Technology Quest. P3 Prospec

SnowMass 99 Fusion Summer Study

Technology Issues Working Group

Subgroup 2: Plasma Support Technology

Topic 3: IFE Target Fabrication and Injection

Can the technologies needed for low cost, cryogenic targets and a high rep-rate injection system be developed?

The goal of these discussions is to consider as many target fabrication and injection technologies as possible for both direct and indirect drive IFE targets and to identify the pros, cons, and issues associated with each. We expect that the results of these discussions will be useful to the subsequent selection and development of a limited subset of these technologies for application to IFE.

This topic will have strong interaction at SnowMass with the Target topic of the Inertial Fusion Concepts Working Group, and the IFE topic(s) of the Energy Working Group, and will require information from the Chamber topics of the Technology, the Inertial Fusion Concepts and the Energy Working Groups.

We envision that this afternoon IFE Target Technologies topic group will focus its discussions on how to do IFE target fabrication and injection, while the companion morning IFE Power Plant Target Concepts topic group will focus on what the IFE targets should be. We request that anyone presenting a target design have given thought to and present their ideas on how it could be fabricated and assembled, how it could be filled and layered (including augmented IR or μW layering), and how it could be handled and injected into a target chamber.

Topic leaders: Ken Schultz (GA)/Warren Steckle (LANL)

Description:

At the heart of an inertial fusion explosion is a target that has been compressed and heated to fusion conditions by the incident driver energy beams. For direct drive, the target consists of a spherical capsule that contains the DT fuel. For indirect drive, the capsule is contained within a cylindrical or spherical metal container or “hohlraum” which converts the incident driver energy into x-rays to drive the capsule. The “Target Factory” at an inertial fusion power plant must produce about 1-2 x 10 ⁸ targets each year, fill them with deuterium-tritium fuel, layer the fuel into a symmetric and smooth shell inside the capsule, and deliver the completed target to the target chamber at a rate of 5 - 10 Hz. These fragile targets must be injected to the center of the target chamber, operating at a temperature of 500 - 1500°C and possibly with liquid walls, without damage. Target fabrication must be done with extreme precision of manufacture, extreme reliability of delivery and for a manufacturing cost four orders of magnitude lower than current ICF target fabrication experience. Target filling and layering must be done with high precision, extreme uniformity of temperature, and with the minimum possible tritium inventory. Target injection must be done with precision, and reliability of delivery and without damaging the mechanically and thermally fragile targets. This challenge does appear to be achievable, but will require a serious — and successful — development program.

Subtopics:

1. Target Fabrication

Targets currently fabricated for ICF experiments have many of the characteristics that will be needed for IFE, although the size is smaller (capsule diameter ~ 0.5 mm for Nova, ~ 1 mm for Omega and ~ 2-3 mm for the NIF vs. ~4-5 mm for IFE). The fabrication techniques used for ICF targets were developed to meet exacting product specifications, to have maximum flexibility to accommodate changes in target designs and specifications, and to provide diagnostic features and a thorough characterization “pedigree” for each target. The current ICF target fabrication techniques may not be — and were not intended to be — particularly well-suited to economical mass production of IFE targets. The cost of current ICF targets certainly is not well suited to IFE power plant operation. Because of constant development required by the small number of any one design that are made, and because of the thorough characterization required of each target, a completed target can cost about $2000. For a power plant to be economically competitive, the target cost must be reduced to about $0.20.

This subtopic must be closely coupled to the Inertial Fusion Concepts Working Group to understand the emerging designs for IFE targets and to provide feedback on the challenges of fabricating various target designs.

1.1 Capsule fabrication.

ICF capsules are currently made using the PAMS/GDP process which may not extrapolate well to IFE. Alternate capsule fabrication techniques must be considered. The microencapsulation process previously used for ICF appears well-suited to IFE target production if sphericity and uniformity can be improved and capsule size increased. Microencapsulation is also well-suited to production of foam shells which may be needed for several IFE target designs. We will pursue the following questions:

• What processes appear promising for fabrication of capsules?

• How can foam capsules or foam-covered capsules be fabricated?

1.2 Hohlraum fabrication.

For indirect drive targets, the capsules are mounted inside a thin metal hohlraum. For current experiments, the hohlraums are a few millimeters in diameter and length. For the NIF these dimensions will be just under a centimeter; for IFE they will be just over a centimeter. ICF hohlraums are currently made by electroplating the hohlraum material, generally gold, onto a diamond-turned mandrel which is dissolved, leaving the empty hohlraum shell. This technique does not extrapolate to mass production. Stamping, die-casting and injection molding, however, may hold promise for IFE hohlraum production but have not been seriously considered. We will address the following question:

• What techniques hold promise for hohlraum production?

1.3 Target characterization.

Precise target characterization of every target is needed to prepare the complete “pedigree” demanded by the ICF experimentalists. Characterization is largely done manually and is laborious. For IFE the target production processes must be sufficiently repeatable and accurate that characterization can be fully automated and used only with statistical sampling of key parameters for process control. We will address the following question:

• What parameters must be monitored and how may they be controlled?

