Electronics Guide

Total Ionizing Dose Effects

Total ionizing dose (TID) refers to the cumulative damage caused by ionizing radiation absorbed over time. In semiconductors, TID primarily affects oxide layers where radiation generates electron-hole pairs. While electrons are quickly swept away, holes become trapped in the oxide, creating positive charge buildup that shifts transistor threshold voltages and increases leakage currents. In MOSFETs, this can cause parametric degradation or functional failure as threshold voltage shifts beyond acceptable limits. Power devices are particularly vulnerable because their thick gate oxides absorb more radiation.

Passive components also suffer TID effects. Insulating materials experience increased conductivity and dielectric breakdown at lower voltages. Optical isolators degrade as radiation damages the LED emitters and photodetectors. Capacitors may exhibit increased leakage and reduced capacitance. Understanding the TID tolerance of each component in a power system is essential for predicting system lifetime in radiation environments.

Displacement Damage

Displacement damage occurs when energetic particles, particularly neutrons and protons, collide with atoms in semiconductor crystal lattices, displacing them from their normal positions. This creates defects that act as recombination centers, reducing minority carrier lifetime and increasing leakage currents. Bipolar transistors are especially susceptible because their operation depends on minority carrier transport across the base region. Current gain degrades progressively with accumulated displacement damage dose.

In power electronics, displacement damage affects bipolar devices including IGBTs, BJTs, and thyristors. It also degrades solar cells used in space power systems by reducing their conversion efficiency. The non-ionizing energy loss (NIEL) metric quantifies displacement damage potential for different particle types and energies, enabling prediction of device degradation from known radiation environments.

Single-Event Effects

Single-event effects (SEE) occur when individual high-energy particles deposit sufficient charge in sensitive circuit regions to cause malfunction. Single-event upsets (SEU) flip memory bits or logic states, causing temporary errors. Single-event latchup (SEL) triggers parasitic thyristor structures in CMOS devices, potentially causing destructive current levels. Single-event burnout (SEB) and single-event gate rupture (SEGR) can permanently destroy power transistors through localized current concentrations or gate oxide breakdown.

Power MOSFETs are particularly vulnerable to SEB when blocking high voltage, as an ion strike can trigger avalanche multiplication and thermal runaway. Protection strategies include limiting drain-source voltage, adding current limiting, and using SEE-tolerant device technologies. In redundant systems, SEU can be masked through voting logic, while SEL requires detection and power cycling to recover normal operation.

Enhanced Low-Dose Rate Sensitivity

Some bipolar devices and integrated circuits exhibit enhanced low-dose rate sensitivity (ELDRS), where degradation is actually worse at the low dose rates typical of space environments compared to the high dose rates used in accelerated testing. This counterintuitive behavior results from different defect annealing dynamics at different dose rates. ELDRS must be considered when qualifying components for space and some nuclear applications, requiring either low-dose-rate testing or application of appropriate safety factors to high-dose-rate test results.

Hardening-by-Design Techniques

Radiation hardening by design employs circuit and layout techniques to improve radiation tolerance without requiring specialized fabrication processes. Guard rings collect leakage currents before they affect sensitive nodes. Edgeless transistor layouts eliminate parasitic edge leakage paths. Enclosed gate structures in MOSFETs reduce TID-induced threshold shifts. Temporal filtering suppresses single-event transients before they propagate. Error detection and correction codes protect memory and data paths from SEU.

At the system level, triple modular redundancy (TMR) uses three parallel circuits with majority voting to mask single failures. Watchdog timers detect and recover from SEU-induced software errors. Power supply current limiting prevents SEL from causing permanent damage. These design techniques can be applied to commercial components to improve their radiation tolerance, though typically not to the level of purpose-built radiation-hardened devices.

Hardening-by-Process Approaches

Specialized semiconductor fabrication processes create inherently radiation-tolerant devices. Silicon-on-insulator (SOI) technology places transistors on an insulating layer, reducing sensitivity to TID and SEL by isolating devices from the bulk substrate. Silicon-on-sapphire (SOS) provides similar benefits with even better isolation. Thin gate oxides in advanced CMOS processes naturally resist TID better than older thick-oxide technologies. Epitaxial layers and careful doping profiles reduce SEL susceptibility.

