Solder-electroformed joint for particle beam drift tubes

The face plates of a drift tube have their joints with a central cylindrical body sealed by utilizating two metallurgical techniques. The first technique involves the soldering of interface surfaces at the joints between the face plates and the body. The soldered joint prevents the entry of electroplating solution into the interior of the drift tube during a second metallurgical procedure involving the electroformation of a copper strap around the joint which provides a permanent seal against leakage of coolant from the drift tube while offering excellent electrical and thermal conductivity across the surface of the drift tube.

RELATED APPLICATIONS 
This invention relates to the technology of co-pending U.S. Pat. 
application Ser. No. 07/522,825 filed May 14, 1990 and co-pending U.S. 
Pat. application Ser. No. 07/507,768 filed Apr. 12, 1990 both in the name 
of the same inventor and assigned to the same assignee. 
FIELD OF THE INVENTION 
The present invention relates to metallurgical bonding techniques, and more 
particularly to a method of soldering a joint together and then 
electroforming a strap over the joint. 
BACKGROUND OF THE INVENTION 
Linear accelerators, or Linacs, are devices which use radio frequency 
energy to accelerate charged particles such as electrons, protons, and 
ions. Charged particles from an ion source enter a cylindrical enclosure 
known as a tank which encloses coaxially spaced devices known as drift 
tubes. RF energy is present in the tank for accelerating the charged 
particles through the gaps between adjacent drift tubes. The construction 
of a linear accelerator is such that the particles become shielded within 
each drift tube from the effects of RF voltage reversals. Thus, as charged 
particles emerge from each drift tube and enter the next gap, they become 
further accelerated. It is necessary to provide means for focussing the 
charged particle beam along the axis of the tank so as to counteract its 
tendency to diverge. Different types of magnets are employed to do this 
and in co-pending U.S. Pat. applications Ser. No. 07/522,825 filed May 14, 
1990, and U.S. Pat. application Ser. No. 07/507,768 filed Apr. 12, 1990, 
the utilization of quadrupole permanent magnets is discussed. These 
magnets are fabricated from rare earth cobalt segments. 
A drift tube includes a central hollow cylindrical body having an annular 
permanent magnet assembly mounted therein. The interior of the drift tube 
is sealed by face plates which form transverse boundaries for gaps between 
adjacently positioned drift tubes. A manufacturing problem arises due to 
the thermal sensitivity of the permanent magnet assembly. Particularly, 
with rare earth cobalt permanent magnets, it is important to avoid 
exposure of the magnet assembly to temperatures over 100.degree. C. Above 
this temperature the quadrupole magnetic properties become diminished. 
With present drift tube assemblies attempts are usually made to perform 
electron beam welding of the face plates. However, this requires careful 
control due to the thermal sensitivity of the magnet assembly. Further, it 
is difficult to use electron beam welding since the electron beam becomes 
influenced by the strong magnetic field of the magnet assembly. 
Japanese investigators at the National Laboratory for High Energy Physics 
in Japan and the Institute for Nuclear Study at the University of Tokyo 
have proposed the utilization of copper electroplating to seal the drift 
tube face plates. An electroplating procedure is extremely desirable due 
to the fact that the process is performed at room temperature. However, 
the attempt to electroplate a seal over the joint created between the face 
plates and the central body of the drift tube, as proposed by these 
investigators, is incomplete without thorough disclosure of preliminary 
joint seal steps which must be taken to ensure that no leakage of 
electroplating bath liquid (typically sulfuric acid) into the drift tube 
occurs. A preliminary joint seal must also create a continuous electrical 
and thermal interface, capable of remaining so after extreme thermal 
cycling. The creation of a gap in the joint would severely affect the 
cooling capability of the drift tube. 
BRIEF DESCRIPTION OF THE PRESENT INVENTION 
The present invention basically employs a two-step process for sealing the 
joint existing between the drift tube face plates and its central body. 
The first basic step is to pre-tin the joint interface surfaces of the 
face plates and the central body with a low melting temperature solder and 
then heat an assembled drift tube so that a soldering operation may be 
completed. The soldered interfaces create a sufficiently strong and 
impervious joint seal so that the drift tube may now undergo a succeeding 
step whereby copper is electrodeposited over the joint to form, in effect, 
an electroformed sealing strap of copper. The creation of a metallurgical 
bond between the solder and the drift tube interface surfaces prevents 
thermal gaps from developing, which might be the situation in the event 
that a wax or silver-filled lacquer seal was employed, as is the case with 
many electrodeposition techniques. 
The solder employed in the present invention is a bismuth-based solder 
having a melting temperature below the critical 100.degree. C. operating 
temperature necessary for preserving maximum quadrupole magnetization of 
the magnet assembly. The resulting soldered-electroformed joint provides 
structural continuity and radio frequency compatibility of the drift tube 
surface. Further, the present technique accomplishes the objectives 
without subjecting the magnet assembly to a heated environment which would 
adversely affect the magnet assembly. Thus, this eliminates the need for 
thermal control instrumentation during manufacture, as is the case when 
electron beam welding is used. Of course, this provides marked decreases 
in the cost and risks of manufacturing failure.

