The effect of ionization and charge multiplication by the electron beam was confirmed by experiments using residual gas, and the following experiment is conducted for ion beams entering the EBIS charge breeder. Before connecting with the ISOL beamline, charge breeding experiments are performed using several test ion beams of the EBIS ion source line. Experiments are conducted using the Rb source and Cs source as test ion sources, and in this experiment, the charge breeding effect for each element is measured. The breeding effect in the actual experiment may differ from the prediction using the current density calculated by the magnetic field depending on other experimental conditions, like a vacuum condition, and the type of ions used. Thus, the effective current density for each element is confirmed by comparing their results and the calculation using CBSIM. It is used to determine the RAON EBIS performance and to predict the electron beam current and breeding time required to produce the ions with the targeted charge state.
4.3.1 Rb Source
At first, an Rb source was mounted as an ion source, and Rb ions are extracted and transported to the EBIS charge breeder. The ion source HV platform was set to 15 kV, so the energy of the extracted beam was 15 keV. In the Rb source, Rb and K are extracted together, and the Rb element naturally has two stable isotopes, which are a mass of 85 and 87, and its abundance ratio is 72 : 28. Through the measurement of the charge-bred ion beam, this can be confirmed. The pulse width and amount
- 4 0 - 2 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 - 1 0 0
0
1 0 0 2 0 0 3 0 0 4 0 0 5 0 0
Beam Current [nA]
T i m e [µs ]
Figure 4.14: Ion beam injected from Rb source measured at switchyard line.
of the bunched ion beam, including 85Rb,87Rb,and39K, were set to 70 µs and∼108, respectively, considering the parameters of the beam from the RFQ-CB, as shown in Fig. 4.14.
When the ion beam was injected, the EBIS HV platform was set to 7.15 kV, and the voltage of the drift tube #04∼07, which are in the breeding region, was set to 8 kV, resulting in a total potential of 15.15 kV. The potential reduction by the space charge effect by the electron beam was considered so that the energy of the ion beam in the breeding region was 500 eV or less. After the ions enter the breeding section, the gate voltage, DT #08, was pulsed from 6.8 kV to 8.5 kV. The charge state of the trapped ions was bred for 30 ms using a electron beam of 1 A. And the EBIS HV platform was set to 42.39 kV at the time of the ejection so that the energy per charge of the withdrawn beam was 50 keV/q.
In other words, ions with A/q of 5 were 10 keV/u.
2 3 4 5 6 7
0
1 0 2 0 3 0 4 0 5 0
Charge / Pulse [pC]
A / q
9 + 1 0 + 1 1 + 1 2 + 1 3 + 1 4 +
1 4 + 1 5 + 1 6 + 1 7 + 1 8 + 1 9 + 2 0 + 2 1 +
8 5R b
8 7R b
3 9K
2 2 +
Figure 4.15: A/q spectrum of charge-bred Rb and K ions.
Figure 4.15, which indicates the x-axis as A/q of each ion and the y-axis as the charge per pulse, shows the results of scanning the extracted ions through the dipole magnet of the diagnostic line, like the residual gas experiment. The black, red, and green marks indicate each position of A/q value for
39K, 85Rb,and87Rb ions. As shown in Fig. 4.15, Rb and K are injected into the EBIS together, and in the case of Rb, two peaks with masses of 85 and 87 are measured together at a ratio of about 7 : 3.
The amount of each ion can be calculated using the charge amount in Fig. 4.15, and the relative abundance can be derived compared to the total amount. The amount of K ions could not be accurately calculated as the peak of ions in some charge state overlaps with the residual gas ions, and thus a histogram of the relative abundance for only Rb is illustrated in Fig. 4.16a. Figure 4.16a shows the
1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1 2 2 2 3
05
1 0 1 5 2 0 2 5 3 0
Relative Abundance [%]
C h a r g e S t a t e
8 5R b
8 7R b
(a)
1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1 2 2 2 3
05
1 0 1 5 2 0 2 5 3 0
Relative Abundance [%]
C h a r g e S t a t e
C B S I M
(b)
Figure 4.16: (a) Experimental and (b) calculated charge distributions of Rb ions.
charge distribution of the charge-bred 85Rb and 87Rb ions and that the highest peak charge state was 18+ and 17+, respectively. However, 87Rb17+seems to include the measurement error compared to the remaining charge states, and the reasonable highest charge state appears to be 18+. The current density of the electron beam calculated by the compression ratio from the magnetic fields, which of the gun coil and the SC magnet are 0.26 and 6 T, respectively, is 166.4 A/cm2. However, the effective current density, about 210 A/cm2, is derived from CBSIM matching [65] with the above result showing that the most abundant state is 18+. And the calculated charge distribution of Rb ions for 210 A/cm2is shown in Fig. 4.16b.