Radiation-hardened foundries offer qualified fabrication processes with demonstrated radiation tolerance. These processes typically lag commercial technology by several generations but provide assured performance in radiation environments. The premium cost of radiation-hardened components reflects the specialized processing, smaller production volumes, and extensive qualification testing required.

Radiation-Tolerant Power Semiconductors

Power semiconductor selection for radiation environments requires careful evaluation of device technology and radiation tolerance. Silicon carbide (SiC) devices offer inherent advantages including wider bandgap that reduces TID sensitivity and greater resistance to displacement damage. However, SiC gate oxides can still accumulate charge under radiation. Gallium nitride (GaN) devices show promise for radiation applications, though their radiation response is still being characterized.

For conventional silicon devices, MOSFETs with radiation-hardened gate oxides are available from specialized suppliers. Bipolar transistors and thyristors, while susceptible to displacement damage, can be selected for applications where the accumulated fluence remains acceptable. Careful derating and margin allocation extend operational lifetime in radiation environments. Testing of actual production lots may be required to verify radiation tolerance, as it can vary between manufacturing runs.

Passive Component Considerations

Passive components for radiation environments require careful selection. Ceramic capacitors generally tolerate radiation well, though some dielectric formulations are better than others. Film capacitors may exhibit parameter changes under radiation. Tantalum capacitors can survive significant doses but require testing to characterize response. Aluminum electrolytic capacitors should be avoided in high-radiation areas due to electrolyte degradation.

Resistors based on metal film or wire-wound construction typically show good radiation tolerance. Carbon composition resistors may exhibit significant resistance changes. Inductors and transformers using ceramic or air cores avoid the magnetic property changes that can affect ferrite materials under neutron irradiation. Optical components including fiber optics and optocouplers require radiation-tolerant formulations to prevent darkening and gain degradation.

Reactor Instrumentation Power

Nuclear reactor instrumentation systems require reliable power to monitor reactor conditions including neutron flux, temperature, pressure, and coolant flow. These instruments provide essential information for reactor control and safety systems. Power supplies for in-containment instrumentation must withstand the radiation environment inside the reactor building, which includes gamma radiation from activated materials and neutron flux near the reactor vessel.

Safety-related instrumentation power supplies must meet rigorous nuclear qualification standards including IEEE 323 for Class 1E equipment. They must demonstrate operability under normal, abnormal, and accident conditions. Seismic qualification ensures continued operation during earthquakes. Environmental qualification addresses radiation, temperature, humidity, and chemical exposure. Redundant power supplies with independent power sources ensure instrumentation remains available even with multiple failures.

Control Rod Drive Power

Control rod drive mechanisms (CRDMs) adjust the position of neutron-absorbing control rods to regulate reactor power. Electric CRDMs require power supplies capable of precise current control for positioning and rapid response for scram (emergency shutdown) functions. Power electronics for CRDM applications must operate reliably in the radiation environment near the reactor vessel while meeting strict safety and reliability requirements.

Magnetic jack CRDMs use sequential energization of lifting coils to move control rods in discrete steps. The power supplies must provide controlled current pulses with precise timing. Linear CRDMs use electromagnetic coils for continuous position control. Both types require power electronics designed for safety-critical applications with appropriate redundancy and failure detection capabilities.

Post-Accident Monitoring Power

Post-accident monitoring (PAM) systems provide operators with information needed to assess plant status and take protective actions following accidents. These systems must continue operating in the harsh post-accident environment including elevated radiation levels from released fission products, high temperatures from decay heat, and potentially steam and water spray. Power supplies for PAM equipment must be qualified for these severe conditions.

The Three Mile Island and Fukushima accidents demonstrated the critical importance of maintaining instrumentation power during severe accidents. Modern reactor designs incorporate diverse and redundant power supplies for PAM systems, including battery-backed power, dedicated emergency diesel generators, and passive cooling to extend equipment survival time. Power electronics designed for post-accident monitoring incorporate extensive radiation hardening and environmental protection.

Emergency Core Cooling Power

Emergency core cooling systems (ECCS) inject water to remove decay heat and prevent fuel damage following loss-of-coolant accidents. Motor-driven pumps, valve actuators, and instrumentation all require reliable power. Power electronics for ECCS applications must function despite radiation from the accident and potential loss of normal power sources. Uninterruptible power supplies, battery systems, and emergency diesel generators provide backup power paths.

