PS/HI Note 96-13 (Tech.)

Further Emittance Measurements on the

CERN Laser Ion Source

R. Scrivens, A. Shumshurov, J. Tambini

Introduction

The Laser Ion Source (LIS) aims to provide beam for acceleration by a four-rod RFQ during the summer of 1996. One of the most important parameters of the source output is the transverse emittance.

During 1995 this emittance was measured at the source exit using a pepper-pot, multi-channel plate (MCP) and CCD camera arrangement. A thorough description of this scheme is given in [1]. These measurements were taken with a target to extraction distance of 620 mm and electrode apertures of 15 mm diameter, giving a total extraction current of ~30 mA (all charge-states averaged over the high charge state pulse).

For injection into the RFQ, the total current extracted should be of the order of 90 mA, so a drift target to extraction distance of 900 mm was chosen with electrode apertures of 30 mm diameter. In this note, measurements of the emittance are given for these conditions.

Other improvements made to the emittance measurement scheme included using a fast high voltage transistor switch to apply the gating voltage to the MCP and data treatment for the calculation an RMS emittance. Furthermore, measurements of the emittance were made for the following conditions:-

  1. Under constant conditions (with random changes of extracted current).
  2. With different delay from the laser pulse to the measurement (time scan).
  3. For different positive electrode potentials.

Apparatus

The full scheme for the measurements is shown in Figure 1.

The distances were d1=30 mm, d2=15 mm, d3= 79 mm, d4=65 mm and the electrodes were 7 mm thick. The target to positive electrode distance was 0.9 m. The CCD camera used a zoom lens to image the phosphor screen. A typical resolution of 4 pixels per mm was achieved with the CCD camera

Figure 1. Schematic of the apparatus required for emittance measurements.

Results

From images integrated in the horizontal direction, the peaks were sufficiently separated to allow the calculation of an RMS emittance. A small error due to the loss of the tails of the peaks, results in a reduction of the calculated emittance of ~10%.

A typical image of the beam after the pepper-pot is shown in Figure 2 and the associated phase space diagram for the horizontal plane in Figure 3. These measurements were made with target potential (Vp)=+79 kV, intermediate electrode potential (Vin)=-10 kV with the distances shown in Figure 1. The resulting emittance was 280 mm.mrad for 4rms. The MCP was gated from 3.0 to 7.2 ms. The calculation of the proportion of the particles lying within an ellipse with the calculated Twiss parameters and emittances of different RMS sizes, shows that approximately 85% of the beam lies within the 4rms emittance.

Figure 4 shows the beam profile calculated from the pepper-pot image, showing that a Gaussian profile is a good approximation.

Figure 2. Typical beam image after the pepper-pot.

Figure 3. Left - Typical measured emittance. e=280 mm.rad (horizontal 4rms). Ions extracted at 79 kV. Average current of 66 mA was extracted (with some electron enhancement). Right - Fraction of particles inside ideal emittance ellipses formed from Twiss parameters and different numbers of RMS emittances.

Figure 4. Beam profile calculated from the pepper pot image.

Current Dependence

Under constant settings for the LIS, there is still a considerable change in the shot to shot characteristics of the beam. Measurements were made of the emittance under constant conditions with a MCP gated from 3.0 ms to 7.2 ms. The emittances are shown plotted against current in Figure 5. The current corresponds to the average value measured from 3 ms to 8 ms. The current is enhanced by electron emission from the pepper-pot.

There is a strong relationship between the extracted current and the measured emittance.

Figure 5. Measured transverse emittance under constant LIS parameters.

Time Dependence

A time dependence of the phase space of the beam has already been measured in [1], where it was shown that emittance was approximately constant during the pulse, but the orientation of a phase space ellipse drawn around the distribution changed during the pulse.

Using the minimum possible gate on the MCP (approximately 1ms), different time delays could be used (from the laser pulse) and a time scan made through the high charge state pulse.

The results of 65 measurements (5 at each time delay) are shown in Figure 6. The extraction conditions were Vp=+79 kV and Vin=-10 kV.

Figure 6. Emittance measured at different times in the high current pulse.

Figure 7. Ideal ellipses for averaged emittances and Twiss parameters.

The average emittance varies from approximately 180 to 300 mm.mrad during the pulse. For comparison, a typical extracted current pulse is shown, measured in a Faraday cup 2.4 cm after the ground extraction electrode (cup has 3 cm input aperture).

Figure 7 shows an emittance ellipse constructed from the average emittance and Twiss parameters from the 5 shots at each time delay. The most obvious feature is the sudden jump in position of the ellipse from 3.5 to 4.0 ms delay. Unfortunately, this also corresponds to a time when the ion source was stopped for 2 hours. The ellipses overlap each other well enough (within the two groups) to suggest that there should not be a large increase in emittance when integrating through the whole pulse.

Positive Extraction Electrode Voltage Dependence

Measurements of emittance at different positive extraction potentials were given in [1] and showed that the emittance decreased up to 80 kV.

In Figure 8 are shown further measurements for the conditions detailed in Figure 1. Insufficient measurements were made to draw a conclusion as to where a minimum of the emittance may lie. At low voltages the current traces (measured from the current to the target chamber) suffered greatly from electron enhancement with a large proportion of the current hitting the extraction electrodes.

Figure 8. Horizontal emittance measured for different target potentials.

Measurements at 59 kV for Ta20+ : Addendum

Following measurements of the charge state distribution during April 1996, the most intense charge state was found to be Ta20+. It was not possible to optimize the source for Ta16+, the lowest charge state that the RFQ can accelerate. For this charge state the positive extraction voltage should be set to ~59 kV. Changes in the design of the source to bring the extraction area to the end of the expansion tank, meant that the target to positive extraction electrode distance was increased to 1 m, producing a lower current at the source outlet. Under these conditions the ~65 mA extracted current is well matched to the extraction voltage (given by P=1-2x10-8 (z/A)1/2 [A/V3/2]).

The emittance was re-measured under these conditions using the same apparatus shown in Figure 1, except that a P47 phosphor screen was used in place of the MCP. The phosphor screen was positioned 97 mm after the pepper-pot. The average emittance and Twiss parameters of 5 shots were e=225 mm.mrad (4 rms), a=-2.6, b=0.7 mm/mrad.

Conclusion

Emittance measurements have been performed on the Laser Ion Source under conditions closer to those required for the forthcoming RFQ experiment. The emittance value has been recorded as a function of the extracted current, the time in the current pulse and as a function of the target voltage.

References

[1] J. Collier, B. Goddard, G. Hall, A. Shumshurov, J. Tambini. Emittance measurements on the CERN Laser Ion Source, CERN PS/HI 95-07.