2. Target Fill and Layering

Targets for ICF experiments are filled by permeation and a uniform DT ice layer is formed by a process known as “beta layering”. By use of very precise temperature control, excellent layer thickness uniformity and surface smoothness of about 1 μm RMS can be achieved. These processes are suited to IFE although the long fill and layering times needed may result in large (up to ~10 kg) tritium inventories. If IFE targets need DT ice smoothness better than ~1μm to achieve high gain, new layering techniques will be needed.

This subtopic must be closely coupled to the Inertial Fusion Concepts Working Group to understand the requirements for layer smoothness in IFE targets and to provide input on the challenges of DT layering and temperature control.

2.1 Target filling.

Permeation filling of polymer capsules appears practical, although very precise pressure and temperature control during the fill process is essential. For thin-walled direct drive capsules with high fill pressure to achieve thick DT layers, the pressure control precision requirements may be a significant challenge and the resulting tritium inventory may be large. For beryllium capsules permeation fill may not be practical. We will address the following questions:

• What techniques appear practical for fill of polymer capsules and what are the implications for tritium inventory?

• What techniques might be used to fill beryllium capsules?

2.2 DT Layering

DT layer smoothness is a potential performance limitation for IFE. The smoothness needed for indirect drive targets appears to be very close to the limits of smoothness that can be achieved by very carefully controlled beta layering. Use of infra-red or microwave energy for enhanced layering may achieve still smoother DT ice surfaces. Since the gain curve is very sensitive to smoothness, a small variation in surface roughness might make a large difference in target gain. If the target gain is significantly reduced or if it is highly variable from shot to shot, this would be a performance limitation for IFE. For direct drive, the surface smoothness needed may be almost an order of magnitude better than has been achieved to date. The technology associated with layering is also a challenge. The individual target must be placed inside a uniform temperature “layering sphere” for several hours to achieve proper layer thickness uniformity and the temperature must be near the DT triple point (~19.5 K) to achieve adequate smoothness. Innovative solutions may be possible. Techniques have been proposed for achieving a liquid surface on the inside DT surface that would be very smooth. These techniques would require development and would entail significant technical risk. We will address the following questions:

• What concepts appear practical for IFE target beta layering in large quantities?

• What techniques are potentially available for enhanced layering and what are their implications for tritium inventory?

3. Target Injection and Tracking

Preliminary design studies of target injection for both direct drive and indirect drive IFE power plants were done as part of the SOMBRERO and OSIRIS studies completed in early 1992. The direct drive SOMBRERO design proposed a light gas gun to accelerate the cryogenic target capsules enclosed in a protective sabot. After separation of the sabot by centrifugal force, the capsule would be tracked using cross-axis light sources and detectors, and the laser beams were steered by movable mirrors to hit the target when it reached chamber center. Target steering after injection was not proposed. The indirect drive OSIRIS design proposed a similar gas gun system without a sabot for injection and crossed dipole steering magnets to direct the beams.

This subtopic must be closely coupled to the Inertial Fusion Concepts Working Group and the Chamber subgroup in the Technology Working Group to understand the emerging designs for IFE chambers and the environments they will present to target injection and tracking.

3.1 Target injection.

A gas gun indirect drive target injection experiment was done at LBNL. The results showed that relatively simple gas gun technology could repeatably inject a non-cryogenic simulated indirect drive target to within about 5 mm of the driver focus point, easily within the range of laser or beam steering mechanisms to hit, but not sufficient to avoid the need for beam steering. Recent results of DT ice layer tolerance of temperature changes indicate that much higher injection speed will be needed for direct drive targets. We will address the following questions:

• Is the gas gun system adequate for indirect drive and will it function in the hostile environment of a liquid wall chamber?

• What approaches should be pursued for high speed direct drive target injection?

• Are there thermal protection schemes that could allow lower injection speed?

• What are the possibilities and trade-offs for reducing the chamber gas pressure on target heating and chamber life?

3.2 Target tracking.

The LBNL experiments showed that for low speed (~100 m/s) indirect drive target injection photodiode detector technology was adequate to detect the target position with sufficient accuracy that the driver beams should be able to achieve the ~ ±200 μm accuracy needed. The case of higher speed direct drive targets must now be considered. We will address the following questions:

• Will photodiode detector technology be adequate for direct drive target tracking?

• Can target tracking signals, when coupled with HIB steering magnet controls and laser steering mirror controls provide adequate time and accuracy to allow the target to be hit on the fly?

Core Working Group:

Gottfried Besenbruch (GA)

Paul Fisher (ORNL)

Esayed Mogahed (U Wisc)

Art Nobile (LANL)

Bob Peterson (U Wisc)

Ron Petzoldt (UCB)

Ken Schultz (GA)

Warren Steckle (LANL)

Rich Stephens (GA)

Plan for Sessions at SnowMass:

We plan to have three 2 hour afternoon sessions to discuss this question during the first week, and one 2 hour afternoon session during the second week to pull together the results. We will devote one two hour afternoon working session to each subtopic. Each afternoon will be structured with a short presentation by the subtopic leader on what the question is all about, followed by presentations by concept advocates on their proposed option(s), followed by open discussion. The wrap-up session will allow about each subtopic to report the results of work done to answer questions raised in the preceding working session and for the subtopic leader(s) to summarize the conclusions of the Summer Study.

Report Outline:

The report will follow the outline above and will record the options proposed in response to each question and any technical conclusions that may emerge from the discussions about each option.

Report Outline Section coordinators

1. Target Fabrication W. Steckle/K. Schultz