These results show that the actual current density applied to the charge multiplication is not the same as the calculated current density by other factors. Therefore, it is necessary to understand the tendency of observed differences through the charge breeding tests on various elements. And it can predict the operating conditions according to the elements and targeted A/q used in future EBIS operations. Despite this difference, the performance for the charge breeding of the EBIS using ions incoming through the beam transport line, not the residual gases, was confirmed by measuring the distribution of the multiply charged Rb ions in the EBIS.
4.3.2 Cs Source: EBIS Efficiency
After the Rb ion experiment, the charge breeding experiment was conducted using the ion source re- placed with 133Cs, having a similar mass to 132Sn, the RI of the Sn element planned to be used for the later ISOL experiment. In the case of the Rb source, the two stable isotopes and K ions were extracted simultaneously, making it difficult to confirm the quantitative performance of the EBIS charge breeder.
Therefore, the efficiency measurement of the EBIS is carried out by comparing the incoming and outgo- ing amounts of ions to the EBIS using the Cs source. And, the experiments are conducted by changing the electron beam current and the breeding time to measure the physical phenomenon of the charge multiplication of the ion beam. Additionally, the beam pulse is very short when a beam is extracted just by opening the gate after finishing a charge breeding process in the EBIS. Such a short pulse beam may have a problem related to synchronizing with the post-accelerator, so the beam with a long pulse may be needed for commissioning. Also, the experimental devices using the multiply charged ions may require a longer beam length. Therefore, the experiment is conducted to demonstrate the possibility of adjusting the beam length in the EBIS by making a long pulse beam by applying the time-dependent voltage to the drift tube when ejecting the charge-bred beam.
The first experiment using the Cs ions is the efficiency measurement of the EBIS. The pulse of the
133Cs+ions used in this experiment is shown in Fig. 4.17, and the pulse width is about 100µs (end-to- end) and the total amount of ions is 4.28×107. These parameters are selected because the beam length
- 5 0 0 5 0 1 0 0 1 5 0 2 0 0
- 5 0
0
5 0 1 0 0 1 5 0
Beam Current [nA]
T i m e [ u s ]
Figure 4.17: 133Cs+ ion bunch injected from test ion source measured at switchyard line to measure EBIS efficiency.
≤100µs and the maximum beam amount∼108 are parameters of the beam ejected from RFQ-CB. In addition, since the energy of the beam sent from the ISOL beamline is planned to be used at about 20 keV, the HV platform of the test ion source is set at about 20 kV in this experiment to meet the same conditions.
In order to check the quality of the beam entering the EBIS, a transverse image of the beam is taken
using a pepperpot emittance meter installed in the switchyard line. As shown in Fig. 4.18, the phase space of the beam is constructed using the emittance calculation code made by the LabVIEW described in Sec. 3.6, and the transverse emittance is calculated. It is confirmed that the calculated phase space and
Figure 4.18: Pepperpot image, transverse phase space and its parameters of injected 133Cs+ ion beam measured at switchyard line.
RMS emittance values (εx=1.77 andεy=1.04π·mm·mrad) correspond to the acceptance condition simulated in Sec. 2.4.
For the charge breeding of this beam, a basic setup, including the SC magnet, was the same as the Rb ion experiment. However, as the energy of the incoming ion beam was changed to 20 keV, the voltage of the EBIS HV platform also increased by 5 kV compared to the previous experiment. Additionally, as the target A/q value is 4.93 and the charge state is 27+, the voltage of the EBIS platform when the ion beam is ejected was set to 41.65 kV in consideration of the voltage of the drift tube and the space charge effect of the electron beam. Figure 4.19 shows the results of the dipole magnet scan of the charge-bred Cs ions with a breeding time of 40 ms by an electron beam of 1 A. It was confirmed that the targeted ions, 133Cs27+satisfying the energy per nucleon of 10 keV/u, accounted for the largest proportion based on the result.
The number of ions having each charge state can be calculated from Fig. 4.19. The blue bar in Fig. 4.20 is the charge distribution of the Cs ions obtained in the experiment, and the amount of ions is indicated on the right axis. The amount of all ions measured was 2.89×107, which achieved an efficiency of 67.52% when compared with the number of ions entered the EBIS. Particularly, the number of targeted 133Cs27+ was 6.95×106, with a relative abundance of 23.9%, and the ratio of 133Cs27+ to the incoming beam was 16.1%. In addition, the theoretical charge distribution was calculated by CBSIM consistent with these results and represented as the red bar shown in Fig. 4.20. At this time, the electron
4 5 6 7
0
1 0 2 0 3 0 4 0
2 0 + 2 1 + 2 2 + 2 3 + 2 4 + 2 5 + 2 6 + 2 7 +
2 8 +
2 9 + 3 0 + 3 1 + 3 2 +
Charge / Pulse [pC]
A / q
Figure 4.19: A/q spectrum of charge-bred Cs ions.