Safety injection pump motor drives may incorporate variable frequency drives for soft starting and speed control. These drives must meet nuclear qualification requirements while providing the motor control functions needed for effective emergency response. The power electronics must be located in areas with acceptable radiation levels or incorporate sufficient hardening for their installed location.

Hot Cell Equipment Power

Hot cells are heavily shielded enclosures used for handling highly radioactive materials including spent nuclear fuel, irradiated reactor components, and radioactive sources. Equipment inside hot cells operates in gamma radiation fields that can exceed millions of rad per hour. Power electronics supporting hot cell operations must either be located outside the shielded enclosure with power delivered through penetrations, or be designed to survive the intense radiation environment.

Hot cell equipment includes manipulators, conveyors, cutting tools, welding equipment, and inspection systems. Power supplies must accommodate the power and control requirements of this diverse equipment while managing the challenges of power delivery through thick shielding walls. Modular power system designs facilitate maintenance by enabling replacement of degraded components without extensive hot cell reconfiguration.

Remote Handling Robot Power

Remote handling robots perform inspection, maintenance, and decommissioning tasks in areas too radioactive for human access. These robots require onboard power electronics for motor drives, sensors, and communication systems. The power electronics must survive radiation doses accumulated during potentially thousands of hours of operation in high-radiation areas. Graceful degradation allows continued operation with reduced capability as radiation damage accumulates.

Robot power systems typically receive power through tethers that also provide communication and control signals. Power electronics convert the tether voltage to the various levels required by motors, electronics, and sensors. Radiation-tolerant motor drives control locomotion and manipulator movements. Careful component selection and conservative derating extend robot operational lifetime in radiation environments.

Decontamination System Compatibility

Equipment used in radioactive environments must be compatible with decontamination processes needed to reduce radiation levels for maintenance or disposal. Power electronics enclosures must withstand decontamination methods including chemical cleaning, high-pressure water spray, and electropolishing. Surface finishes should be smooth and free of crevices that trap contamination. Materials must resist the chemicals used in decontamination without releasing contamination or degrading.

Design for decontamination affects enclosure design, connector selection, and surface treatment. Stainless steel enclosures with welded seams and smooth finishes facilitate decontamination. Hermetically sealed connectors prevent contamination ingress. Cable entries use radiation-tolerant sealing methods. These design features add cost but are essential for equipment that must be maintained or eventually removed from radiation areas.

Accelerator Magnet Power Supplies

Particle accelerators use powerful magnets to bend and focus charged particle beams. These magnets require precision power supplies capable of delivering thousands of amperes with stability better than parts per million. Power electronics for accelerator magnets must operate near intense radiation sources including beam losses, activation of accelerator components, and secondary particle production. The large scale of accelerator facilities means many power supplies operate in varying radiation environments.

Superconducting magnets in modern accelerators like the Large Hadron Collider at CERN require power supplies with special features for cryogenic magnet operation. Fast energy extraction systems protect superconducting coils during quench events. The power electronics must interface with machine protection systems and respond rapidly to fault conditions while maintaining precision current control during normal operation.

Radio-Frequency System Power

Particle acceleration in synchrotrons and linear accelerators relies on radio-frequency (RF) cavities powered by high-power amplifiers. These RF systems require sophisticated power supplies delivering hundreds of kilowatts to megawatts at frequencies from tens of megahertz to several gigahertz. Power electronics providing the high-voltage DC supplies for klystrons and other RF sources must operate in areas where scattered radiation from the beam can be significant.

RF system reliability directly affects accelerator availability, making power supply reliability essential. Modular designs enable rapid replacement of failed components. Built-in diagnostics identify developing problems before failure. Power supplies may be relocated to lower-radiation areas with longer cable runs when radiation at the original location proves problematic.

Beam Diagnostics Power

Beam diagnostic instruments measure particle beam properties including position, profile, intensity, and energy. These instruments often must be located close to the beam where radiation levels are highest. Power electronics for beam diagnostics must survive in these harsh locations while providing clean, stable power to sensitive measurement systems. Radiation-induced noise in power supplies can corrupt measurements, requiring careful filtering and shielding.

Diagnostic power systems increasingly incorporate digital control and communication capabilities that must also be radiation-tolerant. Field-programmable gate arrays (FPGAs) and microcontrollers used for local processing and communication require protection against single-event effects. Radiation testing of complete diagnostic systems verifies that all components, including power supplies, meet requirements for their installed locations.

