高重复率、大批量生产激光惯性约束聚变能的冷冻靶

High Power Laser Science and Engineering 5, e11 (2017)

被喻为“人造太阳”的可控惯性约束核聚变的实现,能够为人类提供一种安全、经济且环保的新型能源。一大批科学技术人员为实现这一“人造太阳”的梦想而不懈努力。惯性约束核聚变装置的设计建造和运行,可以称得上是一项最为复杂的系统工程,涉及到科学研究和技术开发方方面面的协同优化。

作为惯性约束聚变能发电厂的主要元件,含有冷冻氢燃料(固态氢同位素或其混合物)的靶丸需以5-10 Hz的频率传送至靶室中心。照此计算,每天就需要供应5×105-1×106数量的靶丸。因此,只有实现靶丸批量生产和不间断(或按需)供应,方能为未来的反应堆提供源源不断的燃料。发展有效燃料分层方法,是保障靶丸的高重复率和大批量生产的前提;无支撑靶丸传输线则能够保障靶丸的连续供应。考虑到无支撑靶丸传输线将成为所有惯性约束聚变能发电厂的有机组成部分,若能实现将分层方法集成于其上,则可以实现靶丸生产和供应的一体化。

无支撑靶丸传输线的核心要求是实现靶丸向激光焦点的高重复率供应。所有的靶丸设计都包含一个燃料核,其在球形低Z烧蚀层(塑料壳体)的内表面均匀分层。靶丸必须保持无支撑(或不固定),靶丸内部的聚变燃料必须具有超精细结构(近纳米或纳米晶),以此来保障靶丸在靶室中注入和传输过程中燃料层的存活率。

各向同性超精细燃料堪称聚变靶丸制造领域的先进材料,它通过超高冷却速率方法生产制备,能够满足内爆物理的需求。因此,获得具有高缺陷密度或各向同性介质的稳定终态无序结构,对于固态氢同位素或其混合物而言是至关重要的。

俄罗斯科学院列别捷夫物理研究所在高冷却速率燃料分层的技术探索方面取得了重要进展。这些进展证明,“分层+传输”联合方案对于高重复率靶丸制备和传输是十分有利的。无支撑靶丸分层技术与普遍接受的方法(后者通常使用固定靶丸和极慢的冷却速率(~ 3×10−5 K/s),由此导致分层时间长达24小时之久)的根本差别在于它采用自由的和在线移动的靶丸,由此可以便利地制造大量的靶丸并连续(或按需求速率)地将其注入激光焦点。来自列别捷夫物理研究所的I.V. Aleksandrova和E.R. Koresheva对惯性约束核聚变能靶丸研究的当前进展作了透彻分析,并对靶丸制造和传输技术的背后的物理原理进行了详细介绍,相关内容发表在High Power Laser Science and Engineering 2017年第2期上。

这篇文章报道了惯性约束聚变靶丸内部各向同性超细燃料制造的各种有效解决方案,认为无支撑靶丸分层方法是其中较为理想的一个方案。文章对无支撑靶丸分层过程进行了抽丝剥茧,归纳了确保超精细结构均匀固体燃料层形成的几大影响要素:

1. 靶丸在沿(单螺旋或双螺旋)分层通道滚动时,其转动导致液层均匀;

2. 靶丸外部通过壳层壁与分层通道(中空金属管,液氦外部冷却)壁之间小的接触面进行热传导导致液层冻结;

3. 过程中保持高冷却速率(1−50 K/s),以在自由滚动的靶丸内部形成各向同性超精细固态燃料层;

4. 高冷却速率与燃料掺杂(特殊高熔点添加物)相结合导致所得燃料层超细结构的稳定化;

5. 稳定超精细燃料层中D-T混合物的分子成分为:D2:25%, DT: 50%, T2: 25%。在D-T混合物中T2被视为相对于D2和DT而言的高熔点添加物。

文章对无支撑靶丸分层方法的具体实施情况也作了条分缕析(如图1所示):

1. 无支撑靶丸分层模块(LM)一次只处理一个靶丸批次;

2. 传输过程:靶丸通过分层模块的三个基本单元——壳层储存室(SC)-分层通道(LC)-测试腔(TC)注入;

3. 壳层储存室可以是圆柱状或旋转型;分层前其所容纳的无支撑靶丸中燃料处于气-液两态;

4. 基于“分层+传输”方案,分层通道作为主要元件确保靶丸技术开发;

5. 分层的全部时间通常小于15 s,短时间分层亦可为氚的库存最小化带来一些好处;

6. 分层过程中,靶丸在分层通道中自上而下依次快速运动,以此保证冷冻靶丸向测试腔的高重复率注入测试腔;

7. 测试腔完成冷冻靶丸制成品的质量监控:实现精确的单个靶丸断层表征和作为惯性约束聚变能靶丸批次在线质量监控方法的阈值表征;

8. 测试腔是分层模块与靶丸注入器之间的一个典型的接口部件。

由以上讨论可知,移动靶丸经历无支撑靶丸传送线的生产和注入过程的所有阶段。如何来实现靶丸的操控也是一个重要的研究课题,文中对用于靶丸加速的重力或电磁注入器作了讨论,对于利用磁悬浮效应实现冷冻靶丸的新型无接触操控传输系统也作了详细介绍。


图片说明:自由分层方法实现固态超细燃料层的快速均匀化和形成。(a)无支撑靶丸分层模块的示意图。(b)作为低温恒温器特殊插件的单螺旋分层通道。(c)无支撑靶丸分层模块的概貌。实验结果表明,无支撑靶丸分层方法可被认为是构建激光惯性约束聚变能可重复运作的无支撑靶丸传输线的有效途径。
 

High repetition rate and mass-production of the cryogenic targets for laser IFE

Controlled inertial fusion energy (IFE) research is aimed at developing a new powerful energy source which is safe, environmentally friendly and cost-effective.

