Conduction cooling of cryogenic devices and instruments using closed-cycle cryocoolers are fast gain- ing popularity as a pathway to \invisible" cryogenics. The technique comprises direct mechanical coupling of the device/instrument to the cryocooler and the apparatus enclosed in a high-vacuum environment, without convectively contacting any cryogenic liquid or gas. In this contribution, com- mon applications of cryocooler conduction cooling are reviewed to understand the current state of the technology. Three devices are considered - superconducting magnets, accelerator cavities, and sub- Kelvin refrigerators that use closed-cycle cryocoolers for pre-cooling. Thereafter, basic heat transfer topics are described that require further research for better optimization of the conduction cooling technique. These include thermal contact resistance, cooldown time modeling, and exible ther- mal linkages. This contribution is especially helpful for cryocooler developers/researchers who are interested in exploring potential applications of their cryocoolers. 1 Introduction The process of cooling devices and instruments to cryogenic temperatures involves one of the three meth- ods: (1) convective cooling by circulating a cold liquid (cryogen) or gas around them, (2) radiative cooling by exposing them to a cryogenic ambient, and (3) conduction cooling by mechanically connecting them to cryocoolers, which are refrigerators capable of attaining temperatures below 120 K. The convection cooling technique is widely employed to cool large scale devices such as superconducting magnets and ra- diofrequency cavities in science-oriented particle accelerators. Devices and instruments deployed in deep space can employ radiative cooling to take advantage of the surrounding cold ambient, typically near 3 K. Cryocooler conduction cooling is particularly attractive for smaller, compact devices and instruments that do not dissipate substantial heat, require reliable operation over long duration with minimal human oversight, or that need to be deployed at locations where an uninterrupted supply of cryogens is dicult to provide. 1.1 Working principle of cryocooler conduction cooling Cryocoolers are closed-cycle devices that produce cryogenic temperature by utilizing the temperature change associated with compression and expansion processes in real gasses. Helium is the commonly used gas that enables reaching temperatures as low as 2.5 K. An excellent review of cryocoolers, their types, available temperature range and cooling capacities are available in [1]. The conduction-cooling technique involves a mechanical connection of the load (the instrument that is to be cooled) with the cryogenic heat exchanger of a cryocooler. The mechanical connection is established by a thermal link, commonly made of a high thermal conductivity metal such as copper and aluminum. The cryocooler-thermal link-load mechanical connection can be a semi-permanent type such as comprising brazed joints or demountable type realized by pressed or bolted contacts. The thermal link commonly comprises exible elements such as a strap or braid that compensate for the di erence between thermal contraction of the cryocooler body and the instrument body as they cool down from room temperature to the cryogenic operating temperature. The exible elements also provide vibration isolation between the cryocooler (that typically generates vibration) and the load. With conduction-cooling, the cryocooler- thermal link-load operates in a thermally shielded vacuum space, without any cooling liquid or gas surrounding it. 1 1.2 Scope of this review Although di erent cryocoolers and combinations thereof provides access to the entire sub-ambient tem- perature range, this review will focus on devices and instruments operating near or below the normal liquid helium temperature of 4.2 K. This temperature range can be reached using multi-stage Gi ord McMahon cryocoolers, pulse tube cryocoolers, and Joule Thompson cryocoolers. 1. Low Temperature Superconducting (LTS) magnets - typically made of niobium-titanium (NbTi) or niobium-tin superconductor (Nb 3Sn). Applications include magnetic resonance imaging, nuclear magnetic resonance, particle accelerators (for beam bending and focus/de-focusing), x-ray and neutron di raction systems, magnetic energy storage systems, and more [2],[3]. 2. Superconducting radiofrequency (SRF) cavities - typically made of niobium and niobium-tin. Poten- tial applications include electron-beam irradiation treatment of materials, treatment of ue gas and wastewater, medical device sterilization, and more using conduction-cooled SRF accelerators [4]-[7]. 