Continuous Air Monitor Power

Continuous air monitors (CAMs) detect airborne radioactive contamination in nuclear facilities, providing early warning of releases that could affect workers. These monitors require reliable power for air pumps, radiation detectors, and alarm systems. Power supplies must operate continuously with high reliability, as CAM failure could leave workers unaware of dangerous contamination levels. Battery backup ensures continued operation during power interruptions.

CAM power electronics must be electromagnetically compatible with sensitive radiation detectors that measure extremely low activity levels. Switching power supply noise can interfere with detector signal processing, requiring careful filtering and shielding. Some CAM designs use linear power supplies despite lower efficiency to avoid switching noise. Power supply design must balance efficiency, noise, and reliability considerations.

Area Radiation Monitor Power

Area radiation monitors measure gamma and neutron radiation levels throughout nuclear facilities to verify that dose rates remain within limits and detect any unexpected increases. These monitors may be located in areas with significant background radiation, requiring power electronics with appropriate radiation tolerance. Monitor networks can include hundreds of units, making reliable, maintainable power supplies essential for system availability.

Modern area monitors incorporate digital processing and network communication for remote monitoring and data logging. Power supplies must support these electronics while maintaining isolation between power and signal paths to prevent interference. Power-over-Ethernet (PoE) technology enables combined power and data delivery over single cables, simplifying installation, though PoE power supplies must meet radiation tolerance requirements.

Personnel Dosimetry Support Power

Electronic personal dosimeters (EPDs) provide real-time dose monitoring for workers in radiation areas. Dosimeter reading stations and charging systems require reliable power to ensure workers can always verify their dose and maintain functional dosimeters. Power supply design for these systems emphasizes reliability and availability, with battery backup for continued operation during power disturbances.

Advanced dosimetry systems include wireless communication for real-time dose tracking and alarm notification. Base stations that communicate with dosimeters and process dose data require clean power for radio systems and digital electronics. Integration with facility access control systems adds requirements for reliable power to ensure proper radiation protection protocols are maintained.

Gamma Shielding Materials

Gamma radiation shielding uses dense materials to attenuate photon intensity through absorption and scattering. Lead is the traditional choice due to its high density and atomic number, though toxicity and mechanical properties limit some applications. Tungsten provides similar attenuation with better mechanical properties and no toxicity concerns but at higher cost. Steel and concrete provide moderate shielding at lower cost for larger installations.

Shielding thickness required depends on the gamma energy spectrum and required attenuation factor. High-energy gammas from fission products require substantial shielding, with half-value layers of several centimeters even in lead. Shield design must consider weight, space constraints, and the need for penetrations for cables and cooling. Local shielding around sensitive electronics can supplement facility shielding to achieve required dose reduction.

Neutron Shielding Approaches

Neutron shielding requires different materials than gamma shielding because neutrons interact primarily with atomic nuclei rather than electrons. Hydrogen-rich materials like water, polyethylene, and concrete effectively moderate fast neutrons to thermal energies. Thermal neutrons are then captured by materials with high absorption cross-sections including boron, cadmium, and gadolinium. Neutron capture often produces gamma rays requiring additional gamma shielding.

Borated polyethylene combines neutron moderation and absorption in a single material, making it convenient for local shielding applications. Steel and concrete used in structural shielding require careful attention to neutron streaming through penetrations. Power electronics located in neutron fields may require both neutron and gamma shielding, with material selection and arrangement optimized for the specific radiation spectrum.

Shielded Enclosure Design

Shielded enclosures protect power electronics from radiation while providing necessary cooling and access for maintenance. Enclosure design must balance shielding effectiveness against weight, cost, and thermal management requirements. Penetrations for power cables, signal cables, and cooling reduce shielding effectiveness and require careful design with labyrinth paths or supplemental shielding.

Modular shielded enclosures enable standardized designs that can be qualified once and deployed repeatedly. Removable shield panels facilitate maintenance access while maintaining protection during operation. Cooling approaches include external heat exchangers connected through shielded penetrations, radiation-tolerant fans inside the enclosure, or conductive cooling through shield walls. The optimal approach depends on heat load, available space, and radiation environment.

