Abstract
The hydrogen Jet Target can now provide a variable density cluster stream up to 6.5x10¹⁴ atoms/cc.  It is now possible to use the maximum hydrogen pressure, which is its vapor pressure at the nozzle temperature. The maximum jet stream density is limited by the production stage (J1) pumping capacity.

Hydrogen Jet Stream Density
The Gas Jet hydrogen line has been redesigned to be sure that the coldest point of the hydrogen circuit is at the nozzle (Fig.1).  The density is the ratio of the jet flux to the cluster speed.  The cluster speed is known from the nozzle temperature as measured in previous tests.  By sending known hydrogen throughputs into the receiving chamber R1 (Fig.2), the R1 pumping speed has been measured, at varying pressures.  From this calibration, the H2 jet stream flow is determined.  The flux is found knowing the jet stream cross-sectional area  at the interaction point.
 
 

Figure 3 shows the results of the measurements taken with the nitrogen shield cooled to 80K.  The jet density for pressures above 100 psia decreases due to the high local gas density in the area just outside of the nozzle in J1 (Fig.4).  To decrease the local density, the flow of liquid nitrogen through the radiation shield is interrupted.  In Figure 5, the increase in jet density, with increasing shield temperature, is shown.
 
 

The jet density measurements have been repeated with the radiation shield operating at room temperature (Fig. 6 and table).  In this condition, the jet density does not decrease at the increased pressures as previously seen.The same trend is shown with the shield operating at 170K (Fig. 7).
 
 

These results are summarized in Figure 8.  It can be seen that at lower nozzle pressures, the liquid nitrogen cooled shield contributes to a higher jet density.  In these conditions, the hydrogen has a large enough mean free path such that it can thermalize with the cold shield.  At higher nozzle pressures and a cold shield, the local gas density  increases, thus destroying the moving clusters due to their collisions with this background gas.

The jet target efficiency is defined as the ratio of the amount of gas in the interaction region to the total amount of gas in the AA pipe originating from the jet.  In Figure 9 the jet target efficiency is shown for each of the operating conditions.  The jet target efficiency is shown to be above 0.9 except for the conditions of high nozzle pressure and cold shield, when the clusters are destroyed.

In Figure 10, two operating conditions are shown to illustrate the stability of the jet density over a period of time.  In each case, the gas at the nozzle is operating very close to the vapor pressure curve.  Small spikes are seen while operating at these points.  If during the E835 run, these spikes will be a problem, it is possible to slightly increase the operating temperature of the hydrogen, with only a small loss in efficiency.
 
 

Figure 11 shows how the jet density depends upon the temperature. In order to increase the density above that achieved at the vapor pressure curve, it is possible to slightly decrease the temperature with the result of doubling the jet density (Fig. 12).  In this condition, the jet target efficiency falls only a bit below 0.9.  Note that the temperature measured, at a given pressure for which liquid is produced in the nozzle, is the saturation temperature.
 
 

As a result of these measurements, it is recommended that the initial nozzle operating pressure should exceed 100 psia for a period of at least 30 minutes in order to thermalize the machine.  After this period, desired operating conditions may be set.  This can be accomplished without interfering with the antiproton beam by keeping the beam valve closed.

We have produced also a nitrogen jet beam (Fig. 13). This may be useful to test the antiproton cooling system with heavier atoms.