Abstract

Emittance measurements have been carried out on the CERN laser ion source using the pepper-pot technique. A plate with a square array of 80mm holes is placed in the extracted ion beam. Particles passing through the holes impinge on a gated micro-channel plate, the phosphor screen of which is imaged by a CCD camera. A computer program has been developed to calculate emittance from these pepper-pot images - the procedure used by this program is documented here. The un-normalised emittance of a Tantalum ion beam extracted at 70kV was measured (at 5 rms) as 88 mm.mrad.

Experimental Procedure

The pepper-pot technique is based on placing a perforated sheet on the axis of an ion beam to define positions in phase space. Ions pass through the holes in the sheet to arrive at a position sensitive detector, forming a series of images of the defining holes. The size of these images is a function of the spread in particle trajectory in the plane of the perforated sheet which defines their angular position in phase space. The summation of this information from across the whole beam allows one to construct a phase space ellipse, and thus calculate the transverse beam emittance. The use of a pepper-pot consisting of an array of holes allows one to measure the emittance in both horizontal and vertical planes from the same shot.

Figure 1 is a schematic diagram of the laser ion source extraction region. The extraction scheme consists of a drift tube held at the same high positive potential as the Tantalum target, followed by a negatively biased electrode and a ground electrode. The drift distance from target to positive electrode was 62cm, all extraction apertures were of 15mm diameter and the negative electrode was held at -10kV. The pepper-pot consists of a 2mm pitch square array of 80mm holes in a copper foil 100mm thick. The pepper-pot is mounted on a 25mm long collar fixed (in electrical contact) to the rear side of the ground extraction electrode. The micro-channel plate (Philips G60X) is mounted on a chariot 145mm away from the pepper pot, and the phosphor screen imaged at 90 degrees to the beam axis by a CCD camera via a mirror. The CCD camera has an active area of 25.3 x 16.9mm, made up from 576 x 385 pixels. The data is processed by a 12-bit analog to digital converter.

The voltage across the channel plate was gated so that the device was active only for a short period at the time of interest. (Note that if the channel plate was supplied with continuous DC voltage, the desired signal from the ion beam was obscured by very large signals caused by light from the laser produced plasma). The channel plate was gated by a -1.2kV pulse of 2ms duration, triggered 5ms after the laser pulse as shown in figure 2. The part of the signal current pulse overlapped by the channel plate gate includes Tantalum ions of charge states 15+ to 21+ (peak at 18+), with an average current of 30mA. No evidence of saturation of the channel plate phosphor screen, or the CCD camera was seen under these conditions.

Figure 3 shows a typical pepper-pot image as recorded by the CCD camera, and figure 4 an integrated intensity plot across the whole of the same image. From figure 3, one can estimate the beam diameter to be around 16mmm. It can also be seen from figure 4 that the individual images from adjacent rows of pepper-pot holes overlap, which makes it difficult to resolve information and existing calculation methods cannot be used. Due to practical mounting problems in the chamber, it was not possible to move the channel plate closer to the pepper-pot to separate the images, so a program was written to resolve the overlapping images.

Calculation method

The first task in treating the measured data is to resolve the peaks from adjacent holes in the pepper pot. The integrated intensity plot (such as figure 4) is saved as an ASCII file and read into the "Gaussfit" program on Excel. The central peak of highest intensity is defined, then a first estimate is entered for the position, amplitude and width of each peak. The program then fits a set of Gaussian profiles to the measured data by an iterative least-squares process. Figure 5 shows a comparison between data fitted to Gaussians by this method and the raw measured data. The pitch of the pepper-pot array, and the calibration factor in pixels / mm for the CCD image of the channel plate phosphor screen are then input to the program to express the Gaussian peaks in real units, ie. transverse position in the pepper-pot plane.

Next, a Gaussian envelope is fitted around the peaks of all the individual Gaussians, and the amplitude level corresponding to 5 rms is determined, as shown in figure 6. A slice taken through the Gaussian envelope at this amplitude level selects around 92% of particles in the beam. The series of data corresponding to points around the circumference of this Gaussian envelope at 5 rms level is expressed in terms of angular position using the distance between the channel plate and pepper-pot. This data is then fitted by least-squares to an ellipse in phase space, and the area of the ellipse taken as our emittance. Figure 7 shows such an ellipse plotted in phase space.

The development of this calculation method using Gaussian curve-fitting was necessary here in order to resolve overlapping information from individual pepper-pot holes. The technique of fitting a Gaussian envelope around the individual peaks assumes the ion beam itself has a Gaussian profile - this asumption is central to the entire calculation process.

Results

In order to establish the reliability of the emittance calculation method, and the shot-to-shot reproducibility of results, several measurements were taken under the same conditions of 66kV extraction voltage. Table 1 shown below lists the emittance calculated in both vertical and horizontal planes for three such measurements. This data shows that the agreement between emittance calculated in the vertical and horizontal planes is 8%, and the shot-to-shot reproducibility is 12%.

Table 1. Un-normalised 5rms emittance (mm.mrad) in vertical and horizontal planes for three runs under the same conditions of 66kV extraction voltage.

Measurements were then taken at different extraction voltages, as detailed in table 2 below, showing that the un-normalised emittance decraeses with increasing extraction voltage.

extraction   emittance   
  voltage     mm.mrad    

   70kV          88      

   76kV          82      

   80kV          65      

Table 2. Un-normalised 5rms emittance (mm.mrad) for different extraction voltages.

A further experiment was carried out to check on the effect, if any, of secondary electrons ejected from the pepper-pot foil on the measured emittance. The pepper-pot was electrically insulated from the ground extraction voltage to allow an external biasing voltage to be applied. It was found that the measured emittance with the pepper-pot biased at -500V, ground, and +500V was the same to within 10%, suggesting that secondary electron emission from the peper-pot has little effect on emittance measurement.

Figure 1. Schematic of extraction region showing pepper-pot and channel plate

Figure 2. Ion beam signal current with corresponding channel plate gate pulse

Figure 3. CCD camera image of pepper-pot

Figure 4. Integrated intensity plot of pepper-pot image

Figure 5. Comparison between measured (original) and fitted data for Gaussian fit

Figure 6. Fit of Gaussian envelope around individual peaks

Figure 7. Phase space ellipse