TECHNOLOGY AREA(S): Sensors

ACQUISITION PROGRAM: PEO IWS 2: Advanced Offboard Electronic Warfare Program

OBJECTIVE: Develop a high precision alignment technology for affordable manufacture of high power millimeter wave vacuum electron devices.

DESCRIPTION: The generation of high power at millimeter wave (mm-wave) frequencies is expensive and the concurrent need for wide bandwidths at these frequencies creates an extremely challenging problem. The most stringent requirements for mm-wave power and bandwidth can only be practically met by vacuum electronics (VE) technology. This is especially true for applications constrained by considerations of size, weight, and power (SWaP). Design of such devices is no longer the primary roadblock to their development as both theory and analysis tools (for example, modelling and simulation codes) have advanced dramatically over the past two decades. It is now possible to design, model and simulate a mm-wave VE device and predict its performance to a high level of confidence � if the device can be built to the tolerances required.

At present, vacuum amplifiers with the required performance are prohibitively expensive due to the high precision machining and assembly processes involved. Specifically, the devices are constructed of metal and ceramic parts that require extremely tight tolerances be maintained across proportionally large dimensions of assembled piece parts. Therefore, mm-wave device development and deployment are severely impacted by limitations in manufacturing techniques and processes, and devices providing state-of-the-art performance are expensive due to complex manufacturing steps with relatively low yields. Systems applications must therefore accept high component costs or compensate with less than ideal system trade-offs that result in larger, heavier, and lower efficiency systems. Either way, lack of suitable manufacturing techniques manifests itself in greatly increased system cost. This situation will be most severe for future mm-wave electronic warfare (EW) applications where system performance simply cannot be compromised.

Vacuum electronic amplifiers are a critical technology for high power, broad bandwidth, high efficiency mm-wave EW and countermeasures applications where platform volume and weight are highly constrained. In such devices, critical dimensions must scale with the operating wavelength. Thus, as the frequency of operation increases, physical dimensions and mechanical tolerances decrease proportionally. Precision alignment is therefore crucial to future mm-wave device development as the higher frequencies demand tight alignment tolerances to achieve peak performance (i.e., power, bandwidth, efficiency) while maximizing manufacturing yields and minimizing cost and production time. Complicating this problem are the large aspect ratios involved. For example, the beam tunnel in a typical W-band traveling-wave tube is approximately 230 microns in diameter and may have a length of 5 cm or more.

Typically, the electron beam fills 50-80% of the tunnel diameter and must propagate without interception over the full longitudinal distance with less than 0.15 degrees of angular misalignment off the axis. The problem is exacerbated when the assembly is non-axisymmetric as in the case of multiple electron beams or sheet electron beams. Alignment of the component parts for machining and joining is therefore the most critical step in the manufacturing process.
Traditional methods of precision assembly such as alignment pins and in-process machining have accuracies limited to the 10-micron range or above. Furthermore, there are other issues associated with these methods. For example, in-process machining is both labor- and time-intensive, and alignment pins add additional constraints to the assembly process, introducing yet more high tolerance features in the course of achieving the desired overall precision. Recent advances have demonstrated sub-micron level machining of individual parts but the assembly of multiple parts into complete devices, while maintaining the tolerances required, still presents a choke point in the manufacturing process. If a completed assembly is found to be out of tolerance, all the precision machining invested up to that point on the individual parts is lost.

The Navy needs advanced sources of mm-wave power that are affordable. While the cost of individual devices depends on their design and performance, elimination of low manufacturing yields and high rework rates could lower device cost by as much as 50% across all types of mm-wave vacuum amplifiers. In order to achieve this, new approaches to the mechanical design and assembly of critical components must be developed. Specifically, a technology for the high precision assembly of the extremely high tolerance, large aspect ratio components required by modern mm-wave vacuum electronics is desired. The solution should consider (but not be limited to) methods including elastic averaging (EA), kinematic couplings (KC), and quasi-kinematic couplings (QKC). For example, EA techniques offer precision down to approximately 1 micron and work by averaging out errors through controlled compliance between precision surfaces. KC can achieve better than 0.1-micron precision and work on exact constraints where the number of constraint points is equal to the degrees of freedom to be constrained. The process is deterministic and can provide high accuracy and high repeatability. QKC rely on an arc contact as opposed to the point contacts required by KC and can achieve sub-micron precision. Compared with KC, QKC has reduced contact stresses, reduced cost and complexity, and (pertinent to VE applications) has the ability to make a vacuum-tight seal. Other techniques, not anticipated herein, and hybrid techniques combining the best qualities of these techniques are equally of interest. The goal is to develop an innovative manufacturing technology that exceeds the state-of-the-art presented by each of these techniques individually.

