CSS so that they could be used for the electron beam transmission and the ion beam charge breeding experiment in the EBIS.
(a) Pepperpot emittance meter and flange assembly.
(b) Pepperpot emittance meter as- sembly.
(c) Mask of pepperpot.
Figure 3.36: 3D-drawing of pepperpot emmittance meter.
amount of electrons hitting the phosphor screen. The image of the beam passing through the pepperpot can be obtained by reflecting the light to the mirror and measuring it with a camera.
The CCD cameras used as diagnostic devices are IMB-7050G, made by IMI Technology Co., Ltd., and acA2500-14gm, made by Basler, both of which are configurable. In order to control both cameras, the control panel was created using LabVIEW, as shown in Fig. 3.37. After connecting the camera by selecting the session at the top left, the model of the camera is checked, and camera options such as exposure time and gain are set at the top center according to the model. In addition, the trigger mode that activates the shutter in response to a trigger signal generated by the PXIe on the ground platform is also available. It is possible to generate an image by averaging several shots at the top right of the screen, and a continuous mode of continuous image acquisition is also possible. Using a camera during the experiment, the image includes noise unrelated to the ion beam. To remove the noises, the image can be obtained without noise by taking an image with only noise in the absence of the ion beam and then using the image subtraction function. The image taken in this way appears on the big screen. And since the pepperpots are installed horizontally or vertically depending on the installation location, the image should be possible to be rotated to match the transverse direction of the actual beam. As the size of the ion beam is much smaller than the photographed image, only the area where the beam exists can be enlarged and output to the left. The lower left part shows the intensity values in the horizontal and vertical center lines.
The image processing is performed first to calculate the emittance and phase space using the image of the pepperpot measured through the camera. After filtering using the Median filter function provided
Figure 3.37: Camera control screen using LabVIEW.
by LabVIEW, the linear average of X and Y is calculated. The graphs of the red line without filter and the white line without filter are shown in Fig. 3.38a. With the linear average calculated in this way,
(a) Image processing using linear average for each axis. (b) Region used for emittance calcu- lation shown by grid.
Figure 3.38: Widgets used for image processing.
the number of peaks and each position, and amplitude are calculated using the peak detection function of LabVIEW. At this time, a threshold is applied to exclude detecting the noise peak, and the detected peak is displayed as a green line drawn in Fig. 3.38a. The intermediate position and both endpoints between peaks are calculated to divide the calculation areas for each peak, and the determined regions are represented by the grid of Fig. 3.38b. Therefore, the intensity value inside this grid region is used when calculating the transverse characteristics of the beam.
Parameter calculations are carried out using the data in the grid area, which are the mean position
of the beam (unit of a pixel), the position of the hole of the pepperpot (unit of a pixel). In the region, the transverse angle of the beam in each hole, and the emittance and Twiss parameters are calculated.
The mean position in a unit of a pixel,⟨pixel⟩, before conversed to a unit of a mm, is calculated using Eq. (3.2),
⟨pixel⟩=∑Withingridpixel pixel×I(pixel)
∑Withingridpixel I(pixel) = ∑Withingridpixel pixel×I(pixel)
Itot [pixel], (3.2) whereI(pixel), andItot are the intensity at each pixel and the total intensity. The position of the hole on the image must be determined to calculate the angle of the beam passing through the hole of the pepperpot. So, the positions of each hole in a pixel unit are calculated as many numbers of peaks previously determined based on the mean position. Pepperpot’s hole array consists of 1 mm spacing, as previously described, and can be converted into pixels using a conversion factor to express the position of the hole. Since this conversion factor between mm and pixel depends on the setting of the zoom and focus of each camera used, it should be found experimentally. The program for this, described later, is also prepared using the LabVIEW software. When the position of the hole is determined in this way, each hole and the measured peak correspond one by one, and an angle is calculated by Eq. (3.3),
Angle(pixel) = pixel−XHole
Dps ×1000 [mrad], (3.3)
whereAngle(pixel),XHole, andDpsare the angle at the pixel, the hole location, and the distance between the pepperpot and the phosphor screen in a pixel unit, respectively, for each peak region. As shown in Eq. (3.3), the angle of the beam passing through each hole is determined by calculating how much it moved in the transverse direction from the pepperpot to the screen. The distance from the pepperpot to the screen is 60 mm, which is also used in pixels using a conversion factor. After obtaining the angle at each pixel position, the mean angle is calculated using Eq. (3.4),
⟨Angle⟩=∑Withingridpixel Angle(pixel)×I(pixel)
Itot [mrad], (3.4)
in the same manner as Eq. (3.2), and the maximum and minimum values of the angle are additionally found. To calculate the emittance and Twiss parameters of Eq. (2.34) using the results obtained by Eqs. (3.2)∼(3.4), they are changed from pixel unit to mm unit using a conversion factor, as shown in Eq. (3.5),
x− ⟨x⟩= (pixel− ⟨pixel⟩)×fcon [mm], x′−
x′
=Angle(pixel)− ⟨Angle⟩ [mrad],
(3.5) where fconis the conversion factor (pixel→mm). The beam parameters, including the emittances, are displayed on the calculation panel, as shown in Fig. 3.39. In addition, the interpolation function provided by LabVIEW is used to understand the phase space characteristics. First, an angle coordinate in the region existing the value is generated based on the maximum and minimum values of theAngle(pixel) obtained above. Using angle coordinate,Angle(pixel), andI(pixel),I(pixel,Angle)matrix is obtained
Figure 3.39: Screen calculating transverse properties using LabVIEW.
through interpolation. The position in this matrix is in pixel units, so it should be changed back to mm units using the conversion factor. Thus, the mean value in each dimension is used to become a calibrated matrix of I(x,x′). This matrix is drawn as a contour plot so that phase-space plots are illustrated in Fig. 3.39.
