PART I — THE GROWING NOISE PHENOMENON 833 



amplification due to beam rippling. The mechanism is studied in some 

 detail, as its connection with the usually-observed exponential rise of 

 noise is not immediately apparent. In Part II, the UHF noise spectrum 

 and its spatial distribution in beams with large-amplitude, long wave- 

 length ripples, are described. In addition, some of the underlying proc- 

 esses are analyzed. 



II APPARATUS 



As sketched in Fig. 1, the heart of the apparatus consists of an electron 

 gun, drift tube, and movable probe, all enclosed in a demountable, con- 

 tinuously-pumped vacuum system. Outside of the vacuum envelope 

 there is a shielded solenoid, extending the entire 18-inch length of the 

 drift tube. The annular gap between the solenoid pole face and the mag- 

 netic shield about the gun is nearly all taken up by a soft-steel section of 

 the vacuum envelope. 



The electron gun is of the convergent Pierce type, with oxide-coated 

 cathode and a coiled-coil filament heater producing negligible flux at 

 the cathode surface. Surrounding the gun, and inside of the magnetic 

 shield, is a small copper- wire coil that permits variation of this flux over 

 a small range, either aiding or opposing the leakage flux due to the main 

 focusing solenoid. The flux density at the cathode has been approxi- 

 mately calibrated in terms of currents in both coils. Throughout the ex- 

 periments described below, the gun is pulsed with a 1,000 cps square 

 wave of 2,200 volts on its anode, supplying 38 ± 1 ma peak current in 

 space-charge-limited emission. 



The novel feature of the probe is that its annular RF pickup gap 

 couples to a 50-ohm coaxial line leading to the receiver, rather than to a 

 resonant cavity. This permits RF power measurements over a wide 

 range of frequencies. The inner conductor of the coaxial line serves as 

 current-collector, being isolated and biased positively about 40 volts 

 with respect to the outer conductor to prevent escape of slow secondaries. 

 An adjustable vane can be locked in position in front of the probe (whose 

 entrance aperture is 0.100 inch in diameter), so that circular apertures 

 of various smaller sizes are fixed on the probe centerline, about 0.070 

 inch in front of the probe. With these apertures, measurements of col- 

 lector-current variations along the beam furnish a rough picture of 

 beam-ripple amplitudes and locations. In addition, the current-density 

 variation across the beam can be estimated by moving a pinhole aperture 

 in a broad arc through the beam centerline. Both the inner conductor 

 of the probe and the intercepting vane are liquid-cooled. 



The noise powers coupled to the coaxial probe are considerably smaller 