The main element of IFE power plant is a target with cryogenic hydrogen fuel (solid hydrogen isotopes or their mixtures) that must be delivered to the reaction chamber center at significant rates of 5−10 Hz. It leads to the amount of targets (5×105 −1×106) each day, and methodologies that are applicable to mass manufacturing of IFE targets are required for fueling a future reactor. Therefore, the research fields related to the elaboration of the efficient fuel layering methods for IFE applications are rapidly expanding. These methods must be integrated in a free-standing target (FST) transmission line which becomes an integral part of any IFE power plant.

A key aspect of the FST transmission line is the development of scientific and technological base for high repetition rate target supply at the laser focus. All target designs contain a fuel core, which is smoothly layered on the inside of a spherical low-Z ablator (plastic shell). The targets must be free-standing (or unmounted), and the fusion fuel inside the targets must have an ultra-fine structure (near-nano or nano-crystalline), which supports the fuel layer survivability under target injection and transport through the reaction chamber.

The isotropic ultra-fine fuel produced upon extremely high cooling (q >1 K/s) refers to as advanced materials for application to fusion targets fabrication in the form that meets the requirements of implosion physics. Therefore, creation of stable ultimate-disordered structures with a high defect density or isotropic medium is of critical importance for solid hydrogen isotopes or their mixtures.

To fulfill the above requirements, at the Lebedev Physical Institute, Russian Academy of Sciences (LPI/RAS), significant progress has been made in the technology development based on high cooling fuel layering inside moving free-standing targets, which refers to as FST layering method. The aim of these targets is to demonstrate large benefits of a “layering + delivery” scheme for a repetition rate target fabrication and delivery. Thus, a fundamental difference of the FST layering method from generally accepted approaches (characteristic of using a mounted target and extremely slow cooling q of about 3×10−5 K/s resulting in a long layering time of about 24 h) is that it works with the free-standing and line-moving targets, which allows one to economically fabricate large target quantities and to continuously (or at a required rate) inject them at the laser focus. A thorough analysis of the current state of the art in IFE target research and description of the underlying physical principles in the development of target fabrication and delivery technologies were published in High Power Laser Science and Engineering, Vol. 5, No. 2, e11, 2017 (I.V. Aleksandrova and E.R. Koresheva, Review on high repetition rate and mass-production of the cryogenic targets for laser IFE).

This work reports on credible solutions for fabrication of isotropic ultra-fine fuel within IFE targets. The FST layering method is a promising candidate to reach the goal. During FST layering, the following processes are mostly responsible for maintaining a uniform solid layer formation with an ultra-fine fuel structure:

― The target rotation when it rolling along the layering channel (single- or double-spiral) results in a liquid layer symmetrization.

a. The heat transport outside the target via conduction through a small contact area between the shell wall and the wall of the layering channel (metal hollow tube, helium cooled outside) results in a liquid layer freezing.

b. In the process, high cooling rates (q = 1−50 K/s) are maintained to form isotropic ultra-fine solid layers within free-rolling targets.

c. High cooling combined with fuel doping (presence of special high-melting additives to fuel) results in an ultra-fine structure stabilization of the obtained fuel layers.

d. FST layering method gives a unique prospect for the formation of a stable ultra-fine layer from D−Т mixture having the molecular composition: 25% of D2, 50% of deuterium tritide molecules, and 25% of T2. Tritium T2 in D−Т is considered as a high-melting additive with respect to D2 and deuterium tritide.

Figure below schematically shows the operational scenario of the FST layering method:

a. FST layering module (LM) works with a target batch at one time;

b. Transport process is the target injection between three basic units of the LM: shell container (SC) – layering channel (LC) – test chamber (TC);

c. SC can be cylindrical (a and c) or revolving type (b), and before layering it holds free-standing targets with fuel in a “Liquid + Vapor” state;

d. LC is a major element which ensures the target technology development according to the “layering + delivery” scheme;

e. Total layering time is typically less than 15 s, which has a side benefit in the view of tritium inventory minimization;

f. During layering, targets move top-down in the LC in a rapid succession of one after another that allows one to realize a high repetition rate injection of the cryogenic targets to the TC;

g. TC is used for the quality control of the finished cryogenic targets: precise tomographic characterization of individual target & threshold characterization as a way for on-line quality control of IFE target batch;

h. TC is a prototypical interface unit between the layering module & target injector (LM&TI).

Moving targets co-operate all stages of the fabrication and injection processes in the FST transmission line. Gravitational and electromagnetic injectors for application to target acceleration are considered. Recently, the LPI/RAS researchers have started the investigation into magnetic levitation of a target carrier made from high temperature superconductors (HTSC) as an alternative technology of non-contact manipulation, positioning and delivery of the finished free-standing cryogenic targets. In IFE applications, this direction attracts a significant interest due to maglev potential for almost frictionless motion. For this reason, the possibilities to develop a hybrid target injector based on acceleration of the HTSC target carrier over a permanent magnet guideway are analyzed as well.


Graphic description: FST layering method provides rapid symmetrization and formation of solid ultra-fine fuel layers. (a) Schematic of the FST-layering module (100-projection micro-tomograph with a spatial resolution less than 1µm is used for cryogenic target control). (b) Single-spiral layering channel is a special insert in the cryostat. (c) General view of the FST-layering module. The obtained experimental results are direct evidence that the LPI/RAS approach based on the FST layering method can be considered as a credible path for creating a repeatable operating FST transmission line for laser IFE.