Figure 3.40: Screen obtaining conversion factor (pixel↔mm) using LabVIEW.
The precise conversion factor must be used to accurately calculate the characteristics of the beam in
the above program. Therefore, as shown in Fig. 3.40, the program panel was constructed to calculate the ratio between pixels and mm. First, by maximizing the camera’s gain and sufficiently increasing the exposure time, the image as shown in the middle of Fig. 3.40 can be obtained. In this image, the phosphor screen can be clearly taken on the camera, and the internal diameter of the screen is designed to be 46.5 mm, so the conversion factor can be calculated. For an accurate calculation, it is necessary to accurately capture the screen’s boundary to obtain the inner diameter in pixels. To this end, the edge detection function of the LabVIEW software is used to find the boundary of the phosphor screen in the image. After finding the boundary of the screen in this way, the edge detection function calculates the inner diameter of this boundary in pixels. A precise conversion factor may be obtained by comparing this pixel unit’s inner diameter and the actual screen’s inner diameter, 46.5 mm. Since this image depends on the setting of the camera, the conversion factor is calculated for each camera installed at each location and used for the above calculation.
Using this pepperpot analyzer code, the characteristics of the beam entering the EBIS charge breeder can be transported to suit the acceptance, and the efficiency of the EBIS can be optimized. In addi- tion, the pepperpot is also installed on the ISOL beamline. So, the measurement system is prepared to allow the charge-bred ion beam in the EBIS to be stably transported through the beamline to the post-accelerator.
3.7.2 Scan Code with EPICS PV for Dipole Magnet Scan
The Faraday cups with slit using the dipole magnet in the diagnostics line and the A/q separator are used for the A/q spectrum measurements of the highly charged ions produced by the EBIS charge breeder.
The bending radius varies depending on the magnetic rigidity in Eq. (3.1) according to the A/q of the extracted ion beam and the magnetic field of the dipole magnet. Therefore, for the spectrum measure- ments, the current of the Faraday cup should be measured and recorded while changing the current of the dipole magnet. To automate the measurement, the scan code in Fig. 3.41 was created with a sequence using Python developing the GUI using PyQt, one of Python’s packages. While scanning, the PV name of the parameter to be changed (X-PV), the current of the dipole magnet in this measurement, and the PV name of the parameter to be measured (Y-PV), the Faraday Cup current are respectively written. The start value, end value, and interval of the X-PV are set to adjust the scan’s range, and the total number of samples calculated using these settings is displayed. In the case of a dipole magnet scan, it takes some time for the magnetic field to be stabilized after changing the current. So, the delay time is set, and used to wait after changing the value of the X-PV for accurate measurement. In addition, if the beam current is measured only once each time the magnetic field of the magnet changes, the accuracy of the spectrum is reduced due to the measurement error. Thus, the number of measurements is to be averaged so that the experiment can be accurate through the average of several measurements. Since the experiment is conducted with a bunched beam, the time interval between each measurement is determined so that the measurements do not overlap. Finally, when logging the measured results as a file, the header of the file name, which will be made by combining with the date, and time being used, can be entered so that each
Figure 3.41: Scan code screen with EPICS PV using Python.
result is matched for several experiments. After the setup for scanning is completed, the measurement starts by pressing the "Run" button and can be stopped by the "Stop" button. When the measurement starts, the Y-PV value is measured the set number of times to average at each changed value of the X-PV, and the measured value is indicated for each number in the upper right corner of Fig. 3.41. And the av- erage value of these values is displayed in the center graph according to the corresponding X-PV. While measuring in the set range of the X-PV, the calculated average value is displayed in a central graph, and the spectrum can be checked during the measurement. The bottom of the GUI is a status section that allows checking the measurement status, and it can be checked whether it is currently being measured, finished, or stopped. Also, if it is being measured, it displays the value of the X-PV currently set for measuring and shows the percentage of progress and the progress bar compared to the entire range of the set X-PV.
We completed the preparation of the scan code for measuring the A/q spectrum in the charge breed- ing experiment of the residual gas and ion beams of the EBIS charge breeder. The charge breeding effect of the EBIS can be measured using this scan code, and the efficiency of the EBIS can be optimized ex- perimentally. In addition, the scancode as a general-purpose code can be used for scanning using not only the dipole magnet but also any EPICS PV. Henceforth, this scan code is variously applicable to the systems for the measurement in various experiments.
This chapter briefly introduced the mechanical design and manufacturing status of the RAON EBIS
charge breeder. And, the installation status from the E-Gun to the ion transportation line, including the test ion source and the diagnostic line for the charge breeding experiment, was explained. In addition, we showed the configuration and results of the important vacuum system in the EBIS experiment. The electric system was built to apply high voltage to each electrode and platform. The integration with the control sequence using LabVIEW and the ISOL control system using EPICS IOC as the control system for all components for the EBIS performance test and the ISOL beamline test has been completed.
Preparation for the beam test using the EBIS is completed by implementing the measurement system to obtain stable operation parameters of the EBIS by the optimization of the charge breeding experiment and the beam transmission test.
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.