Consistent with the Navy�s objective of producing affordable mm-wave vacuum electronic devices, the solution should specifically demonstrate its utility in the manufacture of mm-wave circuit stack assemblies, electron gun assemblies (cathode to focus electrode positioning), and alignment of the electron gun to the circuit assembly. Successful technologies should demonstrate at least a 10X improvement in precision over existing techniques (better than 1.0 micron tolerance over 5 cm total assembly length), repeatability, vacuum compatibility, and compatibility with other processes common to the manufacture of vacuum devices such as brazing, bakeout (at temperatures up to 500� C), and steady-state operation at temperatures up to 200� C without distortion. The technique should be experimentally demonstrated in at least one of the following areas of relevant W-band vacuum device structure manufacturing: circuit stack assembly; electron gun assembly; or electron gun to circuit assembly.

The prototype solution is expected to produce fixtures, tooling, instrumentation, and the associated process steps (documented by drawings, procedures, specifications, and protocols, etc.), as validated by the prototype device structure. Testing shall include mechanical inspection to validate the precision of the prototype processes and device performance testing, as required to validate the manufacturing techniques. These items, including the prototype structure, will be delivered to the Naval Research Laboratory upon completion of the effort.

PHASE I: Propose a concept for a precision alignment technology for the affordable manufacture of high power mm-wave vacuum electron devices as described above. Propose a specific prototype assembly, on which the technology will be demonstrated. Demonstrate the feasibility of the approach by some combination of analysis, modelling, and simulation. Predict the ability of the concept to achieve the tolerances required according to the parameters of the Description. The Phase I Option, if exercised, will include a device specification and test plan in preparation for prototype development and demonstration in Phase II.

PHASE II: Develop and deliver a prototype precision alignment technology that meets the requirements in the Description. Ensure that the prototype processes and techniques (including fixtures, tooling, and instrumentation) will demonstrate that the requirements of the Description are met by validation of the proposed prototype assembly. Perform testing and validation in the proposer�s facility or in a qualified facility chosen by the company and approved by Naval Research Laboratory (NRL) personnel. Include, in the testing, mechanical inspection to validate the precision of the prototype processes and device performance testing, as required to validate the manufacturing techniques. After testing and validation, deliver to the NRL the process documentation, equipment, hardware, test data, and prototype assembly.

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology for Government use. Since the prototype techniques, processes, and hardware resulting from Phase II are a generic demonstration of the technology, assist in applying the technology for the manufacture of specific VE devices.

Since active mm-wave VE devices require the most stringent manufacturing and assembly tolerances, the technology should be readily applicable to the manufacture of other precision passive mm-wave components such as couplers, diplexers, mode convertors, and circulators. The technology should also prove applicable to other industries requiring precision alignment, such as the laser and electro-optics industry where the precise alignment of optical components over long optical paths is costly and time consuming. The technology resulting from this effort should therefore find a ready commercial market.

REFERENCES:

1. Gamzina, Diana, et al. �Nano-CNC Machining of Sub-THz Vacuum Electron Devices.� IEEE Transactions on Electron Devices 63, October 2016, pp. 4067-4073. https://ieeexplore.ieee.org/document/7536122

2. Culpeper, Martin L. �Design of Quasi-Kinematic Couplings.� Precision Engineering 28, 2004, pp. 338-357. https://www.sciencedirect.com/science/article/pii/S0141635904000054

3. Slocum, Alexander. �Kinematic Couplings: A Review of Design Principles and Applications.� International Journal of Machine Tools and Manufacture, 50, 2010, pp. 310-327. https://www.sciencedirect.com/science/article/pii/S0890695509002090

4. Slocum, Alexander H., et al. �Kinematic and Elastically Averaged Joints: Connecting the Past, Present, and Future.� International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.� http://www.jspe.or.jp/wp_e/wp-content/uploads/isupen/2013s/2013s-1-1.pdf

KEYWORDS: Vacuum Electronics; Manufacturing Techniques; Precision Alignment; Elastic Averaging; Kinematic Couplings; Quasi-Kinematic Couplings