2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1 3 2
05
1 0 1 5 2 0 2 5 3 0
0 .0 0E+ 00 1 .4 5E+ 06 2 .8 9E+ 06 4 .3 4E+ 06 5 .7 9E+ 06 7 .2 3E+ 06 8 .6 8E+ 06 Particle Number (Experiment)
Relative Abundance [%]
C h a r g e S t a t e
E x p e r i m e n t C B S I M
Figure 4.20: Experimental (blue) and calculated (red) charge distributions of Cs ions.
beam current density used in the CBSIM calculation was 285 A/cm2. The difference from 166.4 A/cm2 calculated through a magnetic field was larger than in the Rb ion experiment. This phenomenon may occur because for the heavier mass element, the more ions cannot wholly escape when the ion beam is ejected from EBIS. So, they remain to receive a charge breeding effect in the next cycle, like the current density is shown to be higher. And the distribution shapes are also different from each other. The experiment distribution is broader than the CBSIM result because the cross-section and other parameters in Eq. (2.1) have some uncertainties. The parameters are used as the theoretical values, which do not include detailed experimental conditions such as some electron beam energy loss. These tendencies can be compared with the following experimental results using Sn and Na ions with different masses.
4.3.3 Cs Source: Various Conditions
After the EBIS efficiency measurement was completed, the experiments were conducted on the charge breeding effect under various conditions. First, for a more precise experiment, the heater power and the extraction voltage of the Cs source were raised to extract more Cs ions. Figure 4.21 shows the results of measuring the extracted ions with a Faraday cup at the switchyard line. 1.27×108ions, about three times more than in the previous experiment, were detected, and through this, peaks much more evident than those of the ionized residual gases could be obtained in the charge breeding test.
- 5 0 0 5 0 1 0 0 1 5 0 2 0 0
- 1 0 0
0
1 0 0 2 0 0 3 0 0 4 0 0
Beam Current [nA]
T i m e [µs ]
Figure 4.21: 133Cs+ ion bunch injected from test ion source measured at switchyard line to measure charge breeding effect for various conditions.
For all conditions in the same state as before except for the breeding time, the A/q spectrum was measured by changing the breeding time from 30 ms to 50 ms, and the results are illustrated in Fig. 4.22.
Compared with Fig. 4.19, the peaks of C3+and O4+ with A/q of 4 become much smaller than those of the Cs ions. This makes it possible to distinguish the Cs ions from the ionized residual gases clearly. As the breeding time increases, the ions with a lower A/q, that is, a higher charge state, increase, and the ions with a lower charge state increase when the breeding time is short, as shown in Fig. 4.22.
Each ion’s measured amount of charge was analyzed, and the relative abundance was calculated and shown in Fig. 4.23a. In Fig. 4.23a, the charge distribution is broad when the breeding time is 30 ms.
The reason is that some ions do not entirely overlap the electron beam when injected into the breeding region and are not bred when they get out of the region of the electrons. Figure 4.23b illustrates the charge distribution calculated through CBSIM using 285 A/cm2, which is the effective current density for Cs calculated in the previous experiment. There is a tendency to have a wide distribution in the experiment according to the above reasons when the breeding time is 30 ms, but consistent with the results of CBSIM when it is 40 and 50 ms by comparing these two figures.
Next, the experiments were performed to measure the charge-bred Cs ions by changing the current of the electron beam after fixing the breeding time to 40 ms to see the breeding effect according to the
3 4 5 6 7 8 9 1 0
0
1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0
1 4 + 1 5 + 1 6 + 1 7 + 1 8 + 1 9 + 2 0 + 2 1 + 2 2 + 2 3 + 2 4 + 2 5 + 2 6 + 2 7 + 2 8 +
2 9 +
3 0 +
3 1 +
Charge / Pulse [pC]
A / q
B r e e d i n g T i m e = 3 0 m s B r e e d i n g T i m e = 4 0 m s B r e e d i n g T i m e = 5 0 m s
3 2 +
Figure 4.22: A/q spectrum for charge-bred Cs ions with various breeding times.
