A Beam Size Monitor Based on Appearance Intensities for Multiple Gas Ionization

A method of measuring the spot size or bunch length of intense charged particle beams is proposed. The relation between the size (widths and length) of a charged particle beam and the beam's electric field forms the basis for a submicron beam size monitor. When the beam passes through a low pressure gas of high Z atoms, the collective field of the beam causes multiple ionizations of the gas atoms. Measurement of the appearance of ionized atoms in a given charge state gives information about the field of the beam and hence its size. Sample calculations show that appearance thresholds can indicate the spot size of round beams with 10 nm accuracy or the bunch length of round or flat beams with up to 10 #m accuracy. INTRODUCTION The designs now being proposed for future colliders for high energy physics call for beams of incredibly small dimensions, both in their width and their length. For example, flat beams with transverse dimensions as small as 3 nm and bunch lengths of order 100 #m have been proposed 1. There is an inherent need, then, to develop new diagnostic techniques capable of resolving such small scales. Conventional beam size measurements based on fine wires in the beam path have been used at the Stanford Linear Collider (SLC) to measure round beam spot sizes above one micron. Recently, two new techniques for measuring sub-micron beams have been tested. A laser interference pattern used to produce synchrotron radiation from a test beam at the Final Focus Test Beam (FFTB) at Stanford enabled measurement of a 70 nm flat beam spot size 2. Another technique proposed by a group from Orsay 3 measured the time-of-flight of gas ions ionized by the beam and accelerated by its space charge. The scheme we propose here is complementary to these two. It is similar to the Orsay monitor in that it is based on beam ionization of a gas; however, it differs in that it uses the information about the charge states of the ionized atoms. Multiple ionization has already been observed in the testing of the Orsay monitor, but the information was parasitic and not used to determine spot size. The scheme proposed here can alternatively be used to measure bunch length, typically 9 1995 American Ins t i tu te of Physics 356 down to about 100 #m, if spot size is known. Conventional techniques for measuring bunch length, such as streak cameras, are limited to 3 ps time scales or mm length scales. Our approach for beam size measurement is a modification of the appearance intensity diagnostic developed for use with lasers. Augst et al. 4 have conducted experiments using tunnel ionization to measure the intensity of laser beams. Their work correlates the appearance of the various charge states of a given target gas with the laser intensities required for successive, multiple tunnel ionization of the gas. The appearance intensity technique is useful for measuring laser intensities on the order of 1016 tO 1018 W/cm 2. This diagnostic could be applied to the highly focused particle beams considered for future collider research, which have beam space charge electric fields corresponding to intensities of the same orders 4,5. Figure 1: Proposed experimental setup for beam diagnostic. The beam size diagnostic setup we propose is as shown in Figure 1: After the final focusing, the particle beam is passed through a gas cell. The ions produced from the tunnel ionization are accelerated, by means of an electric field applied across the cell, to detectors which make time of flight measurements. Since the time of flight for an ion depends on the ion's charge state as well as the applied field, the ionization yield for each charge state can be determined. For a round Gaussian beam where the beam number and bunch length are known, the beam spot size can then be found from calculations relating the spot size to the expected numbers of atoms in the charge states. For flat beams (i.e., transverse height ~r v much less than transverse width ax), the peak beam electric field is independent ofay so that this diagnostic does not provide information about the small spot size dimension. However, if the spot size is known from some other measurement technique, the appearance of charge states can be used to compute the bunch length of either fiat or round beams. In this work we calculate ionization yields as a function of spot size by modelling the interaction of the electric field of a bi-Gaussian (in r and z) round beam with various gases. For typical parameters, this yields spot size information down to 10 nm. We also calculate ionization