The only weaponized emp is itself a nuclear weapon. So basically you are talking about bringing down an incoming nuclear weapon with another nuclear explosion. It may or may not work reliably depending on the detonation distance.
USA nukes in service are designed and built to resist EMP and X-rays from detonation of another nuke.
There is no reason to believe that Russian and UK weapons are not similarly protected.
China, Korea, Pakistan Etc. who knows?
The question early on with defense systems was “do you want to be detonating 2 megaton ABM warheads above your homeland?” Many of the targets would only be decoys, so your own response could do more damage than the attack!
This is why modern defense uses “hit to kill” systems with advanced detection and control systems.
-my understanding of EMP vulnerability is that larger-scale devices are more vulnerable than smaller-scale devices. IOW, radio antennas and power lines subjected to an EMP event are going to generate large current spikes that can damage equipment attached to them, but a warhead a few feet across will be much less vulnerable.
-it’s probably not terribly difficult to provide good EMP shielding for a nuclear warhead. If the whole thing is encased in a metal shell, it’s going to be hard for an EMP to cause damage.
-if you’re using a nuclear detonation to generate an EMP, said EMP is not generated in the immediate vicinity of the blast, and you don’t want the blast to necessarily happen right next to the missile you’re targeting. See the link: the blast has to happen well above the earth’s atmosphere, and the resulting EMP will affect everything within hundreds of miles.
Apparently the fear is pretty high. In 1995 Russia thought Norway had launched a missile to use an emp to disable their Command/Control/Radar. Its not clear exactly why their missile tracking is so bad that Russia couldn’t tell the difference between Norway launching a missile at them and the open ocean.
High altitude Electromagnetic pulse (HEMP) devices produce three distinct regimes of pulse, referred to as E1, E2, and E3. Microelectronics are most sensitive to E1, which is due to interaction of x-ray and gamma ray radiation with the rarified upper atmosphere and the geomagnetic field resulting an a nearly coherent, widely distributed pulse. In more dense atmosphere where the the rays are rapidly absorbed and don’t have much free mean length, this pulse is seriously attenuated, and the amount of damage done but the physical effects of the blast (shock and thermal wave) would likely make E1 effects moot. E2 is more like static electricity, and can typically be shielded by using a protected ground or faraday cage type shielding. E3 is energy that is stored in the Earth’s magnetic field (similar to that which comes from coronal discharges and solar flares) and will cause longer term disruption and very high voltage spikes in large arrays like power grids; again, not much of a direct threat to microelectronics, but would likely compromise the power grid the same way a massive geomagnetic storm could (see JSR-11-320 “Impacts of Severe Space Weather on the Electric Grid”, JASON Committee, Nov 2011) resulting in severe long term consequences for the electrical distribution infrastructure.
It is important to understand that EMP is not just a burst of electromagnetic energy; it is a pulse of energy that are quasi-coherent; that is, the wave is in phase as if emitted from a giant free electron maser that is constructed out of the Earth’s magnetosphere. (I’m specifically referring to the E1 component of high altitude EMP, which is what is of concern with regard to damaging semiconductor electronics). EMP is in the radio frequency range and can deliver potentials of up to the 50 kV/m atmospheric saturation range after which the atmosphere will undergo dielectric breakdown as it does with lightning. This pulse develops high potentials in semiconductors that causes them to physically breakdown, regardless of whether they are powered or connected to a power grid. This will affect any device that is not adequately protected, and especially anything with an antenna or other conductive lead that is directly coupled to internal electronics or electrically sensitive components. Much of the effort to make commercial electronics smaller and operate on lower power has also made them much more sensitive to EMP effects.
Modern reentry vehicles have a shell that is a high temperature carbon composite with a pyrolytic graphite tip. They’re conductive and so provide some protection against EMP, but the arming and fusing systems inside of them are designed to be essentially impervious to EMP up to the 50 kV/meter atmospheric breakdown limit. There is some protection against X-ray and neutron impingement but there are obviously physical limits to how much shielding can be provided before the RV payload becomes too heavy.
This was true of the Safeguard ABM system, installed in 1975 at the Stanley R. Michelsen Complex to protect the 321 ICBM Wing based around Grand Forks AFB in North Dakota; the Spartan mid-range interceptors and the Sprint terminal-phase interceptors; both were designed for “enhanced radiation” output. Spartan had a ~5 MT warhead designed for exoatmospheric detonation with high X-ray yield which would vaporize the surface of multiple RVs and cause an internal shockwave which would disrupt the internal systems. Unfortunately, it would also likely cause a significant EMP of its own over American territory. The much smaller Sprint warhead had an estimated yield of 1-2 kT optimized for thermal neutron yield within the atmosphere, designed to disrupt the physics package itself causing a low yield (“fizzle”) detonation. The systems were intended for protection of a hardened Minuteman ICBM installation to provide assured counterstrike capability, thereby theoretically strengthening deterrent capability against a concerted multiple attack; however, the cost and questionable need (over-the-horizon and orbital launch detection systems should give sufficient time to command a launch before the complex came under attack) resulted in the system being shut down the following year. The Soviets had a system which somewhat similar capabilities designed to provide protection for a strike against Moscow.
Current systems use “hit-to-kill” (HTK) kinetic interception capability. While this is more desirable from a public perspective, it is not nearly as reliable, and in fact problems with the infrared seeker sensor on the kill vehicle have plagued missile defense programs in recent years. HTK is best suited for boost phase interception where it can destroy the delivery vehicle under power before the system can deploy individual RVs. However, boost phase only lasts for a couple of hundred seconds, which means that the interceptor needs to be very close to the launch point and forward of the trajectory to be feasible. For this reason most efforts have focused on mid-course interception, after the booster has burned out but before RVs are released and reenter the atmosphere. However, this is when the RVs and warhead bus are moving at the highest rate of speed and are capable of maneuvering in response to an intercept attempt. Terminal phase interception after RVs have been released and are reentering the atmosphere is technically possible (the Sprint system actually had several unintentional kinetic intercepts during testing) but because it occurs only during the last few dozens of seconds of flight a single interceptor site can only protect a very small footprint and there likely isn’t time for a second attempt, so intercept assurance requires the launch of multiple interceptors.
The “hitting a small, moving target” isn’t really be biggest challenge. It is difficult but the capability has been demonstrated by multiple systems. The bigger challenges are detecting and tracking an actual launch, discriminating between a genuine threat and decoys, building reliable sensors for the kill vehicle, and the command, control, and integration systems to tie everything together in a way that lets the system respond to a threat in a sufficiently timely fashion to pose a realistic defensive capability.