**3. Space-charge neutralization**

A high-current ion beam contains ions which mutually repel each other, and a beam of the maximum current will diverge very rapidly, unless steps are taken to control this space-charge blowup. This process was known from the first days of the Calutron; the blowup can be mitigated by trapping electrons in the positively charged ion beam so that the space-charge of the ions is largely counteracted by the electrons. However, for a few decades the details of this process were not widely understood. Bernas [3] showed that the necessary electrons could be generated by collisions with residual gas atoms in several tens of microseconds, depending on the pressure and other conditions. It was understood since the 1960s that the extraction electrodes for a high-current ion beam have to be a triode, as in **Figure 1**, with the intermediate electrode at a more negative potential than the final beam potential, and of course the ion source at a positive potential, defining the final beam energy. The negative electrode, known as the suppression electrode, should repel electrons that find their way into the ion beam, and prevent them from being accelerated toward the ion source.

Ideally the electrons trapped in the positive ion beam will reach thermal equilibrium through multiple elastic collisions, and an unusual plasma will form, comprising the fast beam ions, a component of �room-temperature positive ions from the residual gas, mainly generated through charge-exchange processes with the beam, and a population of electrons in thermal equilibrium, the density of which is roughly equal to the total ion density. Hiroyuki Ito, in his Ph. D. thesis [5], has modeled this population and shown that the positive ions typically occupy a zone in which the potential varies by 0.5 kTe, where kTe, the electron temperature, can be measured and is typically between about 3 and 8 eV in a beam 100% contained in conductive grounded walls, except for a negative suppression electrode as shown in **Figure 1**.

## **Figure 1.**

*A high current source using a Penning Ionization Gauge Trap (HC PIG ion source). Note the convergent ribbon beam formation, triode electrode arrangement with negative electron suppression electrode, and the non-uniformity of the magnetic field. The exit slot is 3 mm wide.*

In commercial implanters it was found necessary to add two further features to most heavy-ion beamlines to achieve and preserve this neutralization: (1) a second negative electrode sandwiched between two grounded electrodes as a triode structure through which the beam passed, to isolate the bulk of the beam from any interactions with the beam target, which was often a silicon wafer covered in insulating photoresist, which could charge to a high potential, and (2) a source of low-energy electrons (such as a plasma flood gun) to suppress significant potentials from developing on insulating surfaces. In many systems, similar electron guns add electrons to the main beam, but these are unnecessary if the beam is fully surrounded by conductive grounded walls.