Redundant Architecture Approaches

Critical power systems in radiation environments employ redundant architectures to maintain function despite component failures. N+1 redundancy provides one spare unit beyond the minimum required, allowing continued operation with a single failure. N+2 and higher redundancy levels protect against multiple simultaneous failures. The appropriate redundancy level depends on failure probability, consequence severity, and the feasibility of repair or replacement.

Diversity complements redundancy by using different technologies or designs for redundant channels, preventing common-cause failures from affecting all channels simultaneously. A radiation-hardened power supply might be backed up by a commercial unit in a shielded enclosure, providing diversity against both radiation damage and design defects. Independence between redundant channels prevents failures from propagating between channels.

Voting and Switching Logic

Redundant power systems require logic to select between available sources and detect failures. Simple two-unit systems may use primary/backup configurations with automatic switchover on primary failure. Three-unit systems enable two-out-of-three voting that masks single failures without switchover transients. The voting and switching logic itself must be highly reliable and radiation-tolerant to avoid becoming a single point of failure.

Hardware voting using analog comparators and power combiners provides inherent radiation tolerance compared to digital implementations. For digital voting, radiation-hardened FPGAs or microcontrollers with appropriate SEU mitigation implement voting algorithms. Self-testing and cross-checking between channels detect degradation before complete failure enables appropriate response.

Graceful Degradation Design

Graceful degradation enables continued partial operation as radiation damage accumulates or components fail. Rather than sudden complete failure, the system progressively loses capability while maintaining core functions. This approach is especially valuable in radiation environments where damage accumulates continuously and replacement may be difficult or impossible.

Power system graceful degradation might involve reducing output power as efficiency drops, switching to backup channels as primary channels degrade, or accepting reduced regulation accuracy while maintaining basic power delivery. Monitoring systems track degradation trends to predict remaining useful life and enable proactive maintenance when possible. Design margins accommodate degradation while maintaining minimum required performance.

In-Situ Monitoring and Diagnostics

Continuous monitoring of power system health enables early detection of radiation-induced degradation and supports remaining life prediction. Monitored parameters include output voltage and current accuracy, efficiency, temperature rise, switching timing, and component-level parameters where accessible. Trend analysis identifies gradual degradation that might not trigger immediate alarms but indicates approaching failure.

Remote monitoring capabilities are essential when power systems are located in areas with restricted access. Communication interfaces must be radiation-tolerant and electromagnetically compatible with power electronics. Data logging captures historical trends for analysis. Integration with facility monitoring systems enables coordinated response to degradation across multiple systems.

Spent Fuel Handling Systems

Spent nuclear fuel handling requires power electronics for cranes, transfer systems, and inspection equipment operating in intense radiation fields. Spent fuel assemblies emit gamma radiation from fission products and neutrons from spontaneous fission and induced reactions. Power systems must function in these harsh conditions while providing precise control for safe fuel movements.

Fuel handling crane drives require precise positioning capability with high reliability. Variable frequency drives control bridge, trolley, and hoist motions with smooth acceleration and accurate speed regulation. The drives may be located in shielded operator areas with long motor cables, requiring attention to voltage drop, motor heating, and electromagnetic compatibility. Redundant controls and interlocks ensure safe operation despite potential radiation damage.

Reprocessing Facility Power

Nuclear fuel reprocessing plants separate useful materials from spent fuel for recycling while managing highly radioactive waste streams. Power electronics support chemical processing equipment, ventilation systems, and material handling in extremely radioactive environments. Many systems must operate continuously to maintain safe conditions, requiring highly reliable power supplies with appropriate redundancy.

Reprocessing facilities present unique challenges combining intense radiation, chemical exposure, and criticality concerns. Power system designs must be compatible with facility safety requirements including prevention of uncontrolled nuclear reactions. Intrinsic safety concepts may be applied to prevent electrical ignition of flammable materials. Equipment must be compatible with remote operation and maintenance approaches required by high radiation levels.

Waste Processing Equipment Power

Radioactive waste processing prepares waste for safe storage and disposal through volume reduction, stabilization, and packaging. Processing equipment including incinerators, compactors, grouts, and vitrification systems requires reliable power for motors, heaters, and control systems. Power electronics must function in the radioactive environment of waste processing areas while meeting reliability requirements for continuous operations.