3. Sub-kelvin cryocoolers such as a dilution refrigerator and adiabatic refrigerator that employ 4 K cryocoolers (commonly the pulse tube cryocooler) for precooling. Dry dilution refrigerators have be- come commonplace in quantum information science, quantum computing development, astrophysics (dark matter search, etc.) and general research at mK temperatures. Cryocooler adiabatic demag- netization refrigerators are routinely used in spaceborne experiments running below 1 Kelvin [8],[9]. 2 Advantages and limitations 2.1 Advantages Unique advantages of cryocooler conduction cooling are the following: 1. The advantages of cryocooler conduction cooling result from the basic fact that no cryogenic gas or liquid leaves the cryocooler body during operation. The working uid is con ned to the cryocooler when it is put together as a commercial package at the cryocooler manufacturer. The overpressure safety and electrical safety devices are integrated into the cryocooler package by the manufacturer such that the end user carries no burden of the cryocooler safety aspects. 2. Since no cryogenic uid leaves the cryocooler package, a conduction-cooled device or its enclosure (known as cryostat) does not make direct contact with a cryogenic uid. As a result, upset con- ditions such as a sudden loss of insulating vacuum [10]-[12] or of beamline vacuum [13]-[17] (SRF accelerators) are easier to manage than if cryogens such as supercritical and liquid helium are present in the cryostat. 3. The absence of cryogenic uid and less severe overpressure condition also simpli es the design of the cryostat. In most cases, the cryostat no longer needs the classi cation as a pressure vessel because it does not contain cold gas or liquid. Design criteria per codes and standards (example, ASME BPVC) then become relaxed compared to those for a pressure vessel containing cryogenic liquid. 4. Cryocoolers have turnkey operation that enables switching it on and o with the push of a button. As a result, conduction cooled devices do not require complex cooldown and warm up procedures. Conduction cooled systems can be operated by personnel who may not have specialized training or deep experience in cryogenic operations. 5. Modern cryocoolers are highly reliable with mean time between maintenance exceeding 20000 hours. This enables continuous system operation for more than 2 years. For contrast, piston expander- based small-capacity helium lique ers typically require yearly maintenance. 2.2 Limitations Some limitations of cryocooler conduction cooling are the following: 1. Cryocoolers that reach liquid helium temperatures (Gi ord-McMahon and pulse tube type) use valves to compress and expand the gas in the cold head. These valves introduce high ineciency in the cryocooler system. As a result, the speci c cooling power of currently available most ecient commercial cryocoolers is 3000 W(electrical)/W(cooling at 4.2 K). This is large compared to 500 2 W(electrical)/W(cooling at 4.2 K) for a mid-size helium refrigerator. As a result, 4 K cryocoolers are cost-prohibitive and impractical for cooling large devices that have a heat load of more than 15-20 W. 2. Cryocooler conduction-cooled devices exhibit long cooldown durations from room temperature to the operating cryogenic temperature. This results from the slow di usive heat transfer mechanism in the thermal conduction links, large heat capacity of the load, and small unit cooling capacity of commercial cryocoolers (2.5-3 W @ 4.2 K). 3. Due to small unit cryocooler capacity, conduction-cooled devices need to be designed to keep static heat leak to as small as practical. Otherwise, a large fraction of the cryocooler cooling capacity is used up for removing the static heat leak. In general, the static heat leak budget of cryocooler conduction systems is very limited compared to systems using convective heat transfer with super- critical/liquid helium. 3 Conduction cooled superconducting magnets 3.1 Design considerations Superconducting magnets are dc (direct current) devices that dissipate negligible heat during operation. Dynamic heat load in superconducting magnet systems to 4 K (in case of NbTi magnets) is therefore negligible. Static heat leak through supports to the ambient stage of the cryostat, current leads, and thermal radiation constitute the major sources of heat load. A thermal conduction link then needs to manage the static heat leak, which can be made small by proper optimization of the system. The considerations include using low thermal conductivity supports made of G10, stainless steel, or titanium alloys with optimal thermal interception at warmer temperature (50-70 K), proper design and installation of multilayer insulation to lower thermal radiation, and optimal design and thermal interception of current leads that minimizes heat conduction and Joule heating during current transport (see [18] and