1 4 1 6 1 8 2 0 2 2 2 4 2 6 2 8 3 0 3 2 3 4
05
1 0 1 5 2 0 2 5 3 0
Relative Abundance [%]
C h a r g e S t a t e B r e e d i n g T i m e = 3 0 m s B r e e d i n g T i m e = 4 0 m s B r e e d i n g T i m e = 5 0 m s
(a) Experiment.
1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1 3 2 3 3 3 4
05
1 0 1 5 2 0 2 5 3 0
Relative Abundance [%]
C h a r g e S t a t e
B r e e d i n g T i m e = 3 0 m s B r e e d i n g T i m e = 4 0 m s B r e e d i n g T i m e = 5 0 m s
(b) CBSIM.
Figure 4.23: (a) Experimental and (b) calculated charge distributions of Cs ions with various breeding times.
electron beam current. The electron beam used in this experiment ranges from 0.5 to 1.2 A, and the results of this experiment are shown in Fig. 4.24. The result shows that as the current of the electron beam increases, the ions in the higher charge state increase.
In addition, the amount of each ion was calculated from these data and compared with the CBSIM calculation results as shown in Fig. 4.25. Similar to previous experiments, the distribution was wide at low electron beam currents. Still, the ions with the highest abundance for each case had the same pattern as the CBSIM results. In particular, the Cs27+, targeted charge state, was the maximum when it was 1A and 1.2A, and the charge distribution at this time was measured similarly to the result of the CBSIM.
These experiments confirmed the effect of the charge breeding in the EBIS charge breeder under various conditions. It was confirmed that the longer the breeding time and the higher the electron beam current, the better the charge breeding of ions, and each tendency was also consistent with the CBSIM
3 4 5 6 7 8 9 1 0
0
1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0
1 4 + 1 5 + 1 6 + 1 7 + 1 8 + 1 9 + 2 0 + 2 1 + 2 2 + 2 3 + 2 4 + 2 5 + 2 6 + 2 7 +
2 8 +
2 9 +
3 0 + 3 1 + 3 2 +
Charge / Pulse [pC]
A / q
E - B e a m = 0 . 5 A E - B e a m = 0 . 8 A E - B e a m = 1 . 0 A E - B e a m = 1 . 2 A
Figure 4.24: A/q spectrum for charge-bred Cs ions with various electron beam currents.
1 4 1 6 1 8 2 0 2 2 2 4 2 6 2 8 3 0 3 2 3 4
05
1 0 1 5 2 0 2 5 3 0
Relative Abundance [%]
C h a r g e S t a t e E - B e a m = 0 . 5 A
E - B e a m = 0 . 8 A E - B e a m = 1 . 0 A E - B e a m = 1 . 2 A
(a) Experiment.
1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1 3 2 3 3 3 4
05
1 0 1 5 2 0 2 5 3 0
Relative Abundance [%]
C h a r g e S t a t e
E - B e a m = 0 . 5 A E - B e a m = 0 . 8 A E - B e a m = 1 . 0 A E - B e a m = 1 . 2 A
(b) CBSIM.
Figure 4.25: (a) Experimental and (b) calculated charge distributions of Cs ions with various electron beam currents.
calculation. Moreover, the effective current density for the charge breeding of the Cs ions was calculated, confirming that this was matched for the experiment. This result shows that EBIS operation can be properly performed by allowing the required breeding time and electron beam current to be predicted, no matter what charge state is targeted in the subsequent experiments using the Cs ions with the ISOL beamline.
4.3.4 Cs Source: Pulse Stretching
As previously mentioned, it is necessary to adjust the length of the beam extracted from the EBIS to a long length according to the needs of the post-accelerator test or other experimental devices. Until now, the voltage of the DT #08, the gate electrode, is instantly opened at almost a step function during extraction. However, to lengthen the beam, the slow extraction method is used to slow the extraction.
Here, there is a method of slowly lowering only the voltage of the DT #08 and adjusting the voltages of DT #04∼07, which are the electrodes of the breeding region, and an experiment of extracting a long beam was conducted using both of them.
DT #04∼07 slowly rise in voltages, and DT #08 slowly falls in voltage to obtain the beam with a pulse width of up to 10 ms, and the voltage distribution in the case of a beam length of 10 ms is illustrated in Fig. 4.26a. The voltages of DT #04∼07 are in the form of a logarithm function over time with different coefficients, as in Eq. (4.1), and the voltage of DT #08 is set to decrease linearly and slowly.
Vslow∝ln
1− 1 e2
t
. (4.1)
The required coefficients of the voltages for each drift tube to have a specific pulse width were experi- mentally determined. Figure. 4.26b illustrates the voltage waveform of each drift tube used to make a pulse length of 10 ms (FWHM).