High-level waste vitrification incorporates waste into glass for long-term storage, using electric melters powered by high-current supplies. Melter power electronics must deliver hundreds of kilowatts while operating in a radioactive environment with extreme temperatures and corrosive off-gases. The critical nature of vitrification operations demands highly reliable power systems with comprehensive monitoring and backup capabilities.

Enclosure and Structural Materials

Materials used in power electronics enclosures and structures must maintain mechanical properties under radiation. Austenitic stainless steels provide good radiation tolerance along with corrosion resistance important in nuclear environments. Aluminum alloys may be acceptable for lower radiation levels but can suffer strength reduction from neutron-induced transmutation. Carbon steels are generally adequate for gamma-only environments.

Polymer materials used in gaskets, seals, and wire insulation vary widely in radiation tolerance. Silicone rubber maintains properties to higher doses than most organic polymers. Polyimides offer excellent radiation resistance for wire insulation. Fluoropolymers may actually be degraded by radiation despite their chemical resistance. Material selection must consider the specific radiation environment and required lifetime.

Insulating Materials

Electrical insulation in power electronics must maintain dielectric strength and mechanical integrity under radiation exposure. Ceramic and glass insulations offer inherent radiation stability. Mineral-insulated cables using magnesium oxide provide radiation-tolerant power delivery. Polymer insulations require careful selection, with polyimide and PEEK offering better radiation tolerance than standard materials.

Transformer and inductor insulation systems must withstand combined thermal and radiation stress. High-temperature insulation systems using ceramic and glass-fiber materials generally tolerate radiation better than lower-temperature organic systems. Insulating oil used in some high-power transformers can undergo radiolytic decomposition, generating gases that may require venting and potentially affecting dielectric properties.

Potting and Conformal Coating Materials

Potting compounds and conformal coatings protect circuit assemblies from environmental exposure and provide mechanical support. In radiation environments, these materials must maintain adhesion and flexibility while resisting radiation-induced degradation. Silicone potting compounds generally offer better radiation tolerance than epoxies, though some radiation-resistant epoxy formulations are available.

Conformal coatings applied to circuit boards face similar requirements. Silicone and polyurethane coatings typically outperform acrylics in radiation environments. Coating adhesion and coverage are critical because radiation can cause accelerated failure at any unprotected areas. Qualification testing should include long-term exposure at relevant dose rates to identify potential degradation mechanisms.

Radiation Testing Methods

Radiation qualification testing exposes equipment to radiation levels representative of the intended application or higher levels for accelerated testing. Gamma testing typically uses cobalt-60 sources that provide a well-characterized spectrum for TID evaluation. Neutron testing uses reactor facilities or accelerator-based sources to evaluate displacement damage. Ion beam facilities provide heavy ions for single-event effect characterization.

Test protocols specify dose rates, total doses, and acceptance criteria appropriate for the application. Nuclear industry standards define qualification requirements for safety-related equipment. Accelerated testing at high dose rates must account for potential dose rate effects, with some devices showing different responses at different dose rates. Testing should characterize both immediate effects and any annealing behavior that might affect long-term performance.

Environmental Qualification Standards

Nuclear power plants in the United States require equipment qualification per IEEE 323 and 10 CFR 50.49 for safety-related applications. These standards require demonstrated operability under normal and accident conditions including radiation exposure. European nuclear facilities follow similar requirements under IEC standards. Space applications use MIL-STD-883 and other military standards for radiation testing, supplemented by project-specific requirements.

Qualification programs include type testing of representative samples, similarity analysis for equipment with minor variations, and analysis to extend test results to different conditions. Quality assurance requirements ensure that production equipment matches the qualified configuration. Maintenance of qualification status requires control of changes and periodic surveillance testing.

Accelerated Life Testing

Accelerated life testing compresses equipment lifetime into manageable test durations by increasing stress levels above normal operation. For radiation effects, this typically means testing at higher dose rates than the application environment. The acceleration factor depends on the dominant degradation mechanism, which may be different for different components. Valid acceleration requires that the degradation mechanism remain the same at the accelerated and normal stress levels.

Combined environment testing subjects equipment to radiation along with other stresses including temperature cycling, vibration, and humidity. These combined stresses may reveal failure modes not apparent from single-stress testing. Test sequences should represent the expected service environment as closely as practical while achieving the necessary acceleration for practical test durations.