Maximum intensity, transmission limited cold electron beams from GaAs photocathode in the eV and sub-eV kinetic energy range

The deceleration of electrons confined by a weak guiding magnetic field has been studied with the goal of achieving an intense ultracold electron beam, suitable for electron cooling and electron collision experiments with ion beams of very low velocity. By application of a beam deceleration technique the electron current extracted from a GaAs photocathode has been increased by an order of magnitude at a few eV and sub-eV kinetic energy, keeping a low energy spread of the electrons. The true kinetic energy of the slow electrons has been determined, taking space charge effects and contact potential differences into account.

Figure 1

Schematic of the TSR electron target in beam deceleration mode. $U_{\mathrm{acc}}$: acceleration voltage; $U_{\text{Pierce}}$: voltage at the Pierce electrode; $U_{\mathrm{ext}}$: extraction voltage; $U_r$: retarding voltage in the deceleration region tube; $I_f$: full transmitted current; $I_p$: fraction of the current cut by a pinhole in the collector used to measure electron beam profiles.

Figure 2

Energy diagram along the electron track. The emission process at the GaAs photocathode is schematically indicated, illustrating the energy gap $E_g$ in the semiconductor and energy distributions of the electrons (gray shaded areas). The work function of the drift tube is denoted by $W_a$ and the cathode work function by $W_c$. The electrons are extracted over a potential barrier of height $\mathrm{\Delta }{E}_c$. In addition to the voltages defined in Fig. 1, $U_s$ denotes the potential at the virtual electron source in front of the cathode, $U_0$ the effective accelerating voltage (see the text), and $U_{\mathrm{SC}}$ the space charge voltage of the electron beam in the deceleration drift tube. The magnetic field strength $B$ along the electron beam axis is also shown.

Figure 3

Deceleration curve of an $8\text{−}\mu \mathrm{A}$ electron beam for an accelerating voltage of $U_{\mathrm{acc}}=-20\mathrm{V}$. The curve breaks at $U_r=-16.12\mathrm{V}$ retarding voltage. Profile A, measured 50 mV before the break, is still flat. In profile B, measured at $U_r=-16.8\mathrm{V}$, the current density in the beam center is strongly reduced through the enhancement of the space charge in this area. The two lower curves (starting near $I_f=4.5\mu \mathrm{A}$ and $I_f=1.2\mu \mathrm{A}$, respectively) demonstrate how the breaking points (arrows) shift when the decelerated current is reduced. The viewing areas of the beam profiles are $20\mathrm{mm}\times 20\mathrm{mm}$.

Figure 4

(a) Electric potential distribution $U(r)$ (schematic) across the drift tube in the presence of a high perveance electron beam and definition of the quantities for describing the space charge (potentials relative to the effective source potential $U_s$). (b) The beam perveance as a function of the space-charge parameter $x$ for the geometrical parameters of the TSR electron target. Thick line: result including the electron density variation; thin line: asymptotic slope $p/(66\mu \mathrm{A}/{\mathrm{V}}^{3/2})=x/[1+2\ln (R/\rho )]$ for $x\ll 1$.

Figure 5

Calibration of the applied retarding voltage at the breaking point ($U_{r,b}$) to yield the actual potential difference $U_0$ between the electron source and the drift tube. The dotted line through the data in the low-current region denotes the fitted reference values ${\widehat{U}}_{r,b}(I)$ using Eq. (9). The drift tube potential $U_{0,b}$ with respect to the virtual electron source is obtained as the difference $U_{r,b}-{\widehat{U}}_{r,b}(I)$.

Figure 6

Perveance law between the minimum effective accelerating potential realized, $U_{0,b}$, and the extracted beam current. Dots connected by the full line: result of the breaking voltage measurements. The given maximum experimental perveance $p_b$ is determined from the fitted straight line.

Figure 7

Working diagram of the beam deceleration scheme. In the lower right diagram a space-charge parameter $x$ is chosen on the curve for a desired axial kinetic energy. Three cases are indicated: 1 (green): extracted current corresponding to the electron-gun perveance, using $U_0$ as the extraction voltage. The beam current (read from the perveance law in the upper right diagram) is $<1\mu \mathrm{A}$. 2 (red): Higher extraction voltage is used at the electron gun; the extraction voltage (higher than $U_0$) is set to obtain an electron current of $\sim 2.5\mu \mathrm{A}$ and for deceleration, $U_0$ is adjusted to $\sim 1.2\mathrm{V}$ as shown on the line intersecting the right abscissa. 3 (blue): Experimental maximum, for a beam current set to $\sim 8.5\mu \mathrm{A}$, a space-charge parameter near the limit $x_b$ is reached for this $E_{k,0}$ and $U_0$ must be set to $\sim 1.7\mathrm{V}$.