1 2
- 3 - 2 - 1
012Beam Current (Cathode) [A]
T i m e [ m s ]
C a t h o d e C o l l e c t o r
- 1 01234 Beam Current (Collector) [A]
Figure 4.6: Single shot electron beam current of 1 ms in 6 T.
The test for the multi-shot operation with the long pulses uses 100 ms dwell time with a repetition rate of 6.6 Hz. Figure 4.7 shows the result in which the cathode current is 0.29 A with an efficiency of 93% using 3.5 kV of Vpp. In this case, Vpp is limited to 3.5 kV because the amplifier of the anode is stopped if trying to increase the voltage above it, and thus the beam current is also limited to less
than 0.3 A This problem results from the loss of the electron beam due to the misalignment between the magnetic field and the drift tubes, so steering coils are used to compensate for it.
- 2 0 0 - 1 5 0 - 1 0 0 - 5 0 0 5 0 1 0 0 1 5 0
- 0 . 7 5 - 0 . 5 0 - 0 . 2 5 0 . 0 0 0 . 2 5 0 . 5 0
Beam Current (Cathode) [A]
T i m e [ m s ]
C a t h o d e C o l l e c t o r
- 0 . 2 5 0 . 0 0 0 . 2 5 0 . 5 0 0 . 7 5 1 . 0 0
Beam Current (Collector) [A]
Figure 4.7: Electron beam current of 100 ms repeated by 6.6 Hz with Vpp = 3.5 kV.
Four sets of steering coils were installed inside the bore of the SC magnet as shown in Sec. 3.1. Each set applies the magnetic field, 1.1 G/A on the axis, in the vertical and horizontal direction at the electron gun side and collector side. The electron beam current of 1.22 A with 96.7% efficiency is shown in Fig. 4.8 without the magnetic field by the steering coils when the cathode heater power is 86.7 W, the dwell time is 1 ms, and Vpp is 9 kV. But the pulse shape is distorted, so it is impossible to increase the pulse length or the beam current.
- 2 - 1 0 1 2 3
- 3 - 2 - 1
012Beam Current (Cathode) [A]
T i m e [ m s ] C a t h o d e
C o l l e c t o r
- 1 01234 Beam Current (Collector) [A]
Figure 4.8: Distorted electron beam current of 1 ms without steering coils when Vpp is 9 kV.
Thus, to transmit more electrons, the direction of the magnetic field should be matched to the central
axis of the drift tubes by using the steering coils properly. Figure 4.9 is the result of adding the 1.1 G by supplying 1 A current to coils in the vertical direction to the previous settings. The exact shape of the beam pulse is measured due to the vertical magnetic field with the cathode current of 1.34 A and the efficiency of 99.3%. The correct alignment on the axis increases the electron beam current and the transmission efficiency.
- 2 - 1 0 1 2 3
- 3 - 2 - 1
012Beam Current (Cathode) [A]
T i m e [ m s ] C a t h o d e
C o l l e c t o r
- 1 01234 Beam Current (Collector) [A]
Figure 4.9: Electron beam current of 1 ms using steering coils when Vpp is 9 kV.
After compensating for the misalignment, it is possible to increase both the pulse length and the beam current. The dwell time and the extraction voltage are set to 50 ms and 10 kV, respectively, so that the electron beam current is achieved at 1.52 A with 98.7% efficiency shown in Fig. 4.10.
- 5 0 - 2 5 0 2 5 5 0 7 5 1 0 0
- 3 - 2 - 1
012Beam Current (Cathode) [A]
T i m e [ m s ]
C a t h o d e C o l l e c t o r
- 1 01234 Beam Current (Collector) [A]
Figure 4.10: Electron beam current of 50 ms using steering coils when Vpp is 10 kV.
To satisfy the operation requirement, the multi-shot test is progressed after the single shot. The
power of the cathode heater to 96.2 W and the voltage for extraction to 12.2 kV are increased, and 1 A current of the steering coils is applied. Using this setup, the electron beam is repeated by 4 Hz, so that 2.03 A is measured as shown in Fig. 4.11 and its efficiency is 98.5%.
- 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4
- 4 - 3 - 2 - 1
0123Beam Current (Cathode) [A]
T i m e [ s ]
C a t h o d e C o l l e c t o r
0246 Beam Current (Collector) [A]
Figure 4.11: Repeated electron beam current of 50 ms using steering coils when Vpp is 12.2 kV and cathode heater power is 96.2 W in 4 Hz repetition.
Through experiments so far, the electron beam transmission in 2 T and 6 T magnetic fields was com- pleted. The electron beam extraction of up to 2 A, which was the original goal for the EBIS operation, was achieved with a transmission efficiency of 98.5%. And also, it was confirmed that the operation of the repetition rate of 4 Hz and duty of 20% was possible. In addition, this experiment confirmed that the effect of mechanical misalignment could be reduced using the steering coils. Thus the electron beam transmission can be performed so that there is no problem with EBIS operation even if the alignment state changes later.