DETAILED DESCRIPTION OF THE INVENTION 
Reference numeral 10 generally denotes a drift tube as employed with linear 
accelerators and previously discussed in the Background of the Invention. 
A detailed description of the structure of such a drift tube is disclosed 
in the previously referred to related co-pending application, U.S. Ser. 
No. 07/507,768 filed Apr. 12, 1990. Basically, the drift tube includes a 
central hollow cylindrical body 12 which encloses a magnet for focusing 
the charged particle beam along the axis of the drift tube. The magnet and 
interior coolant passages are sealed by face plates 16. A stem 14 radially 
appends from the central body 12 and provides means for mounting the drift 
tube to the interior surface of a tank as well as providing conduit means 
for supplying coolant to the drift tube. The purpose of the present 
invention is to seal the joint or interface existing between each face 
plate 16 and an abutting transverse end of the drift tube central body 12. 
As previously discussed in the Brief Description of the Present Invention, 
the sealing is done by first creating a solder bond of the abutting 
surfaces at each interface or joint 18 which then permits an electroform 
strap 20 to be created about the joint, the electroform being in the 
nature of a strap encircling the joint between an face plate 16 and the 
central body 12. 
Reference is made to FIG. 2 which illustrates the interface surfaces 22 
prior to forming a sealed joint 18 with solder. The interface surfaces 22 
are pre-tinned with a low melting temperature solder so as to form layers 
18a and 18b. It may be necessary to heat the joining members before 
pre-tinning so that the interface surfaces 22 become outgassed. Since the 
magnet enclosed in the drift tube is a rare earth cobalt magnet assembly 
(not shown), it is imperative that soldering take place at temperatures 
below 100.degree. C. so that the magnetic properties of the magnet 
assembly are not diminished. A suitable solder comprises indium in 
addition to bismuth and lead components. Such a solder is commercially 
available and is known as INDALLOY 136 which has a melting temperature 
significantly lower than the 100.degree. C. limit. Soldering itself may be 
accomplished by maintaining the face plates 16 in abutment with the 
central body 12 and subjecting the pre-tinned interfaces 22 to an 
oxygen-free oven. After soldering has been accomplished, it is desirable 
to machine the extra solder extending outwardly from the joint 18. The 
previously discussed soldering technique will provide an adequate 
metallurgical bond with the copper of the drift tube and more importantly 
will form an impervious seal when the drift tube is lowered into an 
electroplating bath typically containing sulfuric acid. Clearly, such 
protection is necessary to protect the interior components of the drift 
tube from the sulfuric acid solution. Prior art seals including wax and 
silver-filled lacquers would be inadequate for utilization with the 
present invention since these drift tubes are normally cooled with 
cryogenic temperatures and such would cause thermal gaps to develop in the 
joint 18 thereby severely affecting the electrical and cooling efficiency 
of the drift tube. 
The process of the present invention continues with the electrodeposition 
of a copper strap around each of the joints 18. It is necessary to add 
this strap in addition to the solder bonding inasmuch as the solder joint 
necessarily contains undesirable microporosity and does not offer a great 
deal of permanent strength. 
In order to form the electroformed strap 20, a recess 24 is formed around 
each face plate 16, inwardly of interface surface 22 and over a 
corresponding peripheral area at both transverse ends of body 12, 
immediately inwardly from the transverse ends. The result is a continuous 
peripheral recess 24 formed over the joint 18. With the joint 18 
sufficiently sealed against the sulfuric acid solution of an 
electroplating bath, the drift tube 10 with its machined recess 24, may be 
submerged in an electroplating bath until sufficient copper has been 
electrodeposited in the recess to be flush with the outer surface of the 
drift tube. The result will be a copper electroformed strap which 
metallurgically bonds to the drift tube components and permanently seals 
the joints 18 thereby preventing leakage of coolant out from a finally 
assembled drift tube as well as preventing entry of unwanted gas into the 
sealed interior of the drift tube. 
It should be understood that the invention is not limited to the exact 
details of construction shown and described herein for obvious 
modifications will occur to persons skilled in the art.