3 4 5 6 7 8 9
7 . 5 8 . 0 8 . 5 9 . 0 9 . 5
Applied Voltage [kV]
D r i f t T u b e N u m b e r
0 m s 5 m s 1 0 m s 2 0 m s 3 0 m s
(a) Plot of voltage distribution of each drift tube in breeding section as time goes by.
0 5 1 0 1 5 2 0 2 5 3 0
7 . 8 8 . 0 8 . 2 8 . 4 8 . 6 8 . 8 9 . 0
Applied Voltage [kV]
T i m e [ m s ]
D T - 0 4 D T - 0 5 D T - 0 6 D T - 0 7 D T - 0 8
(b) Function of applied voltage on drift tubes with re- spect to time.
Figure 4.26: Applied voltages on drift tubes with slow extraction.
The experiment was conducted by measuring the pulse of the beam without the pulse stretching and then slowly increasing its length by changing the coefficient of the voltage function of each drift tube.
The pulse of the stretched 133Cs27+ion beam was measured by the Faraday cup after the dipole magnet with the oscilloscope instead of the picoammeter. The pulses of various lengths for the 133Cs27+beams measured in these experiments are shown in Fig. 4.27. The beam with a length of 20.2µs in Fig. 4.27a is the extracted beam pulse without the pulse stretching method. The first stretched beam is the orange line (50.8µs pulse width) of Fig. 4.27a, and the length is extended while maintaining the flat-top shape as much as possible. And it can be extended up to a length of 10 ms, as shown in Fig. 4.27b. In this experiment, since the stretched beam pulse is greatly affected by the noise as the signal weakens when the beam becomes longer, it was measured with the oscilloscope through the noise filter during the measurement. In Fig. 4.27b, the orange line (1.97 ms pulse width) is noisier than others, and the green and red data with a longer pulse width are less noisy because the filtering conditions of the noise filter have changed. By experimentally finding the proper coefficient of the voltage function of the drift tube,
0 . 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 0 . 8 0 . 0
0 . 5 1 . 0
Current / Pulse [µA]
T i m e [ m s ]
F W H M = 2 0 . 2 µs F W H M = 5 0 . 8 µs F W H M = 1 0 5 µs F W H M = 1 9 9 µs F W H M = 5 2 1 µs
(a) Pulse length: 20µs to 500µs.
0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5
0 . 0 0 0 . 0 1 0 . 0 2 0 . 0 3 0 . 0 4
Current / Pulse [µA]
T i m e [ m s ]
F W H M = 1 . 0 3 m s F W H M = 1 . 9 7 m s F W H M = 5 . 1 5 m s F W H M = 1 0 . 1 m s
(b) Pulse length: 1 ms to 10 ms.
0 2 4 6 8 1 0 1 2 1 4
012Current / Pulse [nA]
T i m e [ m s ] F W H M = ~ 1 0 . 1 m s
(c) Pulse length: 10 ms.
Figure 4.27: 133Cs27+beam pulse from 20µs to 10 ms after pulse stretching.
the 133Cs27+ beam of 10 ms was obtained, as shown in Fig. 4.27c. Figure. 4.27c shows that a beam of sufficiently long and flat pulses can be extracted from the EBIS by the slow extraction method, although it is not a perfect flat-top shape.
The primary performance tests of the RAON EBIS charge breeder were conducted through various experiments using the Rb and Cs test ion sources. Using two stable Rb isotopes and additional K ions emitted from the Rb source, the charge breeding effect on several elements was confirmed. And, the effective current density for the Rb ions was calculated to predict the necessary parameters for the Rb ions or the ions with similar mass in the EBIS experiment. Moreover, while scanning and measuring the highly charged 85Rb and,87Rb ions, the setup for the beam diagnostics capable of distinguishing these two types of isotopes was completed. In the EBIS efficiency measurement experiment using the Cs ions, it was shown that the efficiency of the EBIS breeding the ions achieved 67.52%. The performance of higher efficiency could be satisfied through later operation parameter optimization. In addition, the charge breeding effect of the EBIS according to the breeding time and electron beam current was confirmed from the experimental results under various conditions. Also, the effective current density for the 133Cs ions was also derived for future EBIS operation to use it. Finally, through a pulse stretching experiment, the possibility of adjusting the length of the beam extracted from the EBIS using the slow extraction method was confirmed, as a pulse length of∼10 ms of Cs27+ was obtained. In both the Rb and Cs cases, the most abundant ions extracted from the EBIS coincided with A/q < 6 and the energy per nucleon of 10 keV/u, which satisfies the required conditions for the RAON facility. Based on the results using the test ion source, the performance of the EBIS is confirmed by continuing the experiment with the ISOL beamline